sigma antenna system design

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Contents

1 INTRODUCTION..........................................................................................................................................4

2 FUNDAMENTALS OF ANTENNA DESIGN.............................................................................................5

2.1 GAIN.........................................................................................................................................................62.1.1 Polar Plots.........................................................................................................................................6

2.2 MAST POSITION.....................................................................................................................................72.2.1 Omnidirectional.................................................................................................................................72.2.2 Offset Omnidirectional:.....................................................................................................................72.2.3 Sectoral Arrays..................................................................................................................................82.2.4 Directional Arrays...........................................................................................................................11

2.3 RF DOWNTILT......................................................................................................................................122.4 DIVERSITY............................................................................................................................................13

2.4.1 Diversity Gain Explained................................................................................................................142.4.2 Experimental Results.......................................................................................................................152.4.3 Different Diversity Schemes Described...........................................................................................152.4.4 How does Space Diversity work?....................................................................................................162.4.5 How Does Polarisation Diversity Work?........................................................................................17

3 ANTENNA CHARACTERISTICS FOR OPTIMUM PERFORMANCE.............................................18

3.1 HIGH TRAFFIC DENSITY...............................................................................................................................193.2 MEDIUM / LOW TRAFFIC DENSITY.................................................................................................................20

4 GOOD TETRA ANTENNA SYSTEM DESIGN PRACTICE.................................................................20

4.1 RECEIVER ISOLATION FROM TRANSMITTERS.....................................................................................................204.2 OMNIDIRECTIONAL DIVERSITY APPLICATIONS..................................................................................................21

4.2.1 Introduction.....................................................................................................................................214.2.2 Sample Power Balance Calculation................................................................................................214.2.3 ‘Top of Mast’ Omni plus Two Offsets ............................................................................................224.2.4 ‘Top of Mast’ Omni plus Three Panels ..........................................................................................234.2.5 Side Mount ‘Omnidirectional’ Diversity Array...............................................................................264.2.6 Two Sector Hybrid Sector System...................................................................................................27

5 MOUNTING CRITERIA FOR OPTIMUM PERFORMANCE.............................................................28

5.1 ELECTRICAL................................................................................................................................................285.1.1 Background.....................................................................................................................................285.1.2 Dolphin Measurement.....................................................................................................................295.1.3 Practical measurement....................................................................................................................295.1.4 Summary of results..........................................................................................................................305.1.5 Analysis of results............................................................................................................................315.1.6 Test limitations................................................................................................................................325.1.7 Conclusions.....................................................................................................................................32

5.2 PHYSICAL MOUNTING CRITERIA.....................................................................................................................335.2.1 Sectored Panel array.......................................................................................................................33

6 ANTENNA / SYSTEM INTEGRATION...................................................................................................34

6.1 LOW DENSITY SYSTEM.................................................................................................................................346.2 MEDIUM DENSITY.......................................................................................................................................356.3 HIGH DENSITY............................................................................................................................................36

Sigma Wireless Technologies 2 November 2000.

Charts

CHART 1 EFFECT OF GAIN.........................................................................................................................6

CHART 2 OMNI ANTENNA ON ONE METRE MAST AT VARIOUS SPACINGS...............................7

CHART 3 OFFSET ANTENNA ON ONE METRE MAST AT VARIOUS SPACINGS..........................8

CHART 4 BUILD UP OF SECTORED SITE COVERAGE........................................................................9

CHART 5 ILLUSTRATION OF INTERFERENCE IN RE-USING FREQUENCIES...........................10

CHART 6 - AN ELECTRICALLY DOWN-TILTED PANEL ANTENNA MECHANICALLY UP-

TILTED............................................................................................................................................................11

CHART 7 ILLUSTRATION OF THE USE OF A DIRECTIONAL ARRAY..........................................12

CHART 8 ILLUSTRATION OF DIFFERENCE BETWEEN ELECTRICAL AND MECHANICAL

DOWN-TILT....................................................................................................................................................13

CHART 9 VARIABILITY OF THE SIGNAL STRENGTH COMING FROM A MOBILE

TRANSMITTER OVER TIME.....................................................................................................................13

CHART 10 ILLUSTRATION OF DUAL POLARISATION DIVERSITY..............................................16

CHART 11 PROCESS FOR SELECTING OPTIMUM ANTENNA SYSTEM.......................................19

CHART 12 'TOP OF MAST' OMNI PLUS TWO OFFSETS....................................................................23

CHART 13 'TOP OF MAST' OMNI PLUS THREE PANELS..................................................................25

CHART 14 EXTENSION POLE FOR OMNI PLUS 3 PANELS...............................................................25

CHART 15 SIDE MOUNT ‘OMNIDIRECTIONAL’ DIVERSITY ARRAY...........................................26

CHART 16 - SIDE MOUNT ‘OMNIDIRECTIONAL’ DIVERSITY ARRAY GAIN.............................26

CHART 17 TWO-SECTOR HYBRID SECTOR SYSTEM.......................................................................27

CHART 18 - TWO SECTOR HYBRID SECTOR SYSTEM GAIN..........................................................28

CHART 19 - MEASUREMENT CONFIGURATION FOR HORIZONTAL SEPARATION...............29

CHART 20- MEASUREMENT CONFIGURATION FOR VERTICAL SEPARATION.......................30


1 Introduction

The design criteria used in the traditional PMR environment differ significantly from those required

by Digital PMR. The difference between the two platforms demand that a higher priority be given

to the shape and control of the antenna’s radiation pattern.

