MATCHED PAIR CONICAL SPIRAL ANTENNAS Robert E. Metzler ...€¦ · MATCHED PAIR CONICAL SPIRAL ANTENNAS Robert E. Metzler GEOTRONICS, INC. 5718 Columbia Pike Falls Church, Virginia

-- NASA: CR- /;? 'S//

MATCHED PAIR CONICAL SPIRAL ANTENNAS

Robert E. Metzler

GEOTRONICS, INC.5718 Columbia Pike

Falls Church, Virginia 22041

April 1972Final Report

(NASA-CR-122511) MATCHED PAIR CONICAL N72-28172SPIRAL ANTENNAS Final Report R.E. Metzler(Geotronics, Inc.) Apr. 1972 54 p CSCL

17B UnclasG3/07 36543

Prepared for

GODDARD SPACE FLIGHT CENTERGreenbelt, Maryland 20771

Detanl" of Illustrations inthis document may be better

Studied on microfiche

AS/4 --)•3v3

Reproduced by

NATIONAL TECHNICALINFORMATION SERVICE

U S Department of CommerceSpringfield VA 22151

CAR

https://ntrs.nasa.gov/search.jsp?R=19720020522 2020-05-01T13:41:55+00:00Z

Preface

The contract objective was to design, fabricate,

test, and calibrate a matched pair of VHF (220-260 MHz)

conical spiral antennas for use in a rocket-tracking

interferometer array. While gain, bandwidth, impedance,

and pattern measurements met specifications, the phase

match between antennas at low elevations was not equal

to the design goal. Future work in this area might

point toward patterns with more gain at low elevation

angles plus construction techniques to produce pairs

more nearly identical.

Table of Contents

Page

1.0 Summary of Requirements 1

2.0 Design 2

3.0 Test and Calibration 3

4.0 Conclusions

(

List of Illustrations

Figure No. Title

Elevation Pattern

Elevation Pattern, Fast-SpiralAntenna

3-1

through

3-24

Page

4-1

19

through

42

.45

List of Tables

Table No.

3-1

3-2

3-3

through

3-14

4-1

4-2

4-3

Title

VSWR vs Frequency

Gain vs Frequency

Page

. 5

6

7

Phase vs Azimuth through

18

Gain vs Elevation 44

Axial Ratio is Elevation 47

Worst Excursions, Phase vs Azimuth 48

1.0 Summary of Requirements

The application for the matched pair of conical spiral

antennas is an rf interferometer array used in missile track-

ing at the White Sands facility. The cones are to be

mounted with axes vertical and pointed toward the zenith.

In this orientation, it is desired that maximum gain and

circularity, if they vary with azimuth angle, should be

maintained through an azimuth sector of 150° minimum.

The axial ratio (polarization is right hand circular)

'through this sector and between 250 and 85° elevation is to

be 3 dB maximum with a 1.5 dB design goal. Maximum gain

is to fall at 80 ° to 90° elevation (referred to the ground

plane) and is anticipated to be +4dBi. The required fre-

quency range is 220-260 MIHz and the VSWR 1.5:1 maximum

referred to 50 ohms. Calibration and fiducial markings

are required for each antenna of the pair such that they

can be installed to produce equiphase far field planes

(design goal + 30 minutes of arc); this calibration is to

be accomplished at three frequencies and every 360 of azimuth.

Mechanically, the antennas are designed for a wide out-

door temperature range (-55° to +150°F), winds up to 100

mph and humid, salt-corrosive environments.

-1-

2.0 Design

Geotronics had previously designed and constructed

4-arm conical spiral antennas in several physical confi-

gurations. These antennas produced patterns with an over-

head null and maximum gain 30° to 50° above the horizon

and were used for communication with satellites. This inter-

ferometer application, however, required a 2-arm spiral to

produce maximum gain at the zenith.

Geotronics' previously-developed computer program

for conical spirals is usable for either 2 or 4 arm models.

From previous experience, it was felt that a base diamter

.of approximately 24 inches would be required for operation

at 220 MHz. A spiral angle, a, of 84° degrees was selected.

A prototype antenna was constructed to these dimensions and

rotating-dipole patterns were taken across the band. Pattern

shape and circularity were as anticipated, so detailed

design of the deliverable matched pair commenced.

