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PRACTICAL
ANTENNA
GUIDE
2
TÜV
ZERTIFIKAT
KATHREIN WERKE KG
3
PRACTICAL
ANTENNA
GUIDE
PREFACE
The expansion of radio communications, especially for public wireless communications, attracts every day more professionals involved in the technology of the equipment, accessories and infrastructure of wireless systems. The Radiating System as crucial part of the infrastructure is based on Antennas. Their design, manufacturing and application develloped to a technology of its own to which this booklet is dedicated. KATHREIN - the oldest and nowadays biggest supplier of Antennas - is offering with this GUIDE to all interested professionals a comprehensive handbook directed to the day-to-day practice. This guide was oriented by our professional spirit expressed in our slogan:
“ QUALITY LEADS THE WAY”
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Copyright: KATHREIN MOBILCOM BRASIL Ltda. Rua Marcilio Dias, 138 04764-080 São Paulo - SP Brazil Phone: (55 11) 5685-4290 Fax: (55 11) 5685-4292 WEB : www.kathrein.com.br E-Mail : [email protected] Authors : Ing. Eduardo Roberto Huemer Dipl. Oec. Karl-Heinz Lensing Al rights reserved. Total or parcial reproduction is not permitted without specific authorization of KATHREIN MOBILCOM BRASIL LTDA Printed by: Dinâmica Gráfica e Editora Ltda. Phone/Fax: 00 55 11 6947-7788 São Paulo/ Brazil 5th Edition, January 2000 Price: U$ 10,00/ Euro 10,00 This GUIDE is also available in:
- Spanish - Portuguese
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I N D E X Page 1 INTRODUCTION 7 2 MOBILE COMMUNICATIONS 8 3 THEORY 9 4 DEFINITIONS 12 4.1 Polarization 12 4.2 Propagation Patterns 12 4.2.1 Thinking in 3-D 13 4.3 Half-Power Beam-Width - HPBW 13 4.4 Gain 14 4.4.1 Gain References (dBd or dBi) 15 4.5 Front-to-Back Ratio (FB) 16 4.6 Impedance 16 4.7 Return Loss (ROE / VSWR) 17 4.7.1 Connection Effects on the Return Loss 18 4.8 Downtilt 19 4.8.1 Calculating Downtilt for an Omni Antenna 21 4.8.2 Comparison of Mechanical and Electrical Downtilt 22 4.9 Mechanical Details 23 4.10 Intermodulation 24 5 THE RADIO BASE STATION 26 6 BASE STATION ANTENNAS 29 6.1 Comparison of Groundplane and Dipoles 29 6.2 The Influence of Reflections on Radiation Patterns 30 6.2.1 Omni Antenna 30 6.2.2 Directional Antennas with Pannel Reflector 33 6.2.3 Additional Effects with X-Pol Antennas 40 6.2.4 Conclusion 40 6.2.5 Calculating the HPBW 41
6.3 Broadside Arrays 42
7 PARTICULAR TECHNIQUES USED IN CELLULAR SYSTEM 45 7.1 Diversity 45 7.2 Space Diversity 46 7.3 Omni Base Station 47 7.4 Sectored Base Station 48 7.5 Polarization Diversity 49 7.6 Horizontal and Vertical Polarization 49 7.7 Antennas with Dual Polarization 49
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7.8 Kathrein´s Dipole Based X-Pol Antenna Design 54 7.8.1 General Descriptions 54
7.8.2 Outstanding Characteristics 54 7.8.3 Typical Measurements 58 7.8.4 CPR x Azimuth 61 8 SPECIAL APPLICATIONS 62 8.1 Indoor Coverage System 62 8.2 Splitters 63
8.3 Duplexers 64 GLOSSARY 65
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1 INTRODUCTION Constantly we can observe the increase of communication systems based on electromagnetic waves: Turning on the radio or the TV, reading a message on a pager or answering a call with a wireless phone we are surrounded by these systems nowadays. With the development of these different systems in wide scale they became related to some specific frequency ranges which follow a classification. CLASSIFIED FREQUENCY RANGES
WAVE LENGHT FREQUENCY RANGE ABBREVIATION 10 km - 100 km 3 - 30 kHz VLF = Very Low Frequency 1 km - 10 km 30 - 300 kHz LF = Low Frequency 100 m - 1 km 300 - 3.000 kHz MF = Medium Frequency 10 m - 100 m 3 - 30 MHz HF = High Frequency 1 m - 10 m 30 - 300 MHz VHF = Very High Frequency 10 cm - 1 m 300 - 3.000 MHz UHF = Ultra High Frequency
1 cm - 10 cm 3 - 30 GHz SHF = Super High Frequency 1 mm - 1 cm 30 - 300 GHz EHF = Extreme High Frequency
The ranges of VHF, UHF and SHF were defined about hundred years ago. They have their origin in the physical dimensions of the components the pioners of radio based they experiments on and just by chance resonated in these frequencies. In the VHF – frequency band Heinrich Hertz – lateron the indication “Hz” was derived from his name – generated in 1884 waves of 3 meters length connecting an spark transmitter to a dipole terminated with two metal discs as capacitive load. Lodge substituted this circuit by a coesor of Branly with which he increased the coverage of the VHF transmitter to a distance of 30 m. One year later Marconi started his experiments in VHF with wave length of two meters. In UHF, waves length of 30 centimeters were generated by Righi in Italy in 1890. SHF appeared some years later in 1900 with the experience of Bhose in India and simultaneously by other inventors in Italy.
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2 MOBILE COMMUNICATIONS The last years brought an enormous technological jump in the field of mobile communications with the introduction of new mobile communications networks such as CDMA, TDMA, GSM, PCS, etc. The number of worldwide subscribers increased over 300 milions. Below is shown in a general way where some of the communication services in terms of frequency range are allocated. FREQÜÊNCY OPERATOR / COMMUNICATION SERVICE 88 – 108 MHz Broadcasting – FM 108 – 144 MHz Ground to Air Communications 144 – 148 MHz Amateur Radio 148 – 174 MHz Public System in General (Police, Fireworks, Road Services,
etc) 406 – 430 MHz WLL – Wireless Local Loop 430 – 440 MHz Amateur Radio 450 – 470 MHz Public System in General (Police, Fireworks, Road Services,
etc) 806 – 960 MHz Cellular Services (A and B Bands), Trunking, Paging 1850 – 1950 MHz WLL – Wireless Local Loop 3400 – 3450 MHz WLL – Wireless Local Loop 3500 – 3550 MHz WLL – Wireless Local Loop Obs.: The frequencies shown here one generic and do not represent a Frequency Plan; details have to be verified in the specific laws and rules of each country. The requirements for antennas in expanding networks have been continuously risen: - Radiating patterns strictly defined to assure a network planning with high accurancy. - Controlled intemodulation levels for the increasing number of carriers transmitted by
just one antenna - Dual polarized antennas - Electrical Downtilt in vertical patterns. - Design without obstructions. In the next chapters we are going to describe the theory of antennas in general as well as the most important types of antennas.
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3 THEORY Antennas transform wire propagated waves into space propagated waves. They receive electromagnetic waves and pass them onto a receiver or they transmitt electromagnetic waves which have been produced by a transmitter. As a matter of principle all the features of passive antennas can be applied for reception and transmission alike (reciprocality).
The principle of an antenna can be shown by bending a coaxial cable open. a) A transmitter sends a high frequency wave into a coaxial cable. A pulsing electrical
field is created between the wires which cannot free itself from the cable. b) The end of the cable is bent open. The field lines become longer and are ortthogonal
to the wires
Quad-gate
RF-cable
Symmetry
Free space
Antenna
Coaxial cable
Transmitter
Electrical field
Coaxial cable
Transmitter
Electrical Field
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c) The cable is bent open at right angles. The field lines have now reached a length,
which allows the wave to free itself from the cable. The apparatus radiates an electromagnetic wave, whereby the length of the two bent pieces of wire corresponds to half of the wave length.
This simpliified explanation describes the basic principle of almost every antenna – the λ/2 – dipole. Not only is an electrical field (E) created due to the voltage potential (U), but also a magnetic field (H) which is based on the current (I).
Voltage distribution Current distribution
Electric field Magnetic field
Transmitter
Electrical Field
λ /2
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The amplitude distribution of both fields corresponds to the voltage and current distribution on the dipole. The free propagation of the wave from the dipole is achieved by the permanent transformation from electrical into magnetic energy and vice-versa. The thereby resulting electrical and magnetic fields are at right angles to the direction of propagation.
