Agard Flight Test Technique Series Volume 4 Anttenae Patterns and Radar Reflection

8/14/2019 Agard Flight Test Technique Series Volume 4 Anttenae Patterns and Radar Reflection

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AGARD-AG-300-VOL.40M

,,

AGARDograph No.300

AGARD Flight Test Techniques SeriesVolume 4on f.fDetermination of Antennae Patterns and: .

Radar Reflection Characteristics of Aircraftby DIH.Bothe and D.Macdonald ELEICT

Edited byJU7 8jfA.PoolU

DI)TMI-BuTION STATEMENT AAppioed to PubiCes 7.

Ditrbuio

DISTRIBUTION AND AVAILABILITYON BACK COVER


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AGARD-AG-300-Vol.4

NORTH ATLANTIC TREATY ORGANIZATiUNADVISORY GROUP FOR AEROSPACE RESEARCH AND DEVELOPMENT

(ORGANISATION DU TRAITE DE L'ATLANTIQUE NORD)

AGARDograph No.300 Vol.4DETERMINATION OF ANTENNAE PATTERNS AN D RADAR REFLECTION

CHARACTERISTICS OF AIRCRAFTbyH.Bothe and D.Macdonald

A Volume of theAGARD FLIGHT TEST TECHNIQUES SERIES

Edited byA.Pool

This AGARDograph has been sponsored by the Flight Mechanics Panel of AGARD.


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THE MISSION OF AGARDTh e mission of AGARD is to bring together the leading personalities of the NATO nations in the fields of science and

technology relating to aerospace for the following purposes:- Exchanging of scientific and technical information;- Continuously stimulating advances in the aerospace sciences relevant to strengthening the common defence posture;- Improving the co-operation among member nations in aerospace research and development;- Providing scientific and technical advice and assistance to the Military Committee in the field of aerospace researchand development (with particular regard to its military application);- Rendering scientific and technical assistance, as requested, to other NATO bodies and to member nations inconnection with research and development problems in the aerospace field;- Providing assistance to member nations for the purpose of increasing their scientific and technical potential;- Recommending effective ways for the member nations to use their research and development capabilities for thecommon benefit of the NATO community.Th e highest authority within AGARD is the National Delegates Board consisting of officially appointed seniorrepresentatives from each member nation. Th e mission of AGARD is carried ou t through the Panels which are composed ofexperts appointed by the National D elegates, the Consultant and Exchange Programme and the Aerospace ApplicationsStudies Programme. Th e results of AGARD work are reported to the member nations and the NATO Authorities throughthe AGARD series of publications of which this is one.Participation in AGARD activities is by invitation only and is normally limited to citizens of the NATO nations.

Th e content of this publication has been reproduceddirectly from m aterial supplied by AGARD or the authors.

Published May 1986Copyright 0 AGARD 1986All Rights Reserved

ISBN 92-835-1530-7 m%Printedby SpecialisedPrintingServices Limited40 ChigwellLane,Loughton, Essex IGJO 37Z


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PREFACE

Since its founding in 1952, the Advisory Group for Aerospace Research and Development has published, through theFlight Mechanics Panel, a number of standard texts in the field of flight testing. Th e original Flight Test Manual waspublished in the years 1954 to 1956. Th e Manual was divided into four volumes: I.Performance, II. Stability and Control,Ill. Instrumentation Catalog, and IV. Instrumentation Systems.As a result of developments in the field of flight test instrumentation, the Flight Test Instrumentation Group of the

Flight Mechanics Panel was established in 1968 to update Volumes III and IV of the Flight Test Manual by the publication ofthe Flight Test Instrumentation Series, AGARDograph 160. In its published volumes AGARDograph 160 has coveredrecent developments in flight test instrumentation.In 1978, the Flight Mechanics Panel decided that further specialist monographs should be published covering aspectsof Volume I and II of the original Flight Test Manual, including the flight testing of aircraft systems. In March 1981, theFlight Test Techniques Group was established to carry out this task. The monographs of this Series (with the exception ofAG 237 which was separately numbered) are being published as individually numbered volumes of AGARDograph 300. Atthe end of each volume of AGARDograph 300 two general Annexes are printed; Annex 1 provides a list of the volumespublished in the Flight Test Instrumentation Series and in the Flight Test Techniques Series. Annex 2 contains a list ofhandbooks that are available on a variety of flight test subjects, not necessarily related to the contents of the volumeconcerned.Special thanks and appreciation are extended to Mr F.N.Stoliker (US), wh o chaired the Group for two years from itsinception in 1981, established the ground rules for the operation of the Group and marked the ouldines for future

publications.In the preparation of the present volume the members of the Flight Test Techniques Group listed below have taken anactive part. AGARD has been most fortunate in finding these competent people willing to contribute their knowledge andthne in the preparation of this volume.

Adolph, C.E. AFFTC/US.Bogue, R.K. NASA/US.Borek, R.W. NASA/US.Bothe, H. DFVLR/GE.Bull, EJ. A &AEE/UK.Carabelli, R. SAI/IT.Galan, R.C. CEV/FR.Lapchine, N. CEV/FR.Moreau, J. CEV/FR.Norris, E.J. A &AEE/UK.Phillips, A.D. AFFTC/US.Pool, A. (editor) NLR/NE.Sanderson, K.C. NASA/US.

J.T,M. van DOORN, NLR/NE.Member, Flight Mechanics PanelChairman, Flight TestTechniques Group.

ACKNOWLEDGEMENTTlhe authors acknowledge the invaluable assistance provided by many colleagues in the compilation of this paper. Inparticular thanks are due ,o Mr S.C.Woolcook, Manager of the UK Radio Modelling Facility, Dr D.L.Mensa of the PacificMissile Test Center, PMTC, California, Mr L.R.Hughes of Teledyne Micronetics, Messrs C.A.Lupica and J.Micheals ofRADC, Griffiss AFB, Mr C.Barnes of RATSCAT, Mr D.DeCarlo of the US Naval Air Test Center, Patuxent River, MessrsF.N.Stoliker and R.Mahlum of the USAF Flight Test Center and Mr R.W.Borek of the AMES-DRYDEN Flight ResearchFacility, Edwards AFB, M.P.Gaudon et al of CELAR, France, M.Renaudie of C.E.V., Bretigny, France, and M.N.Lapchineof C.E.V. Istres, France. In addition a lecture given by Mr W.F.Bahret in AGARD Lecture Series 59, October 1973 and a

technical note by Mr D.Cooper when with NASA at Edwards AFB, have provided useful material for Chapter 2.2.


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CONTENTS

PagePREFACE iiSYMBOLS AND ABBREVIATIONS viSUMMARY 1GENERAL INTRODUCTION IPART 1 DETERMINATION OF ANTENNA PATTERNS 21.1 DEFINITIONS 21.1.1 Introduction 21.1.2 Antenna Radiation Pattern (ARP) 21.1.3 Polaristion 21.1.4 Aspect Angie 21.1.5 Coordi, ee Systems 31.2 AIRCRAFT ANTENNAS 71.2.1 Aircraft Radio Aids 7

1.2.2 Types of Antennas 91.2.3 Antenna Positioning 121.2.4 Configuistion of Aircraft 131.3 TH E DETERMINATION OFTH E AR P BY MATHEMATICAL MODELI.NG 141.3.1 Advantaijes and Disadvantages 141.3.2 Integral Equation Method 141.3.3 Geomebial Diffaction Method 151.4 THE DETERM INATION OF ARP OF FULL-SIZE AIRCRAFFr IN FLIGHT 171.4.1 Advantages an d Disadvantages 171.4.2 Th e Air-Ground Propagation Channel 171.4.3 Aspect Angie Determination and Flight Profiles 201.4.3.1 Aspect Angie Determination 201.4.3.2 Flight Trajectories for Dynamic ARP Measurements 231.4.4 System Consideratlonw 29

1.4.4.1 Airborne Systems 291.4.4.2 Ground Systems 301.4.5 Puttern Calibration 341.4.5.1 Flyby Procedure 341.4.5.2 Calculation of the Effect of Ground Reflections 381.5 TH E STATIC DETERMINATION OF ARP OF FULL-SIZE AIRCRAFT' ON TH E GROUND 451.5.1 Advantages and Disadvantages 451.5.2 System Consideration 451.6 TH E DETERMINATION OF ARP BY SUB-SCALE MODEL MEASUREMENTS 491.6.1 Advantages and Disadvantages 491.6.2 Model Laws 491.6.3 Far Field Measurements 501.6.4 Anechoic Chamber Measurements 511.6.5 Comparison of Measurements on Full-Size Aircraft and Sub-Scale Models 54

PART 2 DETERMINATION OF RADAR REFLECTION CHARACTERISTICS 572.1 DEFINITIONS 572.1.1 Radar Cross Section (RCS) or Radar Echoing Area (REA) 572.1.2 Angular Noise or Glint 572.1.3 Polarisation 592.1.4 Near Zone, Fresnel Zone and Fraunhofer Zones 612.2 TH E PARAMETItVC DEPENDENCE OF a IN TH E RADAR EQUATION 612.2.1 Derivation of he Radar Equation 612.2.2 The Effect of Ranh Frequency 642.2.3 The Effects of Radar Polarisation 642.2.4 The Effects of Target Aspect Angle 66

2.2.5 Transmitter Waveform and R.ceiver Processing 662.2.6 The Effect of Target Range 672.2.7 The Effect of Partial liumination 672.2.8 The Effect of Pulse Length 67


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CONTENTS

Page2.3 THE DETERMINATION OF RCS OF REAL AIRCRAFT IN FLIGHT 67

2.3.1 Purpose: Advantages and Disadvantages 672.3.2 System Considerations 682.3.3 Existing Measurement Facilities 74

2.3.3.1 The US Naval Research Laboratory (NLR) Dynamic Measurement Facility 742.3.3.2 Edwards AFB, California, USA 752.3.3.3 Griffiss AFB, New York State, USA 762.3.3.4 CE V Bretigny, France 78