Since any appreciable tooling would have been prohibi-

tively expensive for two units on a small fixed-price

contract, the antennas were designed for hand construction.

Three circular discs were machined for horizontal members,

with-oak ribs set into the discs to form the remainder of

the underlying structure. This ribbed structure was covered

-2-

with thin plastic sheet and the seams filled to form a

smooth, continuous skin. The cone was truncated at the

top at the 4 inch diameter point to provide space for

the top of the feed balun. The conductive arms were

pattern cut from aluminum sheet stock, formed onto the skin,

and held in place with rivets and screws. The various

sections of arms were spliced at the joints with aluminum

plates,Devcon aluminum epoxy,and fasteners. The completed

antennas were 24 inches in base diameter with a 27 inch

diameter mounting plate, 50 inches high, and weighed 32

pounds each.

The balun is a choke at 240 MHz, open at the feed

point and shorted one quarter wave below it. Impedance

transformation is accomplished by a quarter-wave section

of high impedance (82 ohm) transmission line.

The completed assembly was covered with 4 coats of Latex

white paint. Following calibration, four fiducial marking

strips and a top cap marker were affixed as an aid to

installation and alignment with surveying equipment.

3.0 Test and Calibration

Four types of testing were performed on the completed

antennas. Impedance (VSWR) measurements, elevation patterns

-3-

with rotating linear source dipole, and gain measurements

with a circularly polarized source antenna were all done

on Geotronics roof-top antenna range. Phase versus azimuth

measurements were made in a Government anechoic chamber at

NASA-GSFC.

Table 3-1 shows VSWR measurements for both antennas.

Table 3-2 lists gain, referred to a right-hand circularly

polarized isotropic antenna, of both antennas at three

frequencies.

Table 3-3 through 3-14 are the results of phase versus

azimuth measurements on the antennas. The various tables

represent measurements at three frequencies, two elevation

-angles, and with both vertically and horizontally'polarized

linear source antenna.

Figures 3-1 through 3-24 are elevation patterns of

the antennas. The first eight patterns were taken at 220

MHz, the second eight at 240 MHz, and the final eight at

260 MHz. In each group of eight, the first four patterns

are of antenna number one and the last four of antenna two.

Each group of four patterns represents a complete 360

elevation cut (0) at four different azimuth angles

( = 45° , 135° , 225° , and 315° ) as measured clockwise from

the "north" foot of each antenna.

-4-

Table 3-1

Antenna VSWR (ref. 50 ohms)

Freq. MHz

220

225

230

235

240

245

250

255

260

VSWR, Ant.#1

1.40

1.30

1.18

1.10

1.08

1.13

1.16

1.15

1.15

VSWR, Ant.,2

1.34

1.22

1.11

1.11

1.17

1.23

1.23

1.18

1.14

-5-

Table 3-2

Antenna Gain

(ref. RHCP isotropic)

-6-

Gain dB, Ant. #1 Gain dB, Ant. #2

Freq.MHz Range Avg Range Avg

220 7.9-8.3 8.1 7.9-8.2 8.0

240 8.0-8.2 8.1 8.0-8.3 8.2

260 8.1-8.2 8.2 8.0-8.2 8.1

Table 3-3

Frequency: 220 MIHz Elevation Angle: 28° Source Antenna PolarizationVertical

PositionerAzimuth-Angle, Deg.

0. 0'36.072.0108.0144.0180.0216.0252.0288.0324.0360.0

Phase,Deg.Ant.#1'

0.139.177.0115.1151.2

-176.5-141.6-101.9- 67.6

3.6.1- 0.3

Phase,Deg.Ant,#2

0.136.875.0112.8148.3

-178.0-142.7-104.4- 68.1- 34.4- 0.2

PhaseDifference, Deg.