Curiosity: The Origin of the Word Antenna For wireless communications, the transmitter has to be connected to a component that radiates the radio frequency under the desired conditions, and on the receiving side another component that captures this radiation under the same conditions. This radiating components are called ANTENNAS The word ANTENNA has a latin background and means very flexible rod. The antenna is not a human invention, but has been used for milions of years by lobsters, shrimps and numerous insects as a sensor in a shape of a flexible rod. It was the Russian physicist Popov who started to use this word for his invention of an electricity captor of atmospheric storms; after this, all physicists using the Maxwell equations adopted this expression.
Electric field Electric field Electric field
Magnetic field Magnetic field
Wave propagation
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4 DEFINITIONS 4.1 – Polarization Polarization can be defined as the direction of oscillation of the electrical field vector. Mobile Communications: Vertical polarization Broadcast systems: horizontal polarization 4.2 – Propagation Pattern In most cases the propagation characteristic of an antenna can be described via elevations through the horizontal and vertical radiation diagrams. In mobile communications this is defined by the magnetic field components (H-plane) and the electrical field components (E-plane). Very often a 3-dimensional description is chosen to describe a complex antenna.
Horizontal Pattern
Vertical Pattern
7o
Horizontal Plane Pattern (H) Upper view, the energy distribution of a directional panel antenna.
Vertical Plane Pattern (E) Observing from the side and making a cut referenced to the horizon line the energy is being distributed this way.
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4.2.1 – Thinking in 3-D To get a total view of how the energy of an antenna is distributed in space it is necessary to join the two patterns (Vertical + Horizontal) and think tridimensional. Below an example of a tridimensional pattern (omnidirectional antenna).
4.3 – Half Power-Beam-Width This term defines the aperture of the antenna. The HPWB is defined by the points in the horizontal and vertical diagrams, which show where the radiated power has reached half the amplitude of the main radiation direction. These points are also called 3 dB points.
Half power beam width
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4.4 - Gain In reality an increment of energy is not achieved via antenna gain. An antenna without gain radiates energy direction. An antenna with gain concentrates the energy in a defined angle segment of 3-dimensional space. The λ/2 – dipole is used as a reference for definig gain.At higher frequencies the gain is often defined with reference to the isotropic radiator. The isotropic radiator is an no-existant ideal antenna, which has also an omnidirecional radiation characteristic in the E-plane and H-plane. The gain is defined as the relation between the irradiated power in its main lobe and the electrical power injected to the antenna, and it is expressed in dB. The increase of energy concentration is obtained by stacking dipoles.
Half power beam width Gain (ref. λ/2 dipole) (1 λ/2 dipole)
(4 λ/2 dipoles)
(2 λ/2 dipoles)
(8 λ/2 dipoles)
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4.4.1 Gain References: dBd or dBi dBi is defined as the reference of gain when measured with an isotropic source; in general used in the specifications of Europeans Manufacturers. dBd is defined as the reference of gain when measured with a half wave dipole; in general used in the specifications of American Manufacturers.
The gain of an antenna is directly linked to the characteristic of its radiation pattern. The gain can be calculated through the half power angles at the horizontal and vertical planes.
Reminder:
dBi = dBd + 2,15
dBd Gain
Vertical half-power beam- width
6,5o
13o
25o
78o
18
16
12
14
0
8
6
4
2
10
45o 90o 360o 180o 270o 60o 120o
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4.5 – Front-to-Back Ratio (F/B) For every directional antenna (Yagi, Panels, etc) this important parameter must be considered. Front-to-Back ratio is the ratio of the gain of the main lobe compared to the gain of the rear lobe. The higher the Front-to-Back ratio, the better the protection against noise or interference behind the antenna. 4.6 - Impedance Characteristic impedance is one of the most important properties of the coaxial cable. Electrically it means the ratio of the voltage between the conductors to the current flowing in the same conductors. In a homogeneous coaxial cable the characteristic impedance is constant along the whole length of the cable. Characteristic impedance is important to be specified, because the cable shall be terminated with an impedance which equals to the characteristic impedance of the cable. In radio communication the most common characteristic impedance is 50 Ohm. Other values, such as 75 Ohm are used in other applications, e.g. in cable TV systems and video systems. All equipment or passive components which are connected to the cable shall have the same characteristic impedances cause mismatch and reflections which distort the transmission.
Back Gain in dB
F/B = (Front) – (Back)
Front Gain in dB
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4.7 – Return Loss (ROE / VSWR ) An impedance of exactly 50 Ohm can only be practically achieved at one frequency. The VSWR defines how far the impedance differs from 50 Ohm with a wide-band antenna. The power delivered from the transmitter can no longer be radiated without loss because of this incorrect compensation. Part of this power is reflected at the antenna and is returned to the transmitter.
The forward and return power forms a standing wave with corresponding voltage minima and maxima (Umin / Umax). This wave ratio (Voltage Standing Wave Ratio) defines the level of compensation of the antenna and was previously measured by interval sensor measurement. A VSWR of 1.5 is standard within móbile communications. In this case the real component of the complex impedance may vary between the following values:
Maximum Value : 50 Ohms x 1,5 = 75 Ohms Minimum Value : 50 Ohms ÷ 1,5 = 33 Ohms
Return ratio Pr/Ur
Forward ratio PV/Uv
Antenna
Standing wave
Umín
Umáx
Coax-cable
Transmitter
VSWR S= Umax/Umin = (1+r) / (1-r) Return loss attenuation a r Factor of reflection: r=Ur/Uv = (s-1) / (s+1) ar [dB] = -20 log r Reflected power Pr/ Pv = 100 r2 [100%]
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The term return loss attenuation is being used more often in recent times. The reason for this is that the voltage ratio of the return to the forward-wave UR/UV can be measured via a directional couplear. This factor is defined as the co-efficient of reflection. Figure shows the relationship between the coefficient of reflection, return loss attenuation, VSWR and reflected power. 4.7.1 – Effect of Connections and Installation Quality on RL Return loss (RL) is a system parameter, which includes the effect of the following factors: - Transmitter mismatch - connector mismatch at the input of the cable - SRL of the cable itself (measured on drum at the factory) - Installation quality - Connector mismatch at the output of the cable - Antenna mismatch Transmitter mismatch and antenna mismatch depend on the difference between the output impedance of the transmitter or input impedance of the antenna and the characteristic impedance of the cable.
Connections and installation quality are very important factors affecting the resulting total RL.
Return loss attenuation in dB
Factor of reflection
VSWR
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Connectors always have serial indutance and parallel capacitance, which cause mismatch and the level of mismatch depends on the connector construction and the mounting methods. Only good quality connectors should be used and the mounting instructions given by the connector supplier should carefully be followed. - When the cable is installed for example on the tower, good workmanship and skill
are required. RL can dramatically be degraded by improper handling of the cable and by poor installation quality.
4.8 – Downtilt The energy in each sector of a Base Station must be tailored in a way to avoid that the signal penetrates in the territory of another cell causing interference. In order to place the energy in a certain target we can slant the main lobe lower than the horizon line, a procedure called Downtilt. The Downtilt can be mechanical:
- just slant the antenna to the desired angle. In practice the typical slant vary from 3º to 15º.
Cell 1 Cell 4
Cell 2
Cell 3
Sector C
Sector A
Sector B
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The downtilt can be electrical:
- The antenna is being adjusted in factory with the main lobe already tilt with a standard that can be 3º, 6º, 9º, 12º for example.
There are some antennas that have variable downtilt:
- On the back of the antenna is a lever with a graduated scale to choose the desired electrical downtilt angle.
741 493
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4.8.1 – Calculating Downtilt for an Omnidirecinal Antenna
Formula: Angle = 1 / tang ((h1 – h2) / d) h1 = Elevation of the base antenna (m) h2 = Elevation of the mobile antenna (m) d = Distance (m) Example: 1 Km = 1000m 9,98 Km x 1000 = 9980,00 m 731,52 / 9980,00 = 0,0732986 1 / tang (0,0732986) = 4,2º
9980 m
3353 m
732 m
2622 m
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4.8.2 – Comparison of Mechanical and Electrical Downtilt
- Downtilt angle varies over the azimuth range: Set down angle only in main direction (0º)
No downtilt in tilt axis direction (+/- 90º from main beam)
- The horizontal half-power beam-width increases with greater downtilt angle. - The resulting gain reduction depends on azimuth direction
Typical Pattern of Mechanical Downtitl
MECHANIC
23
- Constant downtilt angle over the whole azimuth range. - Horizontal half -power beam independent of downtilt angle - Identical gain reduction for all azimuth directions.