2.4 THE DETERMINATION OF RCS OF REAL AIRCRAFT, STATISTICALLY, ON THE GROUND 792.4.1 Purpose: Advantages and Disadvantages 792.4.2 System Considerations 802.4.3 Examples of Established Measurement Facilities 82

2.4.3.1 Holloman AFB, New Mexico, USA 822.4.3.2 Teledyne Micronetics, RCS Measurement Facility, San Diego, USA 822.4.3.3 ONERA, Chalais Meudon, France 822.4.3.4 CELAR, Bruz (Rennes), France 822.4.3.5 DFVLR, Oberpfaffenhofen, Germany 832.5 THE DETERMINATION OF RCS AND GLINT BY SUB-SCALE METHODS 85

2.5.1 General 852 2.5.2 The Principle of Scaled Modelling 852.5.3 Some Implementations of Scaled Modelling 852.5.3.1 Optical Simulation 852.5.3.2 Ultrasonic Simulation 852.5.3.3 Radio Modelling - Thorn-EMI, UK 872.5.3.4 Radio Modelling - Teledyne Micronetics, USA 892.5.3.5 Radio Modelling - Pacific Missile Test Centre, Pt Mugu, USA 902.6 CONCLUDING COMMENTS 90

REFERENCES 93APPENDICES 95L.A DERIVATION OF RADIO WAVE PROPAGATION PARAMETERS 95I.B DERIVATION OF THE TRANSFORMATION EQUATIONS FOR THE ASPECT ANGLE 982.A RADIO MODELLING IN THE UK 101

2.AI Description of Facility 1012.A2 Typical Measurements and Studies 1112.A3 Validity of Data Produced in Relation to Full Scale Dynamic Measurements 11 22.A4 Data Reduction 11 92.A5 Continuing Developments 12 0

Accession ForNTIS (It,?IDTIC T.%3Unanno:,::' c r.:

iJusti"'' -, ":

QUALITY Distr i I:,.:.CINSPECTED Av a il . .' .Dis.~.t


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SYMBOLS AN D ABBREVIATIONSPart 1a radius of earth 6365 km (3437 NM )A effective area of antennaAT longitudinal amplitude taperAR effective area of receiving antennae velocity of lightd horizontal distance aircraft-ground stationd, horizontal distance point of reflection to ground antennad2 horizontal distance point of reflection to airborne antennaD distance between a vehicle and a ground stationD divergence factorE electric field intensityfield strength of direct wavei! resultant field strength vectorf frequencyGM multipath gainGR gain of receiving antennaGs gain of standard gain antenna compared to an isotropic radiatorGr gain of transmitting antennah antenna heighth, height of ground antennah2 height of airborne antennahR height of reflection areahl. height of ground antenna, corrected for elevated reflection areah2u height of airborne antenna, corrected for elevated reflection areak refraction correction factor (k -r 4/3)I distance of antenna from the radio horizonL general losses (e.g. cable)M rotation matrixn index of reflectionPA measured power input of aircraft antennaSPR received powerPs measured power input of a standard gain antennaPT transmitted powerPEA power received from an aircraft antennaPEs power received from a standard gain antennaAP difference of powerr distancereflection coefficientRH reflection coefficient of horizontal polarisationRv reflection coefficient of vertical polarisationS' power densityX coordinates of a cartesian systemY coordinates of a cartesian systemZ coordinates of a cartesian systemU slope of reflection areaPA position dependent angle (see Fig 1.26)PE position dependent angle (see Fig 1.27)CO permittivity of free spaceC, relative permittivity0 phase lag of electromagnetic wave, angular path length difference0 vehicle pitch (nose up is positive)OD depression angle0E earth curvaturewavelengthrotational angle around the X-axis11 permeability1 rotational angle around the Y-axisv rotational angle around the Z-axisp azimuth angle of ground tracking system

E elevation angle of ground tracking systemo conductivityvehicle roll (right wing down is positive)0 phase of reflection coefficient4)A horizontal aspect angleou vertical aspect angle


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%F vehicle heading relative to true Noyth (right turn ispositive)V angle o&. incidencePart 2

AGC Automatic Gain ControlA, receiver antenna capture areaBn noise bandwidthCW continuous wavedB decibel (10 Iog0 P1 /P 2)D maximum linear dimensionE electric fieldEl scattered electric fieldEi or ei incident electric fieldF. noise figureG, gain of receiver antennaG, gain of transmitter antennaH horizontalHCN hydrogen cyanideK Boltzmann's constant (1.38 X 10-23 watts/hertz/kelvin)LL transmit an d receive LEFT HAND CIRCULAR POLARISATIONP powerP. power transmittedP, power receivedP scaling factor0 magnification factorR rangeRt range of radar to targetRME Radio. Modelling EquipmentRPV Remotely Piloted VehicleRR transmit and receive RIGHT HAND CIRCULAR POLARISATIONRL transmit right hand and receive LEFT HAND CIRCULAR POLARISATIONSmin minimum detectable signalS/N signal to noise ratioS/N(min) 'iiinimum value for detectionT temperature in kelvinTHz 1012 HertzV verticalv voltizge

wavelength0 Radar Cross Section (RCS)

=t


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DETERMINATION OF ANTENNA PATTERNS AND RADAR REFLECTIONCHARACTERISTICS OF AIRCRAFTby

Helmut BotheDFVLR/ FF-FLP0 BOX 32673300 BraunschweigGermany

and

Donald MacdonaldRoyal Signals and Radar Establishment

SUMMARY

This AGARDograph is divided into two parts: Determination of Antenna Patterns ofcraft, by D.Macdonald.-APart I describes the different types of aircraft antennas, their radiation characte-ristics and their preferred siting on the airframe. Great emphasis is placed on the var-

ious methods for determining aircraft antenna radiation patterns (ARP) and advantages,disadvantages and limitations of each method are indicated. Mathematical modelling, mod-el measurements and in-flight measurements in conjunction with the applied flight testtechnique are included. Examples of practical results are &g'ven.Part 2 describes methods of determing aircraft radar characteristics, indicating ad-vantages, disadvantages and limitations of each method. Relevant fundamentals of radartheory are included only as necessary to appreciation of the real meaning of radar crosssection (RCS) and angular glint. The measuring methods included are dynamic full-scale,

made to RCS measuring facilities in the USA and Europe and the UK Radio ModellingFacility is used extensively to exemplify the sub-scale technique. ----

GENERAL INTRODUCTIONAlthough the two authors have had regular contacts during the time this AGARDographwas written, each author only bears responsibility for his own part. Mr Macdonald has,in addition, assisted Dr. Bothe by correcting the English language of the first part.

The reason fo r combining the two subjects in one AGARDograph is, that many of thepractice, however, this similarity does not go very far and the research and teexecu-tion of testa for the two subjects is very often done by different institutes. For thatreason the two subjects are treated separately.

Althughtheprimary object of the AGARDograph is to acquaint flight test engineerswith the principles of the flight test techniques for the subjects, the authors wereasked to include brief discussions of other test methods that are available. Flighttesting is very expensive and the flight test engineers should be aware of those othermethods which, though they cannot replace flight testing completely, may significantlyreduce the flight time required. It will be found that both authors discuss the possi-bilities of full-scale ground testing and of reduced-scale model testing. In Part 1 anadditional chapter is devoted to mathematical modelling techniques which has no counter-part in Part 2. Although mathematical modelling is used in the design stage of the air-craft for predicting both antenna patterns and radar cross sections, the author of Part2 is of the opinion that it is not sufficiently advanced to be used in the process ofthe actual determination of the radar cross section. The complex nature of the radarscattering from aircraft engines and their intakes and from radar installations have so0far proved to be intractable to realistic mathematical modelling. Of course, mathemati-results and be checked by actual flight testing.

Now with Thorn-EMI Electronics Ltd


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2

PART 1DETERMINATION OF ANTENNA PATTERNSbyHelmut Bothe

1.1 DEFINITIONS1.1.1 Introduction

Antenna patterns are determined in order to obtain information on the spatial energydistribution of the signals transmitted from radio-frequency antennas and on the sensi-tivity of receiving antennas for signals coming from different directions. For the de-termination of these spatial distributions a number of methods are used:- Mathematical modelling- Dynamic measurements on real aircraft in flight- Static measurements on full-size models or aircraft on the ground- Measurements on sub-scale models.

These methods are described separately in Chapters 1.3 to 1.6.Definitions of the most important notions that are used in this part are given inthe remainder of this chapter. Chapter 1.2 decribes the most important characteristicsof the antennas used on aircraft.

1.1.2 Antenna Radiation Pattern (ARP)An important property of a radio frequency link is the electro-magnetic fiel~d inten-sity at every point in space for a given output power of the antenna. As the propagationof radio frequency waves in free space is well known, the spatial distribution of thefield intensity needs only be measured at one spatial sphere around the antenna. Theinformation is usually given as distributions along the circumference of flat sectionsthrough this sphere: each of these is called an Antenna Radiation Pattern (ARP). SeveralARPs are usually required to describe the complete spatial antenna pattern of an an-tenna.In many applications only the relative energy distribution is of interest. The radi-ation energy is then normalized by dividing it by its maximum in the main radiation di-rection and a relative ARP is derived.