#2 rel. to #1

0.0-2.3-2.0-2.3-2.9-1.5-1.1-2.5-0.5+1.7+-0 .1

-7-

Table 3-4

Frequency: . 220 MHz

-PositionerAzimuthAngle,

0.036.072.0108.0144.0180.0216.0252.0288.0'324.0360.0

Elevation Angle: 28°

Phase,Ant.A1

- 0.136.572.1106.7143.9

-176.9-141.5-108.4- 72.7- 35.9

0.2

Phase;,Ant,#2

0.136.371.3

106.2142.9

-178.9-142.5-108.2- 73.3- 36.8

0.2

Source Antenna Polarization- Horizontal

PhaseDifference

#2 rel. to #1

0.0-0.2-0.8-0.5-1.0.-2.0-1.0+0.2-0.6-0.90.0

-8-

Table 3-5

Frequency: 220 1IHz Elevation Angle: 10°

PositionerAzimuthAngle,

0.03.6.072.0

108.0144.0180.0216..0252.0288.0324.0360.0

Phase,Ant.#1

0.133.767.8

110.4149.0

-179.8-144.6-104.8- 73.7- 40.6

0.2

Phase,Ant,#2.

0.133.566.7104.6143.9179.8

-146.0-110.2- 75.8- 39.0- 0.1

Source Antenna PolarizationVertical

PhaseDifference

#2 rel. to #l

0.0-0.2-1.1

-5.8-5. 1-0.4-1.4-5.4-2.1+1.6-0.3

-9-

Table 3-6

Frequency: 220 MIHz


0.00 . 0 36.072.0

108.0144.0180.0216.0252.0.288.0.324.0360.0

Elevation Angle: 10 °

Phase,Ant.#1

0.138.572.2106.8145.9

-175.9-140.5-103.9- 68.5- 35.8

0.2

Phase,Ant,#2

0.136.872.0

107.4144.2

-178.7-142.4-106.1- 69.8- 35.0

0.3

Source Antenna Polarization

Horizontal

PhaseDifference

#2 rel. to #1

0.0-1.7-0.2+0.6-1.7-2.8-1.9-2.2-1.3+0.8+0.1

-10-

Table 3-7

,/Frequency: 240 MIHz


0.036.072.0

108.0144.0180.0216.0252.0288.0324.0360.0


Phase,Ant.#1

0.134.970.2

106.7144.1

-177.2-139.9-105.8- 71.9- 36.1

0.0

Phase,Ant,#2.

0.136.872.4

108.4146.3

-176.4-142.0-107.9- 72.5- 36.4

0.1

Source Antenna PolarizationVertical

PhaseDifference

#2 rel. to l1

0.0+1. 9+2.2+1.7+2.2

+0.8-2.]-2.1-0.6-0.3+0.1

-11-

Table 3-8

Frequency: 240 MHz

PositionerAzimuth-Angle,

0.036.072.0

108.0144.0 180.0216.0252.0288.0324.0360.0


Phase,Ant.#1

0.235.872.6

109.5144.9179.5

-144.3-106.2- 69.4- 34.7- 0.2

Phase;Ant,#2

0.136.374.8

111.6146.0

-179.9-143.8-106.3- 69.6- 34.4

0.1


Horizontal

PhaseDifference

#2 rel. to 41

-0.1+0.5+2.2+2.1+1.1+0.6+0.5-0.1-0.2+0.3+0.3

-12-

Table 3-9

I. Frequency: 240 MHz


0.036.072.0108.0144.0 180.0216.0252.0288.0'324.0360.0


Phase,Ant.#1

0.14 0 . 440.474.9102.2129.1172.5-132.3- 95.6- 66.3- 35.9

0.2

Phase,.Ant,#2

0.145.079.9109.6142.1

-177.3-133.3- 96.5- 66.3- 37.6

0.2


Vertical

PhaseDifference

#2 rel. to 41

0.0+4.6+5.0+7.4

+13.0. .+10.2- 1.0- 0.9- 0.0- 1.7

0.0

Table 3-10

Frequency: 240 MHz


0.036. 072.0

108.0144.0 180.0216.0252.0288.0'324.0360.0

Elevation Angle:

Phase,Ant.#1

0.00 . 035.572.5108.8143.3177.8

-145.8-108.5- 72.2- 35.4

0.0

10° Source Antenna Polarization

Horizontal

Phase,'Ant,#2

0.135.874.3111.5147.8

-177.2-142.0-104.7- 67.6- 32.8- 0.1

PhaseDifference

#2 rel. to #1

+0.1+0.3+1.8+2.7+4.5+5.0+3.8+3.8+4.6+2.6-O.1

-14-

Table 3-11

Frequency: 260 MHz Elevation Angle: 28°

PositionerAzimuthAngcjle,

0.0 -36.072.0

108.0144.0180.0216.0252.0288.0324.0360.0

Phase,Ant.#1

0.1O . 1

34.770.5

106.5143.2

-179.6-143.9-109.5- 72.8- 35.6

0.0

Phase,.Ant,#2

0.135.371.9

108.4144.7

-178.9-143.3-108.5- 72.1- 35.4

0.2


Vertical

PhaseDifference

#2 rel. to #1

0.0+0.6+1.4+1 . 9

+1.5+0.7+0.6+1. 0+0.7+0.2+0.2

-15-

Table 3-12

,Frequency: 260 MHz


0.036.072.0108.0144.0180.0216.0252.0288.0324.0360.0


Phase,Ant.#1

0.236.073.4

110.6144.7178.4

-145.7-107.'7- 70.6- 35.2

0.4

Phase,Ant,#2

0. .

0.135.773.1110.1144.4178.3

-145.6-107.2- 69.9- 35.1.- 0.1

Source Antenna PolarizationHorizontal

PhaseDifference

#2 rel. to #1

-0.1-0.3-0.3-0.5-0.3.-0.1+0.1+0.5+0.7+0.1-0.5

-16-

Table 3- 13

Frequency: 260 MHz


0.036.072.0108.0144.0180.0216.0252.0288.0324.0360.0


Phase,Ant.1

0.134.566.598.3

139.6-176.6-144.5-113.5- 78.1- 37.7

0.1

Phase,'Ant,#2

0.132.967 . 1,

102.8145.8

-173.4-142.1-110.0- 73.8- 35.2- 0.1


Vertical

PhaseDifference

#2 rel. to #1

O.1-1.6+0.6+4.5+6.2+3.2+2.4+3.5+4.3+2.5-0.2

-17-

Table 3-14

Frequency: 260 MHz


0.036-.072.0

108.0144.0 180.0216.0252.0288.0324.0360.0


Phase,Ant.#1

0.136.675.5

112.5146.3180.0

-143.5-107.1- 69.8-33.8

0.1

Phase;Ant,#2

0.136.475.3112.3145.7179.2

-144.1-106.4- 68.3- 33.0

0.0

Source Antenna PolarizationHorizontal

PhaseDifference

#2 rel. to #1

0.0-0.2-0.2-0.2-0.6-0.8-0.6+0.7+1.5+0.8-0.1

-18-

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4.0 Conclusions

4.1 Gain

.Measured gain, at the only point specified (zenith),

exceeded the requirement by 4 dB. Table 4-1 shows the

typical gain variation with elevation angle from one of the

patterns in paragraph 3. With Geotronics' present under-

standing that the ability to track missiles to impact is a

requirement, a serious question is raised as to the total

suitability of this pattern. The pattern of a conical spiral

is directly controllable by means of spiral angle; for ex-

ample, Figure 4-1 is the pattern of a prototype antenna

designed by Geotronics for another application. Although

the gain at the zenith is only (approximately) +2dBi, this

gain holds within approximately 2 dB down to the horizon.

Such a spiral would therefore provide more gain than the

matched pair delivered under this contract at elevation

angles between about 40° and the horizon. It is not

known, however, what impact this faster spiral rate would

have on the ability to match two antennas in their phase

1versus azimuth characteristics. Dyson and Mayes indicate

that a more linear relationship between phase and azimuth

results from slower spiral rates, but since match rather

than linearity is the requirement, the faster spiral for

broader pattern may be worth investigation.

1. Dyson,J.D., and Mayes,P.E.,"New Circulary PolarizedFrequency - Independent Antennas with Conical Beam orOmnidirectional Patterns", IRE Transactions on Antennas

and Propagation, July, 1961.