4.9 – Mechanical Features Antennas are always mounted at exposed sites. This requires that antennas are designed to withstand the required mechanical loading. Vehicle antennas, for example, must withstand a high wind velocity, vibrations, saloon washing and still fulfil a limited wind noise requirement. Antennas for portable radio equipment are often exposed to il-handling and sometimes even played with by the user. Base station antennas are exposed to high wind speed, vibrations, ice, snow, a corrosive environment and, of course, also extreme electrostatic loading via lightning.
Typical Pattern of Electrical Downtilt
ELECTRIC
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4.10 - Intermodulation The evolution of the technology in mobile communications, especially of digital transmissions (CDMA, TDMA, GSM), pushed another parameter as important into the foreground: intermodulation. WHAT IS INTERMODULATION? Passive intermodulation is built up at the passive elements of the radio communications system. Passive elements are duplexers filters, combiners, connectors, feeders, antennas, etc. The intermodulation is generated at the non-linear discontinuity points of the inferfaces. The space for the aerials is normally limited at the tower of the base station. Thus forces to combine several transmitter and receiver signals to the same feeding line and aerial. High power simultaneous transmitter carriers have tendency to interact together at the non-linear points causing intermodulation interference to the receiving channels. Poor intermodulation characters may disturb receiving channels severely. Passive intermodulation is related to the multiple transmitter duplex communication systems. Non-linearity produces harmonic frequencies. Combinations with the fundamental and their harmonic frequencies will produce intermodulation. All the removable contacts like connectors are potential sources for intermodulation Non-linearity sources of connectors can be for example: • Ferromagnetic materials • Non-linear dielectrics • Dissimilar materials at the contact • Oxidized or improper surface of the contact • Inadequate contact and low contact pressure leading to micro -arcing • Corrosion, dirt, dust, oil, grease, fingerprints The intermodulation can be of various harmonics. Example : 3ª, 5ª e 7 ª harmonic
How to calculate the 3ª harmonic with two frequencies F1 e F2?
IM3 = 2 x F1 – F2,
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CALCULATION EXAMPLE Typical: Level IM7 = Level IM5 – 15 dB = Level IM3 – 15 dB Level Tx = 20 W = 43 dBm System sensibility : - 113 dBm à IM7 < - 156 dBc à IM5 < - 141 dBc à IM3 < - 126 dBc better – 140 dBc ! Specified : - 150 dBc Band – Rx : 835 – 849 MHz Band – Tx : 880 – 894 MHz f 1 : 880 MHz f 2 : 894 MHz
IM3 : 2 x f 1 – f 2 = 866 MHz IM3 : 2 x f 1 – f 2 = 908 MHz IM5 : 3 x f 1 – 2 x f 2 = 852 MHz IM5 : 3 x f 1 – 2 x f 1 = 922 MHz IM7 : 4 x f 1 – 3 x f 2 = 838 MHz IM7 : 4 x f 2 – 3 x f 1 = 936 MHz
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5 THE BASE STATION
The installation of a base station requires special attention! Example of a Base Station and its principal components:
Tower: The special attention starts with the tower needing an appropriate corros ive protection, because corroded metal parts and screws increase the level of intermodulation. Antennas: Remember that the antenna is the most diffiult point to access in installation and maintance; therefore quality, durability and reliability is a must. Connectors: Connectors are always a descontinuing point of the radiating system. Therefore the selection of connectors as well as assembling them to the cable needs special skills and attention. Type N and DIN 7/16 connectors are currently available for all types of coaxial cables. The connectors have to be assembled to the cable following stricly the manufacturer instructions and - when reccomended – sealed with shrink isolation tapes or tubes.
Lightining Jumper
Jumper
Duplexer
Clamp
Shelter
Arrestor
792 951
Antenna AP13-850/065
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Jumpers : Jumpers are the connecting elements between the feeder cables on top to the antennas as well as between the feeder cable and the equipment in the shelter. As adaptable part of the cable run jumpers must be more flexible than the feeder cables. Only flexible coax cable should be used; Jumpers made with standart coax cable with its high rigidness do not attend this requirement; forced into the connections such jumpers may cause distorcion to the tranmission parameters as well as intermodulation. Systematic measurement of VSWR and Intermodulation after assembly garanteering a quality jumper avoids problems after installation. Lightning arrestor: This component protects the system against electrical discharges; typically they are applied next to the antenna and close to the shelter. Coaxial cable Transmiss ion cables
In a coaxial cable the transmission circuit is formed by three functional elements: inner conductor, dielectric and outer conductor. All three elements are concentric, i.e. they have the same central axis. The materials and dimensions of these three elements determine the transmission and the other electrical characteristics of the coaxial cable. Any coaxial cable also have a plastic sheath around its outer conductor and may have some other constructional elements depending on the application of the cable.
Functional elements of a coaxial cable
The power rating of a coaxial cable is defined as the input power at any specified frequency, temperature and pressure which can be handled continuously when the cable is terminated by a load corresponding to the characteristic impedance. The limitation may either be the maximum permissible operating voltage of the cable. Thus the power rating is divided into two categories: - average power rating, limited by the maximum permissible inner conductor temperature. - peak power rating, limited by the maximum permissible operating voltage.
Feeder Cables
Feeder cables are used to connect the transmission equipment to the antenna. The most common applications are: - base stations for móbile networks (AMPS, TDMA, CDMA, GSM etc); - radio link systems; - satellite communications systems.
Outer conductor
Dielectric
Inner conductor
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The function of the feeder cable is to transmit signal power between the transmission equipment and the antenna with sufficient performance.
The most important transmission criteria for cable selection are:
- attenuation - return loss or VSWR - power rating
Frequency
(Feeder length) 150 MHz 450 MHZ 900 Mhz 1800 Mhz 2100 Mhz
< 25 m ½” - 50 Ω ½” – 50 Ω ½” – 50 Ω ? ” – 50 Ω ? ” – 50 Ω
25 ... 50 m ½” - 50 Ω ? ” – 50 Ω ? ” – 50 Ω 1 ¼” – 50 Ω 1 ? ” – 50 Ω 50 ... 75 m ½” - 50 Ω ? ” – 50 Ω 1 ¼” – 50 Ω 1 ? ” – 50 Ω 2 ¼” – 50 Ω
75 ... 100 m ? ” – 50 Ω 1 ¼” – 50 Ω 1 ? ” – 50 Ω 2 ¼” – 50 Ω - 100 ... 150 m ? ” – 50 Ω 1 ? ” – 50 Ω - - -
> 150 m 1 ? ” – 50 Ω - - - - Installation
In all stages of installation it is important to follow the installation instructions and limiting values given by the manufacturer such as: - minimum bending radius during pulling and in final bending; - minimum installation temperature; - maximum pulling force. The minimum bending radius is defined in order to prevent damage of cable structure during bending. Too sharp bending may cause cable kink which results in local changes in characteristic impedance and cause distorcion to the transmission. The limiting values depend on the cable construction and they are given by the manufacturers for each cable type.
Clamps: A careful instalation uses a clamp every meter to fix the cable properly to the tower. Grounding: A very good grounding of the tower and shelter is needed. Note: The lighting arrestors should be connected to the grounding with a grounding strap. Wall Gland: Special plate and rubber isolator to pass the cables inside the shelter avoiding water inside the shelter. Inside the Shelter: Usually another lightning arrestor is connected between the feeder and the last jumper before getting into the system. Duplexer : This equipment is installed whenever the same antenna is to be used for transmission and reception at the same time. Phisically it is installed inside the shelter.
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6 BASE STATION ANTENNAS 6.1 – Comparison of Groundplane and Dipoles The classical omnidirecional λ/2 antennas are of a groundplane or λ /4 – skirt nature.
The names indicate how the antenna is decoupled from the mast.