1.1.3 Pola~risationThe orientation of the electric field vector in the direction of maximum radiation

is defined as the polarisation of the antenna. As an example a vertical dipole or stubwill radiate vertical polarisation while a horizontal dipole will generate horizontalpolarisation. If several antenna elements with different orientations radiate simulta-polarised wave. A circularly polarised wave - a special case of elliptical polarisationisradiated if two perpendicular linearly plrsdfields have a 90phase differ-ence. Then the electric field vector rotates right-handed or left-handed, depending onth nof the 900 phase difference. If an antenna is designed to radiate with a cer-tanpolarisation and additionally radiates in an undesired orientation, this unwantedcomonntis called cross polarisation. Hence the cross polarisation of vertical ishorizontal polarisation and the cross polarisation of circular polarisation is a coun-ter-rotating circularly polarised field. Two counter-rotating fields of differentamplitudes sum to an elliptically polarised field.If the radiating and the receiving antennas of a radio link both have the sane po-larisation, then the polarisation is said to be matched. A polarisation loss of energytransmission occurs if the antennas have unmatched polarisation.In air to ground radio links polarisation is usually unmatched because attitude,height and distance of the aircraft with respect to the ground station vary, which re-sults in changes of relative polarisation orientation. Since the polarisation of theairborne antenna radiation is variable with respect to the earth axes, it is nominally

assigned the polarisation of the ground receiver antenna.1.1.4 Aspect AngleI

The radiation direction of an aircraft antennt with respect to a receiving station(on the ground or in a second aircraft) is defined by the aspect angle. This is theangle between the roll axis and the line of sight (Fig 1.1). In order to cover the wholesphere, the aspect angle is resolved into two parts. The depression angle is measured inthe yaw plane between the roll axis and the projection of the line of sight perpendicu-lar to the yaw plane. In po~lar ARP plots one of these angles is usually taken as theindependent variable, while radiated power is the dependent variable. For static meas-urements of models or full sized aircraft, the aspect angle is readily obtained from theangle readouts of the pedestal. As will be shown later in-flight measurements are morecomplex because the aspect angle depends on the relative locations of the ground stationand the aircraft, as well as on the attitude of the aircraft.


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3

""'" I "4ORIZONTAL

YAW AXIS

GROUND STATION

Fig 1.1 Aspect angle and vehicle coordinate system1.1.5 Coordinate Systems

Spherical coordinates fo r general use in antenna pattern measurements are defined byIEEE standards published in Ref 1 and shown in Fig 1.2..0,o

ISO-, *)

0,*0 "9 go* -go:

0..

-20.. . / _ I _ - 0

4+.o /. I /0.90/

60 180"Fig 1.2 IEEE standard antenna coordinate system


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44

In addition the IRIG hbs published recommendations on how the orientation of an an-tenna-bearing vehicle should be described in the IEEE standardized system (Ref 2). Fig1.3 illustrates ho w the orientation of the vehicle, which is described by its pitch,roll and yaw axes (see Fig 1.1), lies along the Z-axis of the system illustrated in Fig1.2. The complete roll plane is covered by the angle 0 and together with the angle egives coverage over the entire sphere.

X~ 000, go ,

// ,-F 170C i -P) PITCH oA IS G oP ) ( R ) 9

Thiscl euie a 80 rtaioaf heIEE osystemfig 1.2 around the connecTring lined ewe =0, = 0ad0 80

Fi 1.3h osite yaw:yxis is at r = O hi arrutng in Fig 1.4hat~~~~~~~~~~~ ~ ~herigtmingonie iho=0.Ti eursa10 oainof pthe IEE co-

ordinate system of Fig 1.2 around the connecting line between o= 00, 0 = 900 and o= 1800,O= 900. So the positive yaw axis is at e =00. This arrangement illustrated in Fig 1.4will be used throughout this book.


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5

8 1800

8......8,....4' 900 I2700go --0 ----- 6 90"

f 00IIFig 1.4 Orientation of aircraft to IEEE standard coordinate system in dynamic

The depression angle defined in Fig 1.1 will be denoted 0 ' The relations betweenaDand the angle 6 of the IEEE-system are

e 00 900 1800+90" 00

In order to achieve easy comparisions between dynamic and static measurements, thesame coordinate system and notations will also be used for static measurements of modelsor full sized aircraft.Aircraft antenna patterns are often presented in polar diagrams. Horizontal patternsare conveniently recorded by a continuous variation of the horizontal aspect angleswhile the depression angle a is stepped as a parameter. This leads to a movement ofmodels under test on a cone, and the recorded patterns are conic section patterns. Dueto the limited manoeuvrability of a full-sized aircraft in flight, conic section pat-

terns cannot be measured during a complete continuous flight pattern. Nevertheless thistype of mapping is frequently applied to model measurements.The usual polar patterns fo r in-flight measurements are great circle patterns, whichare r'.corded as the aircraft moves in a complete horizontal circle. If the circle isflown with different angles of roll, radiation patterns in the corresponding inclinedplanes through the aircraft roll axis are obtained. This means that the depression anglealso cnmtinges and true oarametric plots are thus no t achieved.A "matrix" plot in the form of a spherical surface projection p',ovides radiationintensity at increments of 0 and e or e over the entire sphere (see Fig 1.5). Radiation

inten1i.ty appears as a plotted number iR each element of the matrix or as contour linesof equal radiation intensity. The first representation allows only a rough angular reso-lution. The second one suffers from poor resolution of radiation intensity.


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aSO ~(edgo*)Spheica Coordincatyitem

far ANTNNA PATTE~RNMeauwernents e906

PATTERN DISPLAY 4) -G10(Rectangular Projection Stxface)

____ ____ ______ _ __ _ ____ ____ ____ __ * * 279,

matrixepresntatho

Inea 000-20a.0*90


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71.2 AIRCRAFT ANTENNAS1.2.1 Aircraft Radio Aids

Aircraft radio aids can be divided into 3 groups: Communication, radio navigationand identification. With one exception -the weather radar- all radio services can beassociated with one of these 3 categories.The aircraft communication services are listed in Tab 1..L.

Tab 1.1 Aircraft communication servicesLA a Large Aircraft, SA - Small Aircraft, H = Helicopter

Function Frequency Trauffit T Trans- NLmber of Polari- I Antenna Positi-I Antenna Typerange Receive R mitter Antennas sation I on(see Fig 1.161Power I to 1.18) 1Voice Ccmmu- IIInication I I II/F 1 3-25 Iz IT R 120 W 1-2 her. LA short stubs atw tipsILA 1 to 4 lon wire

LA 3/4 tail capLA/SA 3 notchVoice Commu-1Inication IIIIIjVHF/FM 130-76 NH z IT R O1W vert. I A 1 to 3 stub

8A 2 stubH 2,86 stub

VH F 118n36 Mt 25 W 2-3 vert.- 1,2,(6),7,8 stubSA 2,(6),7,8 stubSA 5 slotH 1,2,6,7 stubIVoice Coamu-1 I I.'.nication UH F 224-400 z I R 50 W 1 2-3 1 vert. I LA 1,2,(6),7,8 stub11SA 2,(;),7,8 stubSA 5 slot

H ,(2),7 stubTIData Cormm- 1225- 26 &IT R 1100 W 1-2 vert./ I A1,2!,4,6,7,81 suInication 1 1435-1535 MHzl hor. 1 (i),4,(7),(8)I 1/2 deer horn II(Telemetry) 1 2200-2300 M-zI I I (Fig 1.10)112400-25001vi~zI I I SA2467, I~1 2,4,(6),7,81 1/2 deer horn1 5 1 slotI H 1,(2),(6),71 stubI ,I

The VHF communication band is used fo r preference in civil aviation whereas the UHFband is reserved for military applications. Both services are limited to line-of-sightlinks. For long range communication only the HF-band is applicable.

.4C

___


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8Tab 1.2 Aeronautical radio navigation services

LA = Large Aircraft, SA f Small Aircraft, H - Helicopter

Furction Frequency Transmit T I Trans- Minber of Polari- Anterna Positi- I Anterma TypeeI range lReceive R I mitter IAntenmas I sation cIns(see Fig 1.161I IPower I to 1A18)ILong Range 11750-1950WzI R - I I vert. LA 7 small loopIDirection I short morpoleiFiniigLr I IIDirection 190-1750 4zI R 1-2 loop I vert. LA 1,2,7 sIall loop_IFinding (ADF) I I SA 2,7 short mmopole

H 2,6,7IVOR (Bearing) 108-118 NHz I R 1-2 hor. 1A 1,4,(8) Ideer hornLA 5 2 half loopsSA 2,4,(8) deer hornSA 5 12 half loopsILS (Marker) 1 75 Hz R 1-2 1 hor. I LA,SA,H6,7 11/2 deer hornILS(Localizer)I 108-112 MH z Ruses VOR- hor. same as V0 I same as V0RSAntennasILS (Glide 329-335 M1z R 1-2 1 hor. I LA,H9 half loopPath) LA,SA,HI same as VO R Isae as VORIIII I I

1 1IDME/TACAN 962-1214 MzI R kWpeak 1-2 vert. LA 1,2,4,6,7,8 stubI(Distance/ III IISA 2,4,6,7,8 stubIBearinz) III IISA 5 slotH 1, (6),7 stub

IGPS (NAVSTAR) 1575 MHz I R1circ. ILA 1, spiral/SA 2 microstripH (1),2 patch

Radio 14.2-4.4 Cz I1W 1-2 T Iho. LA,SA 7,8 hornlAltimeter I 1 1-2 R IH 6,7 hornI I I II IIIIIDoppler 18.8 G-z I T I1W 11-2 1 hor. I LA,SA 7,8 slotiNavigator I I I I I I H 6,7 slotI I I I I I III I I I I IIMLS I5-5.25 GI-iz R 2-4 vert. 1A 9 horn(Replacing ILSI AI1,4,8 stublin Future) I I I I S 4,8 stubSI IH 1,7 stub

The aeronautical radio navigation services listed in Tab 1.2 are used worldwide incivil and military aviation although VOR/ILS is primarily designed fo r civil applica-tions and TACAN fo r military purposes.


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9The services fo r identification of civil and military aircraft are presented in Tab1.3.

Tab 1.3 Aeronautical identification services

Function Frequency Transmit T I Trans- I Number ofl Polari-I Antenna Positi- I Antenna Type II range Receive R I mtitter I Antennas I sation I ons(see Fig 1.161Power I I I to 1.18) 1ISecondary I 1030/1090 MH z I T R IiKWpeak i1-2 vert. I LA (6),7,8 stubIRar' I SA (6),7,8 stubITransponderl I H 7 stub

IFF 1 1030/1090 i4-zI T R IlKWpeak i 1-2 vert. I LA 1,2,(6),7,8 1 stubI(military) Il I SA 2,(6) ,7,8 stubI I H (2),7 stub* I II III I

1.2.2 Types of AntennasAntennas can be classified into single radiating elements, arrays of such elementsand reflector or lens type antennas, which can use single radiating elements or arraysas excitation sources. With a few exceptions aircraft antennas require wide angular cov-erage, which can be achieved only by single radiating elements, and these are generallyused.The basic elements used fo r aircraft antennas are the dipole, the loop and the slot.Most other types of aircraft antennas fo r wide angular coverage are derived from thesebasic elements.The dipole and its radiation patterns in 3 orthogonal planes are illustrated in Fig1.6. Omnidirectional coverage is achieved only in the y-z plane. In the two other planesthe beam width depends on the ratio between the length of the antenna and the wavelength: If the length of the dipole is short compared to the wavelength the beam widthis 900, a half-wavelength dipole has a beam width of 780 and a full-wavelength dipole awidth of 470. Still longer dipoles will produce additional side lobes, which are notacceptable in most aircraft applications.