-43-

Table 4-1

~~~~~~~~~~~~~Absolute gain vs. el. angle~Absolute gain vs. el. angle

ElevationAngle, Deg. Gain dB, ref

90 (zenith)80706050403020100 (horizon)

r.h.c.p. isotropic

8.17.97.56.34.72.5

-0.4-4.1-8.4-12.4

-44-.

Elevation Pattern, Fast-Spiral Antenna


ATLANTA. GEORGIA

Figure 4-1

-45-

oj'.: h4 0

4.2 Bandwidth.

In general, characteristics of the antennas varied

only slightly with frequency from 220 to 260 MHz. Impedance

match was optimum at 235 to 240 MHz, while pattern,gain,

axial ratio, and phase match characteristics show no signifi-

cant systematic variation across the band.

4.3 Impedance and VSWR.

The matching technique used met the specified require-

ments.

4.4 Circularity.

Axial ratio can be read directly from any of the eleva-

tion patterns. Table 4-2 summarizes the axial ratio at

elevations from horizon to zenith for antenna number one

- at all frequencies and two orthogonal azimuth angles.

The average of the two azimuth readings is generally 1.5 dB

or less above 50 ° elevation and 3 dB or less above approxi-

mately 35° elevation.

4.5 Equiphase Characteristics

Table 4-3 summarizes the worst excursions from phase

match for each of the twelve measurement conditions detailed

in Tables 3-3 through 3-14. Examination of the data shows

better match at 28 ° elevation than at 10°; it was not

possible to make measurements at higher angles in the facility

available. With the patterns showing axial ratios on the

-46-

Table 4-2

Axial Ratio vs. elevation (SN 1)

ElevationAngle, Deg.

Ratio,dB at220 MHz

~=45° c=135° avg.

Ratio, dB, at240 MHz

q=45° p=135° avg.

Ratio, dB,260 MHz

p=45° ~=136°

90 (zenith)8070

0.91.01.2

60 1.050 1.140 1.530 3.220 5.410 8.50 (horizon)13.5

1.81.81.92.02.23.24.96.58.09.0

at

avg.

1.41.41.61.51.62.44.06.08.2

11.2

1.11.0.0.90.51.12 .03.04.76.2

13.0

0.81.01.00.80.31.33.05.18.516.5

1.01.01.00.60.71.63.04.97.4

14.8

1.51.40.80.91.82.84.25.07.514.0

0.60.50.70.71.21.82.84.58.0

17.0

1.01.00.80.81.52.33.54.87.815.5

-47-

Table 4-3

Summary - Worst Excursion from Phase Match

Freq.-MHz

220240260220240260220240260220240260

Elev.,Deg.

282828282828101010101010

SourcePolarization

Vert.Vert.Vert.Horiz.Horiz.Horiz.Vert.Vert.Vert.Horiz.Horiz.Horiz.

ExcursionPhase Angle @ Az. Angle

-- 2.9 °

+2.2 °

+1 . 9 °

-2.0 °

+2.2 °

+ 0.7 °

- 5.8°+13.0°

+6.2 °-2.8°

+5.0 °+1.5 °

@ 144°

@ 72 °

@ 108 °

@ 180°

@ 72 °

@ 288°

@ 108 °

@ 144°

@ 144°

@ 180°

@ 180°

@ 288°

and 144°

-48-

order of 7 to 9 dB at the 10° elevation and gain some 16 dB

down from peak, it should not be too surprising that the

phase match is relatively poor. In general, antenna character-

istics are difficult to control and, in fact, difficult to

measure near nulls in the pattern. It is believed that the

design goal phase match specifications probably would be-met

within the 3 dB beamwidth, had the facility permitted measurement

at those elevation angles. It is possible that a fast spiral

with better control of gain and axial ratio down to the horizon

(see Figure 4-1) would permit the goal to be achieved. It is

not known how the results would have been affected by use of a

circularly polarized source antenna.

As described in paragraph two, the antennas were hand-

built and include splices and fasteners. If facilities existed

for manufacture of this size and type antennas with auto-

matically controlled machinery or photographically-based techniques

(chemical etching, etc.), a better phase match might be obtained.

4.6 New Technology

No new technology was developed in the course of per-

formance of this contract.

-49-

Documents

MATCHED PAIR CONICAL SPIRAL ANTENNAS Robert E. Metzler ...€¦ · MATCHED PAIR CONICAL SPIRAL ANTENNAS Robert E. Metzler GEOTRONICS, INC. 5718 Columbia Pike Falls Church, Virginia