K 51 26 2 146 – 174 MHz
K 55 26 28 164 – 174 MHz
K 75 11 61806 – 960 MHz
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In the first case a conductive plane is achieved via 3 counterweighted poles, in the other case the decoupling is achieved by using a λ/4 skirt. The second type however only works across a very limited bandwitdth, so that for example three versions are needed to cover the 2 m band. The groundplane antenna on the other hand can cover the complete frequency range because it is a wideband antenna. 6.2 – The Influence of Reflections on Radiation Patterns The vertical and horizontal radiation patterns normally given in antenna datasheets apply only to a reflection-free environment (free space propagation). For real installations, this condition is sometimes hard to fulfil. Existing obstructions such as mas ts on flat roofs, or near buildings cause scattered signals, which effect the free-space patterns of the antenna. In certain directions the direct signal from the antenna is superimposed by at least one further signal which has been created by reflections (Fig. 1). The resulting total vector depends on the amplitude and phase of the reflected wave. The amplitude and phase is determined by the reflection performance of the obstruction as well as the difference in distance (running time). Antenna Direct signal
Reflected signal With phase difference
Obstruction
Fig. 1 : Occuring reflections 6.2.1 – Omnidirecional Antennas Omni antennas radiate a constant power across the full azimuth. Disturbances of the free-field conditions therefore have a particularly intensive influence on the horizontal radiation pattern of the antenna. This means that a real circular horizontal pattern can only be created if the antenna is mounted on the tip of a mast. Mounting on the side of a mast, which usually consists of a good reflecting material (steel, concret), changes the pattern considerably.
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Decisive factors are the spacing between the mast and the antenna as well as the mast dimensions. Cylindrical masts As far as a cylindrical mast is concerned, the calculation of the resulting radiation pattern is relatively simple. The uncomplicated and symmetrical reflection surface creates pattern shapes, which are useful for network planning, and vary with the spacing to the mast. In order to consider the resulting patterns independent of the operating frequency, the mast diameter and the spacing to the mast are given in wavelengths λ Radiation Patterns
Side-Mounted / Omni
- Spacing ¼ λ A spacing of 0.25 λ is most frequently used. A smaller spacing should not be chosen due to an increasing and therefore worsening of the antenna’s VSWR. The result is an offset pattern, whose front-to-back ratio varies with the mast diameter. The original omni pattern has changed into a directional pattern with a roughly 2 dB higher gain. (Look at the table).
Spacing Mast diameter 0.04 λ Mast diameter 0.6 λ
1/4 λ
S
M
S
M
32
- Spacing 0,5 λ An increase of the spacing to 0.5 λ creates a bi-directional pattern perpendicular to the line antenna – mast, which becomes more characteristic with bigger mast diameters. This pattern shape provides 2-3 dB more gain and is especialy suitable for the coverage of motorways and railway lines.
Spacing Mast Diameter 0,04 λ Mast Diameter 0,6 λ
1/2 λ
- Spacing 3/4 λ
At a spacing of 0.75 λ a further beam grows in the direction of the antenna; a tri-directional pattern is formed.
Spacing Mast Diameter 0,04 λ Mast Diameter 0,6 λ
3/4λ
33
- Spacing 20 λ The number of beams becomes greater and the depths of the corresponding minima become smaller as the spacing is increased; the pattern changes back into an omni characteristic. However the influence of the mast can still be recognized up to spacings of 20-25 λ.
Spacing Mast Diameter 0,04λ Mast Diameter 0,6 λ
20 λ
Lattice tower The radiation pattern of an omni antenna side-mounted on a lattice tower is much more difficult to determine. Each tower leg, the bracing and existing ladders and cable traces within the tower cause reflections. Therefore larger spacings to the mast always create the risk of unexpected nulls in the pattern. At smaller spacings (0.25 λ / 0.5 λ) the tower leg on which the antenna is mounted, is mainly responsible for the pattern. The principal pattern shapes of „offset“ and „bi-directional characteristic“ still exist, but compared to a cylindrical mast the patterns will have certain irregularities and discontinuities. 6.2.2 – Directional Antennas with Pannel Reflectors Directional antennas only radiate their power into certain space segments, and have a corresponding front-to -back ratio of 20 dB or more. This means that the rearside radiation is relatively low and reflective obstructions in this area only have a very small influence on the radiation pattern. Therefore, mast side-mounted directional antennas provide patterns which are very close to free space propagation in accordance with the datasheet. Wall Mounting Frequently the antenna installation of cell networks and WLL is carried out on building facades, which do not fit with the direction of the cells to be covered.
34
A large plane behind the antenna is thereby illuminated by a significantly broader range of radiation with a power level, which is not sufficiently reduced to be able to neglect the influences on the radiation pattern. A rotation of the antenna increases the radiated power towards the wall and thus the resulting reflections (Fig. 2). Azimuth : 0° 20° 40°
Building fascade
Fig.2.: 65° directional antenna wall-mounted on a building
In the enclosed pages, a series of calculated horizontal radiation patterns for this mounting situation is shown. The spacing, the angle and the half power beam width of the antenna have been individually varied. The calculations consider the wall as ideally reflective. This corresponds to the real situation, e.g. for concrete walls or aluminum covered facades. This method of calculation cannot be applied to brick walls for example, as the factor of reflection may vary on account of rain, etc. According to the calculations (which have been confirmed by measurements), the patterns are more and more damaged with increasing:
- spacing; - angle; - half power beam width;
The following criteria should therefore be considered with wall-mounting of directional anrennas : § Spacing to the wall reduced to a minimum (small phase difference between the
direct and the reflected signal) § A maximum rotation angle of approx. 20° with reference to the wall perpendicular § A maximum horizontal half power beam width of 65°
Radiation against the Reflective plane
35
Radiation Patterns
800 MHz Wall-mounted Directional Antenna
Spa
cing
Panel 65°
Panel 105°
10 m
m
150
mm
300
mm
600
mm
S
Building fascade
Direction 0°
36
Radiation Pattern
800 MHz Wall Mounted Directional Antenna r
Spa
cing
Panel 65°
Panel 105°
150
mm
300
mm
600
mm
S Building fascade
Direction 22.5°
37
Radiation Pattern
800 MHz Wall-Mounted Directional Antenna
Spa
cing
Panel 65°
Panel 105°
150
mm
300
mm
600
mm
Direction 45°
S
Building Fascade
38
It is sometimes suggested to mount the antenna system on the building apex in order to improve the above situation (Fig. 3). However this arrangement is not recommendable for the following reasons: a) The radiation patterns of both Rx-antennas are shadowed by the radial shift of the
Tx-antenna. b-) Mirror imaged reflections create unequal Rx radiation patterns, which have a
negative influence on the diversity performance of the system!
Fig 3:Mounting on a building apex
Mounting above a reflective plane Antennas are frequently mounted on flat roofs. The recommended location for this kind of installation is the roof`s edge, but for optical reasons the antennas are sometimes placed within the roof plane for example on the top of an elevator shaft (Fig. 4).
Fig. 4: Mounting on a reflective flat roof
Rx1 Tx Building Top View Rx2
elevator
Building side view
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Depending on the half-power beam-width of the vertical pattern, the plane between the antenna and the roof`s edge creates reflections, which cause an uptilt of the resulting final pattern. To avoid this effect, the radiated power towards the roof must be limited, that means the antenna has to be mounted with a sufficient height above the roof. In practice the following rule of thumb is approved : The radiated power towards the roof plane should be more than 10 dB less than the maximum radiated power of the main beam !
Fig. 5 : Vertical Radiation pattern of Antenna AP13 – 850 / 065 Gain 15.5 dBi
Horizontal half power beam width : 65° Vertical half power beam width: 13°
The vertical radiation pattern of Fig. 5 belongs to a standard celular system – 800 Mhz - antenna. The radia ted power (gain) is reduced by 10 dB at an angle α of 12° with respect to the maximum power. The required antenna height can then be calculated according to the geometrical relationship as described in Fig.6.