V z zZZ1XxY

Fig 1.6 The dipole and its radiation patternsThe patterns of a small loop antenna, with a circumference which is much smaller

than the wavelength, are the dual of the short dipole and are shown in Fig 1.7. The pat-tern is omnidirectional in the plane of the loop, and if the circumference is enlargedto a full wavelength, the pattern becomes omnidirectional in the y-z plane (see Fig 1.6)(i.e. perpendicular to the plane of the loop).

Y z zIA ,1

Fig 1.7 The small loop and its radiation patterns


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10Fig 1.8 represents an antenna which consists of a 1.rnaar slot in an infinite flatmetallic sheet. If the slot is cut in a sheet of finite dimensions, the x-y plane pat-tern will change drastically, because the fields on the two sides of the plane are equalin magnitude but opposite in phase. Hence there is a null in all directions in the planeof the sheet, and therefore in the x direction of the x-y plane pattern of Fig 1.8.

Z&Yz zxi--

Fig 1.8 The linear slot and its radiation patternsThe aircraft antenna most frequently used is the stub illustrated in Fig 1.9, whichis a monopole fed against a ground plane (the airframe). With an infinite flat groundplane the radiation pattern is identical with half the pattern of a dipole. A finiteground plane causes spillage of radiation into the hemisphere behind the plane.

Fig 1.9 Stub antenna

Another antenna derived from the dipole is the deer-horn antenna (Fig 1.10). Itsradiation pattern is very dependent on the slope of the bent-back arms. The main diffe-rence in the radiation characteristic is in the plane of the elements, where the dipolenulls are filled in by radiation from the bent portion of the conductors. This antennais sometimes used in an asymmetrical configuration similar to the stub antenna mentionedabove. Short asymmetrical deer-horn antennas are often tuned to a particular frequencyby a capacitor at the open end of the conductor.

Fig 1.10 Deer-horn antenna

A half-loop antenna operating against an infinite ground plane (Fig 1.11), producespatterns nearly identical, in half-space, to the patterns of Fig 1.7. If the groundplane is of finite dimensions energy is spilled into the hemisphere behind the plane.

Fig 1.11 Half-loop antenna


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Most linear slot antennas used on aircraft are operated with a backing resonantcavity (see Fig 1.12), which limits radiation to one hemisphere and eliminates radiationinside the airframe.

Fig 1.12 Cavity backed slot antenna ~The annular slot antenna of Fig 1.13 can be regarded as a top loaded stub antennaretracted into a circular cavity. Its radiation pattern is the same as that of a shorthalf dipole or stub.

Fig 1.13 Annular slot antennaIn aeronautical microwave applications, especially if larger bandwidths have to becovered, spiral antennas over absorbing sheets, or cavity backed spirals are useful. Fi g1.14 illustrates the mechanical configuration and radiation patterns of a spiral an-tenna.

z

Fig 1.14 Cavity backed spiral antennaIf higher directivity is desired, horn antennas may be used, the basic radiationcharacteristic of which is shown in Fig 1.15.

y Z

Fig 1.15 Horn antenna


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12For microwave applications microwave-patch ant~ennas are coming into general use. Theyconsist of a thin dielectric layer on one side plated with specially sk.aped conductingareas. The other side must be attached to a conducting surface. For aeronautical ap-plications the thin, flat shape is a great advantage. The principle is that discontinu-ities in th e conducting areas (curves, bends, T junctions, changes in conductor width)will radiate electromagnetic energy, especially if the dimensions of the discontinuitiesare of the order of th e wave length. By choosing different shapes for the discontinu-ities, several types of antennas can be made, especially medium-gain antennas (gain upto 10 dB).

The last columns of Tables 1.1, 1.2 and 1.3 indicate which types of antennas aremost commonly used for the services listed there.I1.2.3ntenna PositioningThe radiation patterns of the different types of antennas discussed in the previoussection are degraded to some extent if the antennas are mounted on an aircraft, sinceenergy is reflected or diffracted by the wings, fuselage and tail fin and, to a lesserdegree, by smaller assemblies such as the undercarriage, flaps and other antennas. Injaddition, the performance of antennas on military aircraft suffers when supplementarytanks. ECM-pods, weapons and other stores are mounted below the fuselage or wings.It is always aL roblem of optimization to find an antenna position where the de-terioration of the antenna's principal radiation pattern is minimized, at least in thatsection of the sphere where the probability of receiving is high. Moreover those sec-tions of th e sphere in which long distance links must be made are most critical. Hencepositioning of aircraft atntennas requires much experience and investigation when a newaircraft type is developed. Nevertheless, there are certain areas on the aircraft struc-ture where, in general, antennas for the different radio aids give the best results.

90~ Fig 1.16 Antenna positioning areas (large aircraft)

The surface of a large aircraft can be divided into 9 different areas fo r antennapositioning, as shown in Fig 1.16. On a small aircraft (Fig 1.17), area 1 is not usablebecause it is occupied by the cockpit and area 9 is normally not available because ofother utilizations e.g. by the engine, or by weapons if military aircraft areconsidered. The usable surface of a helicopter (Fig 1.18) is even smaller. Antennaspositioned in areas 1 and 2 suffer from reflectio~ns from the rotor. Areas 5 and 6 areshadowed by the fuselage in the forward direction, which limits the frequency range ofantennas in this areas to VHF (UHF) , and area 7 is the only one with no severerestrictions.

5

Fig 1.17 Antenna positioning areas Fig 1.18 Antenna positioning areas(small aircraft) (helicopter)


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. U1U'fP- J5 V

13Tables 1.1,1.2 and 1.3 list the different aeronautical radio aids and the viable an-tenna positioning of the prefered types of antennas. Positions are given for large air-craft (LA), small aircraft (SA) and helicopters (H).

1.2.4 Configuration of AircraftEvery deformation of the outer contour of an aircraft influences to a certain degreethe radiation patterns of the antennas, which form part of the radiating structure. Thisincludes all sections, which can be "seen" from the location of the antenna. For thisreason aircraft ARP are first measured in a "clean" configuration and then in one ormore altered configurations, where for example landing flaps and/or the undercarriage

are in the landing position. The antennas of radio aids which are not used during theapproach and landing phase of course need not be measured in the landing configuration.In addition military aircraft , especially fighters, usually carry weapons and fueltanks as outer stores, which leads to many different configurations to be considered forARP measurements. In order to keep down the number of patterns to be measured, a carefulstudy of the interactive effects should be made before planning the test program. Hereearlier model measurements can save many flight test hours during dynamic measurements.Frequently even control surface deflections can influence the ARP of antennasmounted only a few wavelengths away. As an example Fig 1.19 illustrates the radiationpattern of a VHF navigation antenna for VOR and ILS reception. This antenna, a deer-horntype (Fig 1.10), is mounted in position area 5 of a small aircraft (see Fig 1.17). Ascaled model measurement of this antenna indicated by the dotted line in Fig 1.19 showsexcellent symmetry of the ARP (Ref 13). The dynamic in-flight measurement (solid line)illustrates certain ARP asymmetries in the area of 4 = 400 and * - 2200 (Ref 14). Thereason is, that this pattern is measured during a skidded turn of the aircraft, wherethe control surface of the vertical stabilizer has a large deflection and affects theradiation characteristics of the nearby antenna. ~~

noj

Fig 1.19 Radiation pattern of a VHF-navigation antenna at 114 MHz....... scaled model measurement - dynamic (in-flight)measurement

___~~~J ___________


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141.3 THE DETERMINATION OF THE ARP BY MATHEMATICAL MODELLING1.3.1 Advantages and Disadvantages

Modern high-speed computers with large storage capacity have made possible the theo-retical calculation of the ARP of antennas mounted -on complex structures such as air-craft or helicopters.The main advantage of this mathematical method is that, once the shape of the ve-hicle has been rerepresented in the computer, the influence of different positions ofthe individual antennas can be easily evaluated. If the position of an antenna has beenselected on the basis of a computer evaluation, the number of measurements can be cutdown to a minimum.Disadvantages are that the shape of the vehicle can only be modelled approximatelyand that surface parameters such as conductivity and susceptibility are only roughlyknown. Therefore, the calculated results may contain errors and can only be considered

as approximated patterns which usually have to be supplemented by full-scale measure-ments, either statically on ground ranges or by in-flight measurements.

Two different theoretical ARP-computation methods have been developed: the integralequation method and the geometrical theory of diffraction method. Which method must beused will depend on the size of the vehicle compared to the wavelength of the antennaunder consideration.1.3.2 Integral Equation Method

The integral equation method is based on the determination of the surface currents,from which all necessary electromagnetic parameters can be calculated. The vector waveequations are transformed, by means of the second Greens theorem, into two independentequations, the Electric Field Integral Equation and the Magnetic Field Integral Equa-tion. Ref 15 shows that the surface current density depends on the electric Zield andth e magnetic field of the incoming wave from the feed point. Integration must be carriedout over the surface of the ideally conducting body of the vehicle under test. The ap-plication of the Electric or Magnetic Field Integral Equation depends on the shape ofth e model surface.

The solution of the integral equations must be obtained by numerical methods (Ref16). For practical applications, especially for complex structures such as aircraft orhelicopters, wire grid modelling allows further simplification of the integral equa-tions. The surface of the device under test is then approximated by a wire grid, asshown in Fig 1.20. Experience has shown that about 100 segments per square of the wave-patterns. After the determination of the wire currents, Kirchhoff Is laws are applied toth e wire junctions.

.1.0

VHF-Antenno 21 12

110S 15 calculatedIgo mea~ured

Fig 1.20 Wire-grid model of the Fig 1.21 Calculated and measured radia-the helicopter BO 105 (Fig 1.20)Frequency 117.6 MHz


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15At the Institute for Radio Frequency Technology of DFVLR, Germany, the radiationpattern of a VHF antenna mounted on top of a BO 105 helicopter (see Fig 1.21) harileencalculated by this method and was compared to the radiation pattern measured in flight

(Ref 17). It was necessary to model in great detail only the upper part of the heli-copter, including the rotor shaft.1.3.3 Geometrical Diffraction Method

In the case where the aircraft is large compared to the wavelength, the subject isnot totally excited and only currents of certain surface elements contribute to thetotal field at the point of observation. In this case the geometrical theory of diffrac-tion can be applied with success. The complicated shape of the aircraft is divided into

All possible rays significantly contributing to the field must be considered andnt ccount. As an example some important rays contributing to the radiation pattern ofan antenna on the upper side of the fuselage of an aircraft are illustrated in Fig 1.22.

cone of diffracted rays

6. reflected ray

surface ray

Fig 1.22 Examples of different rays on aircraft surface and edges

After all possible ray paths have been determined, the total field can be computedif the individual reflection-, diffraction- and launch coefficients, the attenuationconstant and the divergence coefficient can be obtained. The canonic forms applied toaircraft antenna radiation pattern calculations are the diffraction on curved edges,corners, stubs, half planes and on smoothly curved surfaces. A detailed treatment ofthis method with a presentation of the individual equations is outside the scope of thisvolume, the reader is referred to Ref 8, a presentation of the principle of the geome-trical theory of diffraction, and to Ref 9, which treats the field computation by thegeometrical theory of diffraction.A few results of a comparison between calculated patterns and patterns obtained fromsub-scale model measurements are reported in Ref 10. As an example Fig 1.23 illustratesthe differences between the calculated and the measured radiation pattern of a VHF an-tenna, determined in the roll plane of a HFB 320 aircraft.


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16

VHF-Antenna

6=1800

=9=90-=90* *=270o$.go. 6.70

calculatedS..... measured=o0o0

Fig 1.23 Calculated and measured radiation pattern of a VHF antennain the roll plane of a HFB 320 aircraft


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171.4 THE DETERMINATION OF ARP OF FULL-SIZE AIRCRAFT IN FLIGHT1.4.1 Advantages and Disadvantages

During in-flight measurements of AR? the aircraft antenna under test is part of anair-to-ground radio link. The aircraft flies selected manoeuvres in front of the groundantenna, whereby the following parameters are recorded:- if the transmitter is on board the aircraft, the transmitted power, ifthe receiver is on board, the received power- the (transmitted or received) power at the ground station- the position of the aircraft relative to the ground station- the attitude angles of the aircraft.The gain of T6he aircraft antenna must then be calculated from the range equation Eq(1.9). The flight trajectory for these tests must be chosen carefully to ensure that theother parameters in the equation remain as well as possible constant and can be calcu-lated with the maximum accuracy. The optimum trajectories are discussed in Section1.4.3.2.

The advantage of this method is that the antenna gain is measured under actual con-ditions, without any errors due to modelling imperfections. The effect of moving parts,such as propellers, helicopter rotors and stabilizing rotors is fully taken into ac-count. Measurements of this kind are necessary for the certification of new aircrafttypes, even if model calculations. and model measurements have been carried out.The main disadvantage of this method is the high cost of the flying hours that arerequired. For that reason the flight tests usually are the final stage of a long process

of modelling. Additional problems of the in-flight measurements are:- the flight characteristics of the aircraft limit the choise of aspect~angles at which measurements can be made

tests and it is difficult to eliminate these effects.The effects of these disadvantages can be reduced by careful planning of the trajecto-ries flown during the tests, as is described below.1.4.2 The Air-Ground Propagation Channel

In a free space radio-frequency link with an isotropic radiator as a transmitting4antenna which has no preferred direction of radiation, the power density Sr at the re-ceiver at a distance r is

s= T (1.1)4wnr

where P is the transmitted power and 4irr2 is the total surface of the sphere on whichthe pow;r density Sr is measured. If the transmitting antenna radiates in a preferreddirection, the ratio of the maximum power density to that of an isotropic radiator withthe same total output power is called the gain GT of the transmitting antenna, and Sbecomes

=r 4T *GT(1.2)

If the receiving antenna has an area A., the received power P can be expressed asPR = 3r *A R (1.3)

The following relation between effective area A, gain G and wavelength X. applies to allantennas:

A X2(1.4)

Equations (1.2) and (1.3) can be combined as

R0 (1.5)AR 471r2


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18

and, introducing Eq (1.4) with th e gain of th e receiving antenna GR

PRG 2 (1.6)GR 41wr 2 4v

As x = c/f where c = velocity of light and f = frequency, Eq (1.6) can be writ ten asPR GR GT '2-- = (1.7)PT - (4*r)2 f2

Eq (1.7) can be rewritten as a "gain-loss" equation and can be completed by including aground reflection multilath gain GM and general losses L, e.g. cable losses.

PTGRGTGM=PRf 2 r 2 ( 1.8

In logari thmic units of measure this becomes

PT + GT + GR + GM = PR + L + 20 lo g r + 20 lo g f + 2.54 dB (1.9)It isP = output power of transmitter in dBW (dB above 1 W)G = gain of t ransmit t ing anteina in dBGR = gain of receiving antenna in dBGM = gain of reflecting ground in dBPR = input power of receiver in dB m (dB above 1 milli Watt)L = line losses (between power measuring terminals and antennas) in dBr = distance between t ransmit t ing and receiving antenna in km (if th e distance ismeasured in NM, th e last term (2.54 dB) must be replaced by 7.8 dB)f = t ransmit t ing frequency in MH z

The basic transmission loss designated by th e right-hand side of Eq (1.9), dependsvery much on th e geometrical relationships between th e t ransmit t ing and receiving anten-nas and th e earth below, as discussed in more detail in section 1.4.5.2. According toRef 11 th e airspace above th e earth may be separated into three different regions: th eline-of-sight, th e diffraction and th e scatterregion. Due to its stable, predictablepropagation condit ion only th e line-of-sight region, which offers a direct path betweenth e two antennas, is suitable for in-flight antenna measurements. The separating linewhich limits th e line-of-sight region can easily be derived from Fig 1.24:

Fig 1.24 Geometrical relationship in an air-to-ground radio link(d, d1 and d 2 are distances measured along th e surface of th e earth)


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19

r = 11 + 12 = (ka+hl)2 - (ka) 2 + (ka+h 2 )2 -(ka) 2 (1.10)1and 1 are the distances from the antennas to the radio horizon, hl and h are th etenna eights, a is the radius of earth (a = 6365 km), and k = 4/3, a factor whichcorrects fo r refraction, assuming that rays are straight lines bu t that the earth has aradius 4/3 timeo .a large as its actual value. To evaluate Eq (1.10) accurately requiresa high computat'.onal accuracy. For practical applications where 1 and 1 are computedby a pocket calculator the problem can be reformulated using the rianlgg sine rule inorder to eliminate squares and roots. It is

11 = (ka + h1 ) sin cos-1 (ka/(ka + hl))12 = (ka + h 2 ) sin cos-1 (ka/(ka, hA

Fig 1.25 shows the limits of the line-of-sight region fo r different antenna heights h1and h2 .4000Ift hi=

_______h ____ 1 30f3000V Line -of -sight

2000h2 101000 diffraction /scatter

20 40 60 80 100 120 140 km 160Fig 1.25 Different regions of radio wave propagation

As mentioned cbove, the ARP (i.e. the variation of the gain of the antenna undertest) is determined from the output signal of the receiver in the radio link set up forthe test. Because the dynamic range of the received signal is large, the receiver musthave a logarithmic characteristic, oi , an automatic gain control circuit ,As a consequence of the reciprocity theorem of antennas the transmitting and re-ceiving patterns of an antenna are the same. It, therefore, makes no difference whetherthe antenna under test is the transmitting or receiving antenna in the radio link se t upfo r pattern measurements. Up to frequencies of several GHz the transmitter is usuallysmaller and weighs less than the receiver. Therefore it is convenient to mount thetransmitter in the aircraft. At much higher frequencies the transmitting equipmentbecomes heavy and volumirnous. Then the receiver is usually mounted in the aircraft, andeither on-board pattern recording is used, or a telemetry system must transmit the

measured signal to a ground processor.The transmission equation (Eq 1.8) contains two parameters which usually alter therecorded antenna gain signal and therefore have to be compensated by computation.1. The variation of the received signal due to a change of the distance r of theaircraft to the ground station has to be compensated by multiplying the lossesby r .2. The ground reflection multipath gain GM has to be considered.

This term is investigated in more detail in section 1.4.5. If possible, distance, flightlevel and receiving antenna height should be 3hosen such that GM remains nearly constantduring the test flight pattern.

N __ _


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201.4.3 Aspect Angle Determination and Flight Profiles1.4.3.1 Aspect Angle Determination

As mentioned in section 1.1.3 the radiation direction of an aircraft antenna withrespect to a receiving station is defined by the aspect angle illustrated in Fig 1.1. Todetermine the aspect angle during tests, the following parameters have to be considered:Location of the ground stationLocation of the aircraftAttitude (pitch, roll and heading) of the aircraftEarth curvature

In a plane system where the earth curvature is neglected, the aircraft and groundantennas are at almost the same elevation and the pitch and roll angles of the aircraftare small, the determination of the horizontal aspect angle is very simple. As shown inFig 1.26, the horizontal aspect angle O'A becomes

1800 (1.11)if T is the heading angle of the aircraft under test and 8 3600 - the azimuth angleof the aircraft as seen from the ground system. If the &ircraft is tracked from theground station, 8 can be measured directly. If no such tracking equipment is availableat the ground sta?,on, OA must be calculated from the outputs of radio-navigation orinertial systems on board the aircraft (see e.g. Ref 12).