Max. Power (main direction) Power: -10 dB
α = 12°
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Building side view
Fig. 6 : Calculation of the antenna mounting height Example: For a spacing L of 14 m to the roof edge the antenna height must be at least 3m above the roof. 6.2.3 – Additional Effects with X-Pol Antennas So far the effects on vertical polarized antennas have been discussed. These kind of antennas are a standard in 900 MHz and 1800 MHz mobile communication networks using space diversity. The latest technology of polarization diversity systems base on so-called X-pol antennas, which provide two slanted polarizations at an angle of +45° and -45°. These polarizations can be seperated into vertical and horizontal components of equal amplitude. Depending on the orientation of the obstructions, these components are effected differently. Vertically orientated structures such as towers or building fascades will have a higher influence on the vertical component, while a flat roof will change the horizontal component more. Therefore reflections do not only destroy the radiation patterns of X-Pol antennas but the polarisation direction as well, which may result in a reduced diversity performance. 6.2.4 - Conclusion Antenna site planning should consider the aspect of a reflection-free radiation. If reflexions are expected due to existing obstructions, it is recommended to consider the possible qualitative effects on performance.
elevator
H = L x tan α α
L
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6.2.5 – Calculating HPBW Touchdown Point
Formula (*) : Distance = H / tang ( HPBW / 2 ) H = Height of antenna from the ground (m) HPBW = Vertical half-power beamwidth (deg) Example: 70º (HPBW) / 2 = 35 tang(35) = 0,7002 Distance = 60,96 m / 0,7002 = 87,06 m (*) This formula is for level terrain calculations only.
60,96 m
87,06 m
70º
Soil
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6.3 – Broadside Arrays Directional antennas whose mechanical features are orthogonal to the main radiation beam are called “Broadside Arrays”. Panels and corner reflector antennas are typical for this type. Panels and corner reflector antennas are typical fo'r this type (Fig.l4). Panel antennas are made up of several dipoles mounted in front of a reflector so that gain can be achieved from both the horizontal and vertical plane. This type of antenna is very well suited for antenna combinations. The reflector plate of a corner reflector antenna is, as the name suggests, not straight but bent forwards. The chosen angle influences the horizontal half-power-beamwidth, normaily the angle is 90o. The corner
K 73 12 21 400 – 700 MHz
730 684 890 – 960 MHz
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reflector antenna is only used singly, for example: for the coverage of railway lines and motorways. Special applications which cannot be realized by using a single antenna are very often achieved via antenna combinations. The combination is made up of several single antennas and a distribution system (power splitter and connecting cable). Very often a combination is designed in order to achieve a higher gain. Many different antennas are also used to achieve a wide range of horizontal radiation characteristics by varying the number of antennas, the azimuth direction, the spacing, the phase and the power ratio. Figure below shows 3 single examples.
Distance A = 20 mm 947 MHz
Antenna 730 360
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A quase-omnidirecional pattern can also be produced. The required number of antennas increases with the diameter of the tower. For examples 8 panels are required at 900 MHz for a mast with a diameter of approximately 1.5 m. The calculation of such radiation patterns is achieved via vector addition of the amplitude and phase of each antenna. The amplitude of each pattern can be read from the data sheet but the phase is only known by the antenna manufacter. However the phase is the most important factor for the calculation because a rough estimate using only the amplitude can lead to completely incorrect results.
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7 PARTICULAR TECHNIQUES USED IN CELLULAR SYSTEM 7.1 - Diversity Diversity is used to increase the signal level from the mobile to the base station (uplink). The problem with this path is the fact that the mobile telephone only works with low power and a short antenna. Diversity is applied on the reception side of the base station. A transmitted signal extremly rarely reaches the user via the most direct route. The received signal is very often a combination of direct and reflected electromagnetic waves.
The reflected waves have differing phase and polarization characteristics. As a result there may be an amplification or in extreme cases a cancelling of the signal at specific locations. It is not unknown, that the reception field strength may vary 20-30 dB within several meters. Operation in a canyon-like street is often only possible by using these reflections. These reflections from buildings, masts or trees are especially common, because mobile communications predominantly uses vertical polarlzation.
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7.2 – Space Diversity This system consists of two reception antennas spaced a distance apart. One antenna has a certain field strength profile with maxima and minima from its coverage area, the other antenna has a completely different field strength profile although only spaced a few meters away. Ideally the minima of one antenna will be completely compensated by the maxima of the other
The improvement in the average signal level achieved with this method is called diversity-gain. Diversity antennas are not RF-combined because this would lead to an unfavourable radiation characteristic. Both antennnas function separately on different reception paths, whereby the higher signal per channel and antenna is chosen by the base station.
Signal
Signal
Level signal in dB
Distance
Composed signal
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7.3 – Omni Base Station This typical Omni Base Station is made up of 3 antennas.
- one transmitting antenna (TX) - two receiving antennas (RX) The transmitting antenna is mounted higher and in the middle in order to guarantee a cleaner omni-directional characteristic. Furthermore the influence of the Rx and Tx antennas on each other is reduced (higher isolation). The two receiving antennas are spaced at 12-20 λ to achieve a diversity gain of 4 -6 d B.
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7.4 – Sectored Base Station Omni base stations are mainly installed in regions with a relatively low number of subscribers. For capacity reasons the communications cell is divided into 3 sectors of 120o in urban areas. Directional antennas, for example panels, are used to cover these sectors. All 3 antennas per sector can be mounted at the same height because directional antennas have higher isoiation in comparison to omnidirectional antennas. 7.5 – Polarization Diversity The reflections which take place within urban areas are not all of the same polarization, i.e. horizontal components also exist. Furthermore a moibile telephone is never held exactly upright which means that all polarizations between vertical and horizontal are possible. It is therefore logical that these signals be also used. Space diversity uses 2 vertically polarized antennas as reception antennas and compares the signal level. Polarization diversity uses 2 orthogonally polarized antennas and compares the resulting signals. 7.6 – Horizontal and Vertical Polarization The dipoles of both antenna systems are horizontally and vertically polarized respectively. A spacial separation is not necessary which means that the differently polarized dipoles can be mounted in a common housing. Sufficient isolation can be achieved even if the dipoles are interlocked into one unit so that the dimensions of a dual-polarized antenna are not greater than that of a normal polarized antenna. 7.7 - Dual Polarization Antennas It is well known that in a cellular network móbile phones have considerably lower transmission power than the transmitters of the base stations. This means that the transmission from the mobile phone to the base station (Uplink) is much more unfavourable in relation to the transmission from the base station to the mobile phone (Downlink). To compensate this, a way of improving the quality of reception at base stations had to be found. Due to the multi-path signal propagation of radio signals, which particularly occurs in urban areas, so called Space Diversity Reception has been introduced which provides goods results in the field. Space Diversity Reception is based on the following Idea: The signal transmitted by a mobile phone is multi-reflected in the propagation field and reaches the base station via different paths. The resulting signal at the base station receiving antenna is the sum of various vectors with different amplitudes, phases and polarizations.
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If two receiving antennas are located at certain horizontal (or vertical) distance then it is highly likely that one of them will provide the required signal strength (principle of uncorrelated signals). A logic unit permanently ensures that the higher signal level of the two receiving antennas is fed into the receiving system. Depending on the individual situation in the propagation área, the use of a Space Diversity Receiving System will produce a divesity gain of 3-5 dB, as compared to using only one single receiving antenna.
fig. 1: Sector antenna system using Space Diversity Reception
3 antennas, 3 feeders The space diversity antenna configuration certainly provides good electrical results, but the number of antennas required is a negative factor with regard to the resulting optical appearance, the increased space requirements and the greater amount of mechanical hardware and feeder cables needed. In view of the great difficulties involved in trying to obtain permission from the authorities concerned and also in getting approval from property owners for the installation of antenna systems, an advanced antenna system with a low-level optical impact had to be found. Extensive investigations and trials have shown that so-called Polarization Diversity Reception is equivalent to or almost equivalent to Space Diversity reception. Polarization Diversity Reception means that the reception levels of two orthogonally polarized antennas are compared and then the stronger signal is led to the receiver.
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The object of development was to design dual-polarized antennas that have the same outer dimensions as single polarized antennas and also equivalent gain figures and radiation patterns. The number of individuall antennas can thereby be considerably reduced, resulting in improved optical appearance, reduced space requirements and less mechanical hardware required.