A HEADING ANGLE

y ROLL AXIS*..-----ASPECT ANGLE

FLIGHT PATH ..- POSITION ANGLE

AZIMUTHANGLE

GROUND STATION)

Fig 1.26 Determination of the horizontal aspect angle


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21The determination of the vertical aspect angle * is very simple if the test flightis conducted in a vertical plane which also intersects the ground station (Fig 1.27).The vertical aspect angle 0E then becomes

0E = 0E + e (1.12)where e is the pitch angle of the aircraft. The position-dependent anglp B equals th eelevation tracking angle of the ground system p . Again 0 can also be comptted from a-vailable on-board information, derived from ragio or ineitial navigation equipment andaltitude measurements (see Ref 12).

PITCH ANGLEROLL AXIS

*---POSITION ANGLE

04-.GROUND STATION:

Fig 1.27 Determination of the vertical aspect angle

If the above-mentioned restrictions apply, Eqs (1.11) and (1.12) are useful fo r thedetermination of the horizontal and vertical aspect angle. However in many of the flightprofiles discussed in the next section the horizontal and vertical position angle varysimultaneously. Also many flight profiles require changes of the angle of roll whichinfluence the horizontal aspect angle.

A universal equation, which takes account of all parameters necessary to compute th etwo components of the aspect angle *And *E in the general case, is derived in Appendix1.B In many applications the simplified equations, Eqs (1.14) or (1.21) and Eq (1.15),are used for on-line data processing and quicklook possibilities or to save computingtime.

The angle of the earth curvature aE fo r a horizontal distance d between the aircraftand the ground station is given by

Stan-ld(! (1.13)

The earth radius averages 6365 km, so e stays below 0,90 if the distance d is limitedto 100 km. Therefore the earth curvaturi has to be considered only if measurements re-quire high angular accuracy. If eE is neglected, then the error in o is of the order ofOE. In the component oA the effect usually is smaller, Rxcept when the angle of roll ofthe aircraft 4 exceeds 150 and simultaneously 0A approaches 450, 1350, 2250 or 3150.

If the earth curvature is neglected, the conditional equations fo r 0 A and 0 E asgiven in Appendix B, Eq (1.B11) and Eq (1.B12) reduce to

OA ;an- (Ay(1.14),x/


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22

=tan-/A \* 5where

Ax - -C0S8E cos(Y-P)cose - sinflEsinO (1.16)

Ay w COSE (sin(Y-p)cosO-cos(Y-.)sinesin0) + sinfBEcosOsinf (1.17)AZ = -COSOE (cos(V-p)cosecos + sin(Y-p)sinO) + sin$Ecosecos$ (1.18)

40

t30 elo :O0 23"100.; 200020

10I-,t

00 200 400 600 800Aspect Angle %A

Fig 1.28 Error in the horizontal aspect angle *A if it is no t correctedfo r ,Atch angle e ($ - angle of roll)Further simplification is possible if the pitch angle remains small during the testflights. Fig 1.28 illustrates the error of horizontal aspect angle 0*, if a pitch angleof 10 0 is neglected in the computation of 0^ . The error increases w ih the angle of rollS.f test flights are performed at low-titudes or large distances from the groundstation, so that th e vertical posit ion angle 0 also does not exceed 100, Eq (1.16) andEq (1.17) can be further reduced to~~Ax - -COS(Y-P) (.9

SAysin(Y- P)coso (1.20)and th e horizontal aspect angle 0 becomes after Eq (1.14)

= an (tan(Tl-P) (121A = a- coso 1.1


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23

so1,60

#

so \

40 50

90 40A

-20 2100 110 120 13 14 18 16 17 180s

"10"-40lo 14-- 20- 30

__-70

Fig 1.29 Error eA of horizontal aspect angle A if it is no t corrected for rollanglessAs Fig 1.29 illustrates, a correction of the horizontal aspect angle #A as a conse-quence of roll angle variations is no t always necessary. If the roll angle can be limi-ted to a maximum of 200, the maximum error does not exceed 3.20.

1.4.3.2 Flight Trajectories fo r Dynamic ARP MeasurementsWhen measuring an ARP it is no t usually necessary to cover the whole sphere aboveand below the antenna under test. The aspect angle zone of interest depends very much onthe manoeuverability of the aircraft and on the kind of radio aid under consideration.Once the angular range to be covered by the measurements is defined, the flight profilescan be selected. Continuous flight trajectories which allow complete continuous radia-tion pattern recordings are very efficient with respect to flying time and data process-ing. Such profiles also provide excellent quick-look opportunities if the received poweris recorded as a function of the variable angle, e.g. heading. Unfortunately, certainangular areas of the sphere can no t be covered by continuous flight trajectories, andmeasuring requirements ma y call for additional dis6ontinuous flight trajectories.The flight test trajectories fo r ARP can be divided into two main categories,straight trajectories, usually with constant attitude and height, and curved trajec-tories which always reqtire attitude variations and sometimes also height variations.

radialJ slant

parallel fly overI//prlel,/\\ I ..... ///\/

I\ /1d \

/S/

/d/ \ I \\\ //

\\

/

Fig 1.30 Straight flight test trajectories fo r ARP measurementd - distance; h = he0.ght


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24The straight trajectories illustrated in Fig 1.30 are very simple to fly, becauseattitude and height remain constant during the profile, preferably controlled by anautopilot. No complex attitude measuring system arnd data recording or transmitting sy-

stem are necessary on board the aircraft. On the other hand one parallel flight providesonly part of the data required for a complete 3600 polar antenna trajectories plot. Itmust be supplemented by additional slant and radial flights, and the total flight timeis very high. If, in addition, a large range of depression angles must be covered, thesame profiles must be flown at different heights, which further increases flight time.While radial, slant and parallel flights are suitable for horizontal antenna patternmeasurements, the fly-over trajectory is better suited to vertical antenna patterns, atleast for those below the fuselage of the aircraft, A problem with this profile is thatthe distance between the ground station and th e antenna under test varies greatly, sothat the measurements suffer from ground reflections as the height can be optimized onlyfor certain sections of the trajectory.

1800 ROLL SPLIT S

FIGURE 8 RACETRACK - OPNCIRCLE------------0--------------------------

5=--7 -7 7-77777 ,o777777777-r --

Fi .1Curved flight Thereforj Fig.~atturedfitude tettajsei-tories for ARP measurement, tories for ARP measurement,fixed height, d = distance variable heightTecurved flight test trajectories for ARP measurement, usually flown at a fixedhihare shown in Fig 1.31. The variable attitude and heading of the aircraft flyingthes trjecoris,ualy erivd fom n MS (ttiudeand heading reference system)

or an IRS (inertial reference system), must be recorded or transmitted.The circle or obttrajectory isvr ayto fly, the aircraft circles inaskdding turn or at a constant hank angle, each turn covering a complete 360' great circleantenna radiation trjcois hrfr hstrajectories is one of the mostefienprofiles for ARP measurements as far as flight time is concerned. In a flight at greatdistance and at low altitude the coverage of the depression angle in the nose and tailarea of the aircraft is poor. The coverage of the depression angle can be improved byflying the aircraft at higher altitudes. The disadvantage of this flight test trajectoryis, that the drift of the AM-RS or inertial system will increase when the aircraft iscontinously turning in the same direction. Therefore, alternating left and right handturns are recommended in order to compensate the drift forces.This requirement is also met by the figure of eight trajectory of Fig 1.31, whichallows the aircraft to fly left and right hand turns during the same manoeuver. In thedashed section of this figure no measurements are recommended because the roll rate ofthe aircraft is very high. To complete a 3600 antenna pattern, the manceuver has to berepeated under conditions where the figure of eight is turned 900 with respect to theground station.


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25The racetrack trajectory, very much alike the circle pattern, allows a gyro rea-lignment during the straight course. runs connecting the semicircles. The discontinuitiesat the 00 and 1800 azimuth points, where the aircraft has to change from banked to levelflight, comprise a disadvantage of this profile because of the great roll rate of th eaircraft during the transition period.If only the nose and tail horizontal patterns of an aircraft antenna need to bemeasured, the horizontal S flight trajectory illustrated in Fig 1.31 is convenient.Platform drift is minimized by the alternate turns, bu t the angular range of measurementis limited to , at most, + 400 in the horizontal plane.

The candidate curved flight trajectories fo r ARP measurements in the vertical planeare shown in Fig 1.32. For a spl i t S trajectory the aircraft starts from straight hori-zontal flight, then performs a 1800 roll and finally reverses it s flight direction div-ing to a lower flight level. This manoeuvre gives a nearly 1800 coverage of the nosetail elevation out but can be performed only by highly agile aircraft. The fast rate ofchange of the pitch angle will make it necessary to use a high sampling rate.The looping trajectory, where the aircraft performa a 3.500 vertical turn, extendsthe coverage to a complete 3600 vertical radiation pattern. The same constraints men-tioned in conjunction with the split S trajectory apply. Due to the short duration ofthese fast manoeuvers the drift of the attitude measuring gyros will be negligible.The vertical S or porpoise trajectory, during which the aircraft alternately dives

and climbes, gives a limited coverage of the depression angle in the nose and tail area.If the distance is large it can even provide data on small negative depression angles.Again pitch rate has to be examined carefully, bu t gyro drift will be negligible.In many cases a combination of the previously mentioned trajectories is used to a-

chieve a rational and economic flight test programme. Three such combination patternsare dicussed below.

000

WM-" IN11 1970 flgh 'ligh flight 1o l

0- 20 No317T 10 f.ght ' I 0ftt.p IR

Fig 1.33 Radial run flight profile Fig 1.34 Horizontal aspect anglefo r antenna measurements at coverage area of radial runNATC, USA flight profile (Fig 1.33)A typical combination is the radial run flight profile which is used on several US

test ranges and is illustrated in Fig 1.33. This profile, composed of several straighttrajectories, covers a complete 360F horizontal aspect angle range as shown in Fig 1.34.The fl ight area where antenna test data is recorded must be chosen carefully so that themultipath conditions fo r the radio link are acceptable. Moreover, the aircraft must bevectored - usually by a ground radar - to the desired positions and headings. Even asmall error in the preuietermined flight trajectories can lead to missing sections in the3600 ARP.