Fig. 2: Sector antenna system with Polarization Diversity Reception 2 antennas, 3 feeders
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Duplexador
Tx1 Rx1 Rx2
Fig. 3: Sector antenna system with Polarization Diversity Reception using a duplex filter, 1 antenna, 2 feeders, 1 duplexer
Generally it can be Said, and this of course also applies to dual-polarized antennas, that the isolation between neighbouring transmiting antennas, as well as between a transmiting antenna and a receiving antenna, must amount to at least 30 dB in order to avoid: - Interfering intermodulation products; - Blocking of the receivers; - The activation of the transmitter´s VSWR monitoring system by an adjacent transmitter; Initially dual-polarized antennas with horizontal and vertical polarization were preferred since this antenna concept easily provides the required 30 dB isolation figure between the horizontally polarized system and the vertically polarized system. Practical results with horizontal/ vertical dual-polarized antennas are fairly positive. However, there is also a weakness. Since the mobile station antennas (i.e. on cars or mobile phones) mainly operate in a vertically polarized mode, the propagation efficiency is more favourable to the vertical system of a horizontal/ vertical dual-polarized base station antenna than to the horizontal system. Thus horizontal polarization is not really suitable for transmitting purposes.
Duplexer
52
However, with +45º/-45º dual-polarized antennas both systems are equivalent as regards their propagation efficiency. The two systems can therefore also be used with good results for transmitting and receiving purposes. Moreover, this antenna concept allows simultaneous transmission from two transmitter without the use of a transmitter combiner.
Duplexador
Tx1
Duplexador
Tx2 Rx1A + Rx2A Rx1B + Rx2B
fig. 4: X-polarized antenna for 2 transmitting and 2 receiving channels 1 antenna, 2 feeders, 2 duplexers
Radiation patterns and half power beam widths of X-pol antennas Vertically or horizontally polarized antennas show Constant polarization regardless of the azimuth angle at which the antenna is observed. However the polarization of a 45º polarized antenna is not always 45º, it varies with the azimuth angle. This concept is easy to understand if one considers the angles of orientation when a slanted dipole is viewed from different perspectives.
Duplexer Duplexer
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A dipole which is set at a slant of +45º when viewed from the front will appear to be vertically polarized when viewed from the side.
Dipolo
Polarizado c/ - 45o
Polarizado c/ + 45o
PolarizadoVerticalmente
fig. 5: Varying polarization of a dipole set at a slant of 45º. The vector of the radiated electrical field strength is fully descibed by a pair of two orthogonal vector components, in other words, the vector is described by a rectangular coordinate system. According to Fig. 6, any field strength vector can be described either by a coordinate system, defined by the EV vector and EH vector, or with the same preciseness by a coordinate system defined by the E+45
o vector and the E-45o vector.
Vertical
Horizontal
+45o
E+45o
E
EV
E-45o
EH
45o
α
-45o
fig. 6: Polarization: EH=EV=E+45o- 3dB, E-45
o=0 Separation of a vector E into orthogonal components
Vertically polarized
+45º polarized
Dipole
-45º polarized
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This means that the exactness of the 45º polarization of na X-polarized antenna can be tested by measuring the radiation pattern in vertical polarization and in horizonal polarization and in – 45º polarization. If the field strength vector is exactly +45º, the co-polar value is 100% and the cross -polar value is 0%. In other words, if the polarization is almost to 45º, the radiation patterns measured in vertical polarization Ev and in horizontal polarization EH will also be quite close to each other, or referring to the second coordinate system, the cross-polar value E-45
o of the
field strength will be much smaller than the co-polar one E+45o.
If the polarization is exactly 45º, then the half power beam widths of the radiation patterns measured in vertical polarization and in horizontal polarization are all of the same value. 7.8 Kathrein´s Dipole Based Xpol-Antenna Design 7.8.1 General Description Electrical: Xpol antennas consist of two independently working slanted dipole systems, one for +45º polarization and the other for –45º polarization. The dipoles are symmetrically positioned in front of a reflector screen. Both the power distribution and the impedance transformation are carried out by a low loss cable harness. Additional elements for beam-shaping and isolation perfect the design. Mechanical: The radome consist of a completely closed self-supporting fiber-glass profile, into which the metal parts are inserted.There are no drill-holes at all in the profile, which is closed by two end caps with short sealing rings. This concept offers ideal permanent protection against environmental influences and increases the mechanical stability. The improved separation of the electrical and the mechanical function facilitates the optimization of particular performances. 7.8.2 Outstanding Characteristics Symmetrical construction Xpol antennas are available with horizontal half power beam widths of 65º and 90º. Starting from a standard vertical polarized antenna, the required dipole-pair for 65º and the single dipole for 90º are rotated by +45º and –45º, resulting in orthogonal polarizations (see fig. 1)
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While the dipoles of the 90º type form an “X” on which the expression Xpol antenna is based, the basic 65º dipole system is a rhomb. Both designs are fully symmetrical referred to the center line of the reflector screen, which is the basic condition for symmetrical horizontal radiation patterns. 65° Half-power Beam Width 90° Half-power Beam Width -45° +45° -45° +45°
Fig. 1 : General construction of Xpol-antennas Beam - shaping The dipole technology offers a high flexibility in modeling the radiation patterns. Beam width and shape are defined by the dipole position to the reflector and the reflector dimensions. Particular the vertical edges of the reflector screen have a decisive influence on vertically polarized components. So the quality of the resulting pattern is improved regarding sidelobes and gain, and the required number of single elements is minimized. (see item “Low-loss power distribution by cables). In addition, with the separate adjustability of the vertical and the horiz ontal components, the resulting polarizations are controllable. Orthogonal polarizations provide the best polarization diversity gain results, therefore the horizontal radiation patterns for the vertical and the horizontal component are standard measurements for Xpol antennas.
Reflector
Dipole system
Feeding harness
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If the patterns half power beam widths and thereby the gain values resp. the amplitudes are identical, the polarizations are orientated +/- 45º and consequently orthogonal. (fig. 2) a) Equal amplitudes (V=1/H=1) à orthogonal polarizations +45° -45° 90° V V H -H b) Different amplitudes (V=1/H=0.7) à non-orthogonal polarizations 70° +35° V V -35° H -H
Fig. 2 : Vertical (V) and horizontal (H) components and resulting polarizations A perfect polarization orthogonally results in a high cross-polar ratio (CPR), which is determined by measuring the horizontal radiation patterns with the operating polarizations +45º and –45º. The CPR compares the level difference between the similar polarized signals (co-polar) and the dissimilar polarized signals (cross-polar) of the radiated wave. A high CPR stands for a high uncorrelation of the two signals and consequently fo a good polarization diversity performance. The dipole design provides excellent values also apart from the main direction (coverage sector width +/- 60º) and even at +/- 90º. High isolation between the two antenna systems The polarization diversity technology assigns both systems of an Xpol-antenna to work in the Rx and Tx mode simultaneously. Therefore a minimum isolation of 30 dB between the antenna inputs is required. Kathrein´s dipole design guarantees a min. isolation of 32 dB. Measurements of each antenna during the production show a typical value of 35 dB!
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Low-loss Power Distribution by Cables Low-loss flexible semi-rigid coax cables distribute the power to each dipole and take care of the impedance transformation. The diameter of the cables (and the corresponding attenuation) varies with the application, diameters of 0.250”, 0.141” and 0.085” are in operation. This system produces only a minimal attenuation, which will become apparent by comparison it with a printed circuit solution. As a standard the corresponding cross -section of the conductive lines is between the 0.085” and the 0.141” cable. In addition these lines are open and radiate a part of the power, which causes further losses. Another advantage of the cable harness is the flexibility regarding versions with electrical downtilt. The required variation of the phase relations between the radiating elements is carried out easily by changing the length of the cables. It is not necessary to redesign the entire antenna. Low Intermodulation Products Since more than 15 years Kathrein is doing research on the reduction of intermodulation (IM) products. There was already a self-designed measuring device for IM products at 450 MHz with a dynamic range of 160 dB in operation, when such a device was not available on the market. The extremely valuable experiences flowed into the antenna design and determine for example the applied material, the possible material combinations and how a contact between two parts should look like. Kathrein antennas provide a typical 3rd order Im -products attenuation of –150 dBc using two transmitters with an output power of 20 W (43 dBm) each.
Continuance of the Electrical Parameters against Enviromental Influences Antennas are confronted with all the envvironmental influences such as cold and hot temperatures, rain, ice, snow, lightning and high Wind velocities. KATHREIN antennas are well prepared, the mechanical design is based on the environmental conditions to ETS 300 019-1-4. Regarding the deviation of the electrical parameters, especially rain, ice and snow on the radome may cause problem because of their dielectric parameters. Due to the fact that the antenna depths became smaller and smaller, this dielectric load is very close to the radiating elements, working as an additional capacity. Consequently the operational frequency range is shifted, which goes together with the deterioration of electrical parameters like VSWR, isolation and CPR.
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The KATHREIN dipole technology is highly resistant against rain, ice and snow. Dipoles are very slim structures with a small surface and therefore the ocurring additional capacity is relatively low. Due to their larger surface, the capacity influence on patches is much higher. For example, a wet radome can change the isolation of a patch antenna significantly, while a dipole antenna reacts much more good natured. 7.8.3 - Typical Measurements The following antenna parameters have a decisive influence on the network and are important for the judgement of antennas: - Half power beam width for co-polar polarization; - Half power beam width for vertical/ horizontal polarization; - Front-to-back ratio – co-polar; - Front-to-back ratio – total power; - Cross-polar ratio. For a high cross-polar attenuation the half-power beam-widths of the three polarization components co-polar, vertical and horizontal are similar. This feature is perfectlly performed by Kathrein´s Xpol-antennas and consequently there is no need for network planning reasons to differentiate between the above polarization components. These measurements also provide the front-to-back ratio, which is na important feature for the network planning. The front-to-back ratio can be determined as the worst case of either the vertical or the horizontal polarized components. It is only required to calculate the total power, if the two components have similar levels. In case of identical levels, the total power value is 3 dB less compared to the individual components. Xpol dipole antennas provide typical front-to-back ratios of 24-30 dB total power. The following figures show the co-polar and cross -polar as well as the vertical and horizontal polarized patterns of 65º and 90º antennas
Beside the symmetry of the patterns, the scalar printout with a linear scale in dB shows clearly the cross -polar ratio in each azimuth direction. The dipole design provides excellent values also apart from the main direction and even at +/- 90º! Please note as well the high front-to-back ratio for the co-polar and the cross-polar signal.
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Fig. 3 : Typical horizontal co -polar and cross-polar pattern for 65º beam width (measurements)
Fig. 4 : Typical 65º horizontal pattern of vertical and horizontal polarized compent (measurement)
XPol A-Panel 800/900 65° 17dBi horizontal radiation pattern
-40
-35
-30
-25
-20
-15
-10
-5
0
-180 -120 -60 0 60 120 180
azimuth [deg]
rela
tive
gai
n [
dB
]
co-pol
cross-pol
120°-sector
XPol A-Panel 800/900 65° 17dBi horizontal radiation pattern
-40
-35
-30
-25
-20
-15
-10
-5
0
-180 -120 -60 0 60 120 180
azimuth [deg]
rela
tive
gai
n [
dB
]
hor. polarized
vert. polarized
120°-sector
60
Fig. 5 : Typical horizontal co -polar and cross-polar pattern for 90º beam width (measurement)
Fig. 6 : Typical 90º horizontal pattern of vertical and horizontal polarized components (measurement)
XPol A-Panel 800/900 90° 17dBi horizontal radiation pattern
-40
-35
-30
-25
-20
-15
-10
-5
0
-180 -120 -60 0 60 120 180
azimuth [deg]
rela
tive
gai
n [
dB
]
co-pol cross-pol
120°-sector
XPol A-Panel 800/900 90° 17dBi horizontal radiation pattern
-40
-35
-30
-25
-20
-15
-10
-5
0
-180 -120 -60 0 60 120 180
azimuth [deg]
rela
tive
gai
n [
dB
]
hor. polarized vert. polarized
120°-sector
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7.8.4 - CPR against Azimuth As already mentioned, the dipole design provides excellent CPR values not only in main direction but even at +/- 90º. It´s important for the coverage of a standard sector, to rely on high CPR values and consequently on high diversity gains also at the sector edges , where the antenna gain is already considerably reduced.
Fig. 7 : CPR values against azimuth (according patterns fig. 3 and 4)
XPol A-Panel 800/900 65° 17dBi Cross Polar Ratio
0
5
10
15
20
25
30
-90 -60 -30 0 30 60 90
azimuth [deg]
CP
R [
dB
]
120°-sector
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8 SPECIAL APPLICATIONS 8.1 Indoor Coverage System With the public wireless cellular systems and the need of calling and talking from everywhere a real challenge for all operators is to cover large indoor areas such as Shopping, malls, buildings, parking lots, etc. This indoor coverage was made possible by using microcells, repeaters and bi-directional amplifiers which are connected to small discreet antennas displayed in strategic spots to capture and send signals from inside to outside. Typical example of indoor distribution with discreet antennas.
Outdoor antenna
Repeater
Coupler
Indoor antenna Level 3
Level 2
Level 1
738 749
738 573
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8.2 Splitters Low-loss power splitters and tappers are used for combining antennas to obtain particular radiation patterns or to set up indoor distribution networks. - Low-loss coaxial-line transformation
- high power rating
- equal (splitters) or unequal (tappers) power rating
- suitability for indoor and outdoor use
- extremely small dimensions
- multi-band versions for 800-2200 MHz
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8.3 Duplexers Duplexers are filters with ressonant cavities was the principal function to isolate two frequencies or frequency beam widths from each other. To this device the antenna is connected to its output and on the inputs the transmitter and receiver are connected, each to its designated port. This way it is possible to transmit and receive radio frequency signals with only one antenna. Special requirements to choose a best fitting duplexer are: - Maximum input power; - Frequency spacing between Tx and Rx. I.e. for 800 MHz celullar 45 MHz spacing; - Isolation between Tx and Rx. A good duplexer has it better than 70 dB; - Special attention to the intermodulation specifications.
Antenna
Transmitter Receiver
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GLOSSARY
ACI - Adjacent Channel Interference: Interference caused in a channel by its adjacent channel. A / D: Conversion from Analog to Digital. Adjacent Channel: Near channel next or behind. AGC - Automatic Gain Control: A feedback control circuit which maintains the gain or output power level of an amplifier constant over a wide range of input signals levels. Airtime: Time spent on a cellular phone call, which is usually billed to the subscriber on a per minute basis. Amplifier: Device increasing the intensity of a signal. AMPS - Advanced Mobile Phone System: Standard for analog cellular phone systems. Analog: Continuously varying electrical signal in the shape of a wave, transmitted electronically in a form analogous to the spoken word; a form of information which is represented by continuous wave forms which vary as the source varies. Analog Driver: An accessory circuit for an oscillator of filter which permits its frequency to be changed by a continuously varying signal. ANSI - American National Standard Institute (EUA). Attenuation: Decrease of the power signal expressed in dB. The reduction of a signal from one point to another. For an electrical surge, attenuation refers to the reduction of an incoming surge by a limiter (attenuator). Wire resistance, arresters, power conditioners attenuate surges to varying degrees. Attenuator: Component that causes attenuation of a signal. Azimuth: Angular diference measured against the horizon. Band: A certain frequency range of the electromagnetic spectrum. A Band: Frequencies alocated for the public wireless system in USA and South American Countries in 800 MHz. Rx – 824-835 MHz (ERB), Tx – 869-880 MHz B Band: Frequencies alocated for the public wireless system in USA and South American countries in 800 MHz. Rx – 835-849 MHz (ERB), Tx – 880-894 MHz Bandwidth: Range of frequencies a transmission line or channel can carry: the larger the bandwidth, the larger the information - carrying capacity of a channel. For a digital
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channel this is defined in bit/s. For an analog channel it is dependent on the type and method of modulation used to encode the data. 3 dB Bandwidth: The frequency span (in Mhz) between the points on the selectivity curve at which the insertion loss is more than3 dB greater than the minimum insertion loss. Also called 3 dB passband. Base Station: Fixed transmitter/receiver with which a mobile radio transceiver establishes a communication link to gain access to the public -switched telephone network. Baud: Measure of signal changes per second. Often used incorrectly instead of bps (bits per second).. Beam: Main lobe where most of the energy is concentrated. BER - Bit Error Rate. Bidirectional: Network that allows signal traffic simultaneously in two directions. BIT/BITE: Built-in Test/Built-in Test Equipment - Some products have provisions for connection to customer-supplied test or test equipment that is a part of the system in which the products are used. Generally, a military/aerospace term for equipment that contains an automatic self-testing function. Broadband: Technologies communications channels that are capable of carrying a wide range of frequencies. Broadcast television, cable television, microwave and satellite are examples of broadband technologies. These technologies are capable of carrying a great deal of information in a short amount of time, but are more expensive in use than technologies like telephone which require less bandwidth. Cable Loss: Reduction of the signal level/ power through the internal resistance of the cable. Carrier: A high-frequency radio signal which is modulated to carry information at long distances through space or via cable. Carrier signal: The underlying frequency or frequencies that are to carry information. They are modulated through one or more modulation techniques to impose information Cavity: Metallic enclosure to resonate at a desired frequency. Primarily used to describe a cavity filter, which is a highly-selective tuning element that may be used as the frequency-determining element of an oscillator or as a lowpass, bandpass or highpass filter. Generally of fixed frequency or mechanically tunable over a very limited frequency range. CCITT - Consultative Committee for International Telephony and Telegraphy