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26The major disadvantage of this combination trajectory is the flight time required tocover the complete 3600 aspect angle, which requires at least 32 runs (see Fig 1.33). Itis only worth-while if a number of antennas radiating at different frequencies aretested simultaneously. No special test equipment such as an AHRS or an inertial systemis necessary on board the aircraft, which makes this profile particularly useful fo rsmall aircraft.

GROUNDEliD STATION

Fig 1.35 Cloverleaf fli'qht pattern for ARP Measurement

Another combination of straight flights is the cloverleaf flight trajectories Fig1.35) used in the past by several US test ranges without the aid of an automated meas-urement system. Range and altitude are chosen so that the depression angle of interestis covered and that the multipath conditions are optimal for the radio link. Each timethe aircraft passes the point of measurement (the centre of the pattern) one point ofthe radiation pattern is recorded. This procedure provides only a rough graphic repre-sentation of the antenna pattern with e.g. 24 plotted points, and this is only accept-able at lower (VHF) frequencies. At higher frequencies many more straight runs are nec-essary in order to cover the fine structure of a pattern, which makes this method in-practical above the VHF frequency range. These measurements do not require additionalon-board equipment and can be performed without radar support if landmarks or radionavigation are used. The flight time for a 24 point cloverleaf is very high, of theorder of several hours. If the area of data collection is enlarged and the headings areProperly selected, a nearly complete 3600 pattern can be recorded. But then radar sup-port and data processing are necessary. Tab 1.4 shows selected headings and angular cov-erage of measurements, when a ratio of 1:3 is assumed between the radius of the areawhere data is collected and the distance from the central point of this area to the re-ceiving station.Table 1.4 Angular coverage of' modified cloverleaf pattern

selected 900 530 280 140 70 30 0heading

agular 71,50 40,50 210 10,30 5,30 2,30overage 108,50 71,50 40,50 20,80 10,50 4,50 00

Fig 1.36 Flight trajectory for vertical ARP


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27A flight pattern frequently used by DFVLR in Germany is illustrated in Fig 1.36 andconsists of the straight flyover flight (Fig 1.30) combined with two porpoise patterns(Fig 1.32). This method ensures an uninterrupted measurement of a vertical radiationpattern below the aircraft usually starting at a depression angle 6 of -100. When usingthis trajectory, probable ground reflection problems must be carefully considered asseveral points of wave cancellation may be met during a test flight. At frequencies ofabout 1 GHz and higher a highly directive ground antenna can reduce this problem con-siderably. An alternative, also helpful at low frequencies, is to make use of the groundas a reflector for the receiving antenna. In order to obtain well-defined conditions,the surroundings of the ground antenna are covered with a metallic mesh to a distance of

several wavelenghts. This antenna arrangement has only one lobe, (see Fig 1.37), whichcovers the whole test flight, but corrections must be applied for the variations of gainwithin that lobe. The large distance variations during the test flight require addition-al corrections.

Fig 1.37 Radiation pattern of receiving antenna, distance ofreflecting Ground d X/

Several of the above-mentioned flight profiles can add up to a complete 3600 coy-erage of the horizontal aspect angle during one test flight. To cover a larger range offlight tests for, vertical pattern recordings, vertical coverage charts can be used (Ref13). They are given in Fig 1.38 for ground ranges of up to 40 NM. The correct values forsample point A in Fig 1.38 are

Ground Range 8.65 NMSlant Range 10 NMEi 'evation Angle 30"Earth curvature G 0.140Altitude 30.43 FTDepression Angle e 30.140

If any two of the first five variables are given, the others can be found by using thechart. Given the depression angle and any one of the first four variables, the otherscan be estimated. For ground ranges greater than 40 NM (vertical coverage chart of Fig1.39), the slant range i. s within 0.6 NM of the ground range.


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28ELEVATION AN6LE

900800 700 600 500 450400 350 30o 25 0 200190180 170 160 150 140 13 0 120 110I I tII I I//I I I' // / / / //, - ., "~10040 go

35'go

0A

=U-- /,50

.205

S15-0

13K10 "R- Xz10

55 100 35 M 70010 20 GROUND RAN E-- 0 5 m 7p I , t, 1I I, I

.1o .20 .30 .40 .06e1 ( Add to Elevation Angle to get Depression Angle IFig 1.3B Vertical coverage chart (ground range up to 40 NM )

900 45'300 200 150iII I I I I 1 109 70 ELEVATION ANGLE_

30,-2.2

25.. 2

26 -+10

10 - .4 0

' l~l llll - Do

20 40 60 0 100 120 140 150 190 14 2'10 0 150 GROUND RANGE - 250 TO km0.50 1.0k 1.50 2.00 2.50 3.00ei I Ad d to Elevation Angle to get Depression Angle)I

Fig 1.39 Vertical coverage chart (ground range up to 200 NM )


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291.4.4 System Considerations

The system configuration for in-flight antenna measurements depends very much en thefrequency range the system has to cover and on the size of th e vehicles under test,which range from small drones to wide-body aircraft.An important question is whether the test system should work as an air-to-ground ra-die link or vice versa. In other words, should the antenna under test act as a transmit-ting or receiving antenna. If measurement fre~uencies exceed several GHz, the transmit-ters become large and heavy, and power consumption increases. In this situation a &round

radiating concept has important advantages. In small aircraft, such as fighters, it isoften necessary to take out the operational equipment to provide space for the testtransmitter. This has, however, the advantage that the existing cabling can often beused for th e test installation.Usually the distance between the airborne and the ground station will be between 20and 40 kin, in order to limit the required transmitter power, the required dynamic rangeof the receiver, and to reduce the effect of field intensity variations with distance.The total pewer budget of the antenna measuring link is calculated using Eq (i.9.)Usually the received power should be about 40 dB above th e noise level of the receiversystem, in order to ensure that a 40 dB dynamic range can be attained in the patternrecordings.

1.4.4.1 Airborne SystemsDepending on whether receiving or transmitting the airborne system of an in-flightantenna measuring device has to contain the necessary HF transmitters or receivers whichcover the frequency range under test.On-board teut transmitters are either single-frequency units, which can often re-place the original equipment, or wide-band devices, consisting of a frequency synthe-sizer, a frequency multiplier and power amplifiers, the latter usually one for eachoctave of the frequency range. As mentioned in S3ection 1.4.4 a transmitting power ofabout 1 to 10 watts is required, which for frequencies of up to a few GHz can be pro-vided by semiconductor circuits. At higher frequencies travelling-wavt tube amplifiersare used. The stability of the transmitter amplitude must be controlled or monitoredcarefully to avoid fluctuations corrupting the measured ARP.On-board test receivers usually have a wide frequency range,with switchable crystal-controlled oscillators converting the incoming signal to the frequency range of thefirst intermediate-frequency (IF) amplifier. In order to make th e receiver suitable fo rall frequencies that must be received, the second oscillator which converts the firstintermediate frequency to the second IF is often a programmable frequency synthesizer. Alogarithmic amplifier covering an adequate Oynamic range of 60 to 80 dB is usually em-ployed as the second intermediate-frequency amplifier. The accuracy of its logarithmiccharacteristic should be within .5 to 1 dB deviation for the full dynamic range.As shown in section 1.4.3, several parameters must be measured on board the testaircraft to determine the aspect angle. These are attitude parameters and altitude, withthe addition of parameters from navigational aids if the position-dependent part of theaspect angle is not measured by ground-based equipment.For the on-board acquisition of aircraft position data two approaches are possible:the use of the operational navigation equipment already available in the aircraft or theuse of a special flight test package. In the first approach only the data recorder orthe telemetry transmitter with their signal conditioning ar~e added to what is alreadyavailable in the aircraft, but connections must be made to the operational navigationequipment of the aircraft. If safety reasons make that unacceptable, a completely inde-pendent sensor package must be installed. If th e data recorder or telemetry transmitteris included in that package, the additional cabling in the aircraft will be reduced to aminimum.Usually ARP measurements require on-board and ground data. If data are recorded onboard, only pest flight processing and evaluation of the ARP is possible. If real tinemonitoring is desirable, a telemetry data link has to be installed. This considerablyspeeds up and improves the handling of the test flights.A typical example of an ARP measurement system which derives on-board data from theavionic systems of th e aircraft is illustrated in Fig 1.40. This system, used at theNaval Air Test Center, Patuxent River, Ma, USA, employs telemetry for data transmissionand radar tracking for space position data acquisition.


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30

RF HORIZONTALLY POL. WRECEIVER GROUND RECEIVERF SIGNAL ]NENAMPL ITUDE ROTODOMEPOSITION

ROTODOME POSITION RECEIVER IPC -4-ROLLTELEMETRY A/D PITCHHEWLETTTRANS-

PACKARD SPACE " HEADINGJM-SERIES JPOSITION "ALX CT R -------IMINI- PARAnLLAX TRACKINGIICOMPUTER l CONVERT]ERI RADARISYSTEM I"" 'TA E STORAGE DISPLAY AND CONTROLRAW TERMINAL

Fig 1.40 Inflight antenna measurement range of NATCA system of the NLR, Netherlands (Ref 14 ) makes use of separate on-board und grounddata recording with synchronization accomplished by a radio link between the airborneand ground data acquisition units. The data processing scheme of this system is shown inFig 1.41.A third example, of DFVLR Germany, is described in Re f 15. It consist of one com-plete black box which contains all data acquisition units, including radio navigation

systems (VOR/DME) fo r space position data acquisition (see Fig 1.42).A typical airborne receiver fo r ARP measurements, developed by DFVLR Germany forflight tests, is shown in Fig 1.43. Depending on the oscillator frequencies, four dif-ferent test frequencies in the range 0.6 to 20 GHz can be selected. The receiver is ofmodular construction and can be configured in almost any shape, to adapt it to differentconditions of use. The front end of the receiver is preferably mounted close to the an-tenna terminal, remote from the basic chassis, to minimize cable losses at very high

frequencies (Ref 16).1.4.4.2 Ground Systems

As mentioned above, ground systems fo r ARP measurement receive or transmitt the sig-nals of the radio-frequency test link. The necessary ground antennas fo r transmission orreception must be selected carefully, and their bandwidth must be matched to the fre-quency range of the measurements. Even if log-periodic devices with a wide frequencyrange of e.g. 1:n0 are provided, the total frequency range usually requires more thanone antenna.

To reduce ground reflection disturbances, the siting of the ground antennas must bedone with great care. The necessary conditions, especially the adjustment of the antennaheight above the ground, are outlined in more detail in section 1.4.5.At frequencies above 1 GHz an adequate effective area of the antenna is achievednly by high gain antennas like horns or parabolic dishes. The half-power beam width ofthese antennas is so small, that they must be made to track the vehicle under test. Inmany systems this is done by slaving the pedestal of the measuring antennas to a track-ing radar or to a telemetry tracking system used fo r data transmission.

At high frequencies receiver preamplifiers or transmitter power amplifiers should beas close as possible to the feed systems of the antennas to avoid sensitivity reductionor power loss in cables.

!2


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31

A GROUNEBASEDEC I AIRBORNELDATA ACI. UNIT DATA ACQ ANITCONVERSION INTO jCONIERSION INTO

COMPTERMPAIBLETAERP TO AN CORMPUTIRO MPTBETPCALIBRATIONFL T CALIBRATION O

FIEAZIMUTH HEADINGpnc

VATINRANGE SYNCHRONIZATION ROLLFIELDSTRENGTH / TACAN POSITION( INS POSITION)INTERPOLATION ALL FORMATS

II ARNENTOF DATA PESRONCALCULATION OF TH E ASPECT NORMALIZATION OF THEANGLE WITH RESPECT TO THE RECEIVED POWER WITHPOLAR AIRCRAFT COORDINATE F RESPECT TO DISTANCESYSTEMSSTORAGE OF DT

OF ALL FI GHT

SARRANGEMENT OF DATA PERSELECTED SPHERE ELEMENT(Ae. 60)

OUTPUT 1) DISTRIBUTION PRINTS OF- NUMBER OF DATA POINTS PER SPHERE ELEMENT- PATTERNS OF JAM TO SIGNAL RATIO, ANTENNAPOWER. RADAR CROSS SECTION- STANDARD DEVIATIONS PER SPHERE ELEMENT2) GRAPHS OF PATTERNS IN SPECIFIC CROSS

SECTIONS

Fig 1.41 Data processing scheme of NL R system

PITCH SYNCHRO/DC P/L- BANDGYRO SYNCHRO/DC VCO 3

7 COUPLER HK~SIN OuRDsESIGNALCONDI!T.HK- G 10M _-V01'-70 -16D0 M~Hz

1 V 0 R ~~~FINE ECTI IIRETTASM COUPLER HKVCO~'4C'UIGN.COND. TLRANSN'. CULEUR K.":,'"HK- G COURSE VC 15'

HK -HOUSEEEPING PARAMETERSVCONUMBERSNDICATING IRIG-SUBCARRIER CHAMNELUMIBERS

Fig 1.42 Airborne system for antenna pattern measurements (DFVLR)


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32

Lo w Band FI p Deviation

F2 F21 F1 Field StrengthF3F23[ - --- Outp .zF41 I Bo2MHz

High Band F3Input 1.F 2. IFF4 1,5GHz 70MHz

Fig 1.43 Airborne receiver fo r ARP measurementsA typical configuration of a ground antenna system fo r ARP measurement is shown inFig 1.44. Four parabolic dishes, each covering part of the frequency range, are combinedwith a log-periodic crossed dipole antenna fo r 'VhF measurements. The antenna pedestalalso carries the RF equipment.

Fig 1.44 Antennas of precision antenna measurement system (PAMS) ofUSAF/Rome Air Development CenterFig 1.45 shows a different concept. Here a section of a parabolic dish is mecha-nically connected to a telemetry tracking antenna. The surface of the dish is designedto operate at up to 20 GHz. Matching to different frequeocy ranges is accomplished byexchanging the antenna feeds, which illuminate a smaller section of the dish if the fre-quency is increased. Again a rack on the pedestal holds all RF equipment.


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33

LA.... W..n A

44

V1,R- 4"

Fig .45 F-rceptontenae fr A? mesureent at FVL

IN A R~:IGIFOMTO

Fig 1.45 n-reighti antennas o Reasurements ayteq im nDf VNAT


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34Tw o data processing systems, one with on-board data recording and synchronization ofground data (Fig 1.41) and one with telemetry transmission (Fig 1.40) have been men-tioned already. The Naval Air Test Center ground system of Fig 1.40 is shown in moredetail in Fig 1.46. The frequency management of the receiving system is automated to ahigh degree. A third system with on-line data processing and quick look presentation ofresults used by DFVLR Germany, is illustrated in Fig 1.47. The FM-subcarrier system hasnow been replaced by a PCM system.

Ph- BAND70 -1600 MH z

DETODULATORS CH, 1-18

RECORDER RECORDER RECORDER

Fig 1.47 Receiving and daLa processing station fo r antenna measurementsof DFVLR1.4.5 Pattern Calibration1.4.5.1 Flyby Procedure

The methods described in Section 1.4.4 provide telative radiation patterns bu t donot give quantitive information on the efficiency of the on-board system, i .e. on theratio between the power actually received and the power that would be received from anisotropic radiator excited by the same power and frequency and measured at the samedistance as the antenna under test. The actual power measured at the receiving antennadepends no t only on this efficiency, but also on the reflected power that varies withthe shape and nature of the terrain between the transmitter and the receiver. Usually,this efficiency is calculated fo r the geometric relationship between transmitter andantenna of the actual flight test by calculating the power that is reflected by theground in that case. This reflected power can then be subtracted from the received powerto obtain the power that would be obtained in an environment without reflections. Thiscalculation is described in Section 1.4.5.2.DFVLR (German Aerospace Research Establishment) has recently developed a method formeasuring this efficiency using the flyby procedure described in this section. Thismethod is based on a comparison between the signals at the ground receiver received fromthe aircraft and from a standard gain antenna placed on a tower near the aircraft tra-jectory. The ratio between the signals from this antenna and from a fictitious isotropicantenna is known. The radiation pattern of the isotropic antenna is a circle which DFVLRcalles the "isotropic circle". The isotropic circle is usually plotted together with themeasured pattern in order to indicate the gain of the antenna under test in each sectionof the polar plot (see e.g. Fig 1.53). To compare the aircraft under test with a stand-ard gain antenna the flyby procedure shown in Fig 1.48 is conducted. For calibrationpurposes the aircraft flies past the ground measuring station At a short distance (about250 m) and at a low height (between 16 and 20 m). Just before this flyby, a tower with astandard gain antenna is erected in the test area. Thus the radiation of the aircraftantenna can be compared to the radiation of the standard gain antenna if both antennasradiated from nearly the same position. The input power to both antennas must be meas-ured. For flight security, the standard gain antenna tower must be removed before theaircraft flights.


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35

................................ ................ ::l..... dB1A'

AIRCRAFT ANTENNAEST FLIGHIT -21

"-22. .l.,.. 6........... ..... le

REFERENCE MEASUREMENTITH STANDARDAINANTENNA 10 12 14 16 IBM H

Fig 1,48 Flyby procedure fo r comparison Fig 1.49 Received power as a functionwith standard gain antenna of height H of standard gainantenna (f - 335,5 MHz)

Again care must be taken to achieve stable propagation condit ions. This means thatthe antenna heights must be chosen careful ly to avoid large power reductions at the re-ceiver due to ground reflections. Propagation conditions have to be nearly th e sameduring the tower and th e aircraft derived measurements. Investigations have shown thatthe aircraft antenna and th e standard gain antenna have to radiate from th e inside of afictitious cube which measures about S m from edge to edge. Within this cube, spatialUifferences between the antennas of the comparative measurements ca n be interpolatedlinearly (Ref 17). Fig 1.49 shows an example of th e relative received power from th estandard gain antenna as a function of height. It shows that th e heights of aircraft andstandard gain antennas must be known within a tolerance of 40 cm if th e error is not toexceed 0.2 dD. Fo r th e aircraft this high precision can only be achieved by accurateposition measurements with kine-theodolites or laser trackers, and it is convenient tomeasure th e position of th e standard gain antenna in th e same manner. Fig 1.50 shows aphotograph of an aircraft under test taken by a kine-theodolite with azimuth and eleva-tion angle information, which has to be corrected for th e displacement of th e aircraft'sreference point from tho cross-h&irs . A measurement installation at Braunschweig Airport(Fig 1.51) shows two theodolites installed for position measurements.

Fig 1.50 Evaluation of theodolite measurement


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36

y

FLIGHT PATH STNDARD GAIN ANTENNARUNW~AYS 09/27/ - /" /x

- /

. ///- / 0 GROUND ANTENNA

THEODOLITE 2/

THEODOLITE 1

Fig 1.51 Measurement installation at Braunschweig AirportThe fl ight path with respect to the position of the standard gain antenna of a testis illustrated in Fig 1.52. These curves are used to interpolate the received power ofthe standard gain antenna to the actual fl ight path level. The height of the standardgain antenna is varied in steps of 2m to match the fl ight path in x-direction asclosely as possible. Deviations in y-direction are much less sensitive with respect toan alteration of received power.

YA

io0/0

FFLIGHT PATH14 OSTANDARD GAIN - o...

ANTENNA0

0 10 20 30 40 50m X 10 20 30 40 5oM X

Fig 1.52 Flight path with respect to standard gain antennaAs an example, Fig 1.53 shows th e radiat ion pattern and isotropic circle of a glideslope antenna measured fo r an HFB320 aircraft. The difference A P between th e

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Agard Flight Test Technique Series Volume 4 Anttenae Patterns and Radar Reflection