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CDMA - Code-Division Multiple-Access : A digital technology using a low-power signal "spread" across a wide band-width. With CDMA a phone call is assigned a code instead of a frequency. Using the identifying code and a low power signal, a large number of callers can use the same transmission group of channels. The Telecommunications Industry Association (TIA) has awarded CDMA interim standard approval (IS-95). CDPD - Cellular Digital Packet Data Cell: The geographic area served by a single low-power transmitter/receiver. The area of service of a system is divided into multiple “ cells”. Cellsite: Place/ installation of a Radio -Base-Station. Channel: The width of the spectrum band taken up by a radio signal, usually measured in kilohertz (kHz). Most analog cellular phones use 30-kHz channel. Channel Capacity: The maximum number of channels that can be distributed in a system. Circuit: Interconnection of the number of electrical elements and/or devices, performing desired electrical function. CNR - Carrier-to-Noise Ratio : Value that quantifies the quality of a signal. Coax Cable: Cable to conduct signals in a shielded environment, composed of internal conductor covered by an isolator/ dieletric and outer conductor. Convergence: Technical and market tendency of integrating various different services such as voice, vídeo and data transfer. Conversion Loss: The ratio (in dB) of the intermediate frequency output power of a mixer to the RF input power. All conversion loss measurements and specifications are normally based on the mixer being installed in a system with wideband 50 resistive terminations on all ports and a stated low output signal power level being applied. Cross Modulation Distortion: The amount of modulation impressed on an unmodulated carrier when a signal is simultaneously applied to the radio frequency port of a mixer under specified operating conditions. The tendency of a mixer to produce cross modulation is decreased with an increase in conversion compression point and intercept point. CW - Continuous Wave: Signal of constant amplitude. D / A: Conversion from Digital to Analog. dB (Decibel): A unit of gain equal to ten times the common logarithm of the ratio of two power levels or 20 times the common logarithm of the ratio of two voltage levels. dBc: Decibel related to the signal carrier level
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dBi: Measured in dB reference to an isotropic source ( eg.: antenna ). dBm: Decibels related to 1mW - the standard unit of power level used in microwave work. For example, 0dBm= 1mW, +10 dBm = 10mW, +20dBm=100 mW, etc. dBw: Measured gain or loss in dB of the power of an equipment. DC - D irectional Coupler DCC - Digital Cross Connection Demodulation: Recovery of an original signal transmitted modulated. Digital: Digital signals, including pictures, sounds or computer data, are represented by a code of "on" and "off' signals. Since this system ignores all but these two signals, it is more precise and less susceptible to interference than the analog system. Digital Compression: Compression technique of digital signals where equal and redundant bits are agregated, reducing the bandwidth needed to transmit the information. Digital Driver: Accessory circuit for an oscillator or filter which permits its frequency to be varied by changing a digital "word." A digital driver is also an accessory circuit interfacing a switch or attenuator to a digital command circuit. Digital Modulation: Method of transmitting a human voice using the computer's binary code. Digital transmission offers a cleaner signal than analog technology. Cellular systems providing digital transmission are currently in operation in several locations. Digital Signalling: An electrical signal in which the signal state is discontinuous with time and is characterized by abrupt changes. The conversion of voice or data into a stream of binary information for transmission to a remote location, as opposed to Analog Signalling. Digital Transmission: A code of discrete binary signals (on and off; zero and one, high and low, etc.), as opposed to continuously variable analog type signals. Digital transmission is expressed by numbers of bits per second, or data rate. Downlink: Signal from antenna/ base-station to cellular handset. Downtilt: Inclination of main lobe of antenna. DSP - Digital Signal Processing (or Processors). EAMPS - Expanded Advanced Mobile Phone Service. EIA - Eletronic Industries Association ( USA). EIRP – Effective Isotropic Radiated Power.
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E-TDMA - Enhanced TDMA Access ETSI - European Telecommunications Standards Institute. FCC - Federal Communications Commission. The U.S. government agency responsible for allocation of radio spectrum for communication services. FDM – Frequency D ivision Multiplexing. Feeders: Transmission lines supplying power to a distribution system. Frequency: The rate at which a current alternates on a telecommunications medium, measured in Hertz (Hz).
Frequency Range: Usually presented as the minimum and maximum frequencies between which a particular component will meet all guaranteed specifications. GSM - Global Systems for Mobile Communications (originally called the Groupe Speciale Mobile). Digital cellular standard for Europe; widespread also in Africa and Asia; few systems in the America. Handoff: Cellular systems are designed so that a phone call can be initiated in one cell and continued in other cells. The transfer to the next cell, called a handoff, is designed to be transparent to the cellular phone user. During a cellular conversation, assign, computers in the network assign another tower in the next cell to provide the phone with continuing service. Harmonic Intermodulation Distortion: Ratio (in dB) of distortion to the intermediate frequency output waveform caused by mixer-generated harmonics of the radio frequency and low output input signals. Harmonic Signals: Signals which are coherently related to the output frequency. In general, these signals are integer multiples of the output frequency. Hertz (Hz): Unit of measuring frequency signals (one cycle per second). HF – High Frequency IF - Intermediate Frequency: In superheterodyne receiving systems, the frequency to which all selected signals are converted for additional amplification, filtering and eventual direction. Impedance: Forces which resist current flow in A.C. circuits, i.e. resistance, inductive reactance, capacitive reactance. IN – Intermodulation Noise Insertion Loss: Transmission loss measured in dB at that point in the passband which exhibits the minimum value.
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ISO - International Standards Organization Isolator: Device that permits microwave energy to pass in one direction while providing high isolation to reflected energy in the reverse direction. Used primarily at the input of communications-band microwave amplifiers to provide good reverse isolation and minimize VSWR. Consists of microwave circulator with one port (port 3) terminated in the characteristic impedance. Modulation: Imposing na information signal on a provider signal by changing the signal´s frequency, amplitude and/ or phase. N-AMPS - Narrowband Advanced Mobile Phone Services Output Power: The minimum and/or maximum output power at the output frequency under all specified conditions. Usually the specified conditions are temperature, load, VSWR and supply voltage variations. It is typically expressed in dBm or milliwatts (mW). PCS – Personal Communication System (3rd generation of Cellular Systems). Power Divider: A passive resistive network which equally divides power applied to the input port between any particular number of output ports without substantially affecting the phase relationship or causing distortion. Return Loss: When expressed in dB is the ratio of reflected power to incident power. It is a measure of the amount of reflected power on a transmission line when it is terminated or connected to any passive or active device. Once measured, it can be converted by equation to reflection coefficient which can be converted to VSWR. RF - Radio Frequency: Generally referring to any frequencies at which the radiation of electromagnetic energy is possible. Roaming: Using a cellular phone in another area than the one in which it is subscribed. S / N (Signal-to-Noise Ratio): The ratio of noise to actual total signal, and it shows how much higher the signal level is than the level of noise. It is expressed in decibels (dB) and the bigger the value is, the more crisp and clear the picture and sound will be during playback. SCC - Switching and Control Center. Sensitivity: The normalized change in YIG component's center frequency resulting from a change in tuning coil current, specified in MHz/mA. Signal loss: Weakening (or attenuation) of a signal, measured in decibels. Simplex: Network that transmits in only one direction only. TDMA - Time D ivision Multiple Access: Digital transmission standard based on time division access.
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Trunk: A large-capacity, long-distance channel used by a common carrier to transfer information between its customers. VSWR – Voltage Standing Wave Radio : Relation of the stationary wave in relation to the output power and the reflected power. Wave lenght: Distance of one cicle wave calculated by dividing the propagation speed by its frequency. WLL - Wireless Local Loop: Fixed wireless telephone system.
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