In the past, system designers attempted to maximise coverage from each site while balancing the

return path for the expected type of mobile terminal. Frequency re-use was less of an issue and

overlap between sites was managed by the ‘capture effect’ of standard FM receivers. Management

of calls was less sophisticated as dropped calls were more acceptable in a dispatcher oriented PMR

system.

However, because of the influences of Cellular technology expectations from users have been

raised and TETRA must adopt new RF planning principles. Coverage from new systems needs to

be balanced for in-door portable use and call management needs to be managed by the

infrastructure software.

The following summarises the changes to antenna and network design:

1. The traditional PMR antenna range has been expanded to include new panel antennas products

to allow sectorisation. Techniques enabling polarisation diversity to improve receiver gain in

multi-path environments must also be considered.

2. TETRA antennas need to have optimum electrical performance in cellular dimensions. The

influence of cellular network planning is strong, imposing electrical and dimensional

expectations on all new products developed for this application.

3. Call handling system software is used to manage channel changeover in overlap areas.

4. Receiver diversity is used to enlarge cells as much as possible, while maintaining the balance

between fixed and mobile devices.

Underpinning all of these changes from standard PMR antenna designs is the need to precisely

control the radiation pattern, in terms of envelope shape and electrical tilt.

Antenna systems, when properly designed, will yield reduced costs to the network operator and

clear communication to network users. The following document is divided into five sections aimed

at describing the fundamentals of antenna design at TETRA frequencies, highlighting the key


trade-off’s when designing a TETRA antenna system and finally to pass over some tips on good

antenna system design. This document will not focus on hand portables or mobile transceivers.

2 Fundamentals of Antenna design

The main challenges of antenna design are concerned with defining and controlling the shape of the

radiation pattern. The ability to do this well ensures that RF signals are directed into the right area

and at the appropriate strength level. It also ensures the minimisation of unwanted signals in key

areas. The TETRA standard requires that a 17dB differential be maintained between carrier and

the interference level. In a frequency re-use scenario, it is important to be able to plan the network

using realistic antenna patterns, which may be used to:

• Amplify signals for range or building penetration purposes.

• Direct radiation in a controlled manner, omni, sectored, directional.

• Reduce/enlarge coverage using gain and diversity.

• Optimise performance in a range of mast fixing arrangements.

The key factors affecting the shape of the RF envelope are as follows:

Omnidirectional: Gain, mast position and down-tilt.

Sectoral: Gain, beamwidth, front-to-back ratio and down-tilt.

Directional: Gain, beamwidth.

The antenna patterns used to characterise an antenna are the E-Plane and H-Plane. The E-plane is a

cross-section of the antenna pattern and is a ‘side on’ view. The main information given is the

depth of the main beam plus any side lobes produced. These side lobes may be a source of

interference to other sites and need to be controlled. Panel antennas invariably have unwanted

lobes at the rear of the panel. These need to be minimised and controlled. Failure to control this

may result in interference to some other site. The H-Plane is the top down view of the radiation

pattern and defines the direction of the pattern in relation to the antenna. Omnidirectional antennas

have a circular pattern, while panel and directional antennas tend to focus the RF energy in a

particular manner. The focussing of this energy results in gain.


2.1 GAIN

An Omnidirectional antenna is used in circumstances where frequency re-use is not an essential

issue due to medium to low traffic density requirements. This type of antenna is constructed using

a spiral dipole array or a collinear design and generates a circular pattern when viewed from above.

The following diagram is an E-Plane view of an omnidirectional antenna and shows the effect of

gain on the shape of the main lobes. When the gain level is increased from 3dB gain, using two

dipoles, to 6dB, using four dipoles, the distance covered is increased and the lobes become thinner.

The Chart 1 shows the effect of gain at the TETRA frequency band.

0 -3 -6 -10

-15-20

-30

dB

0

90

180

270

Chart 1 Effect of Gain

2.1.1 Polar Plots

Sigma uses log-dB polar plots to display their antenna patterns. The ARRL (the American Radio

Relay League, the US national organisation of Amateur Radio operators) log-dB scale is widely

used in amateur publications. It provides a convenient scale to compare the patterns of antennas

with those of existing designs. It also yields patterns with familiar shapes.

The ARRL log-dB scale dedicates approximately half of the area of the plot to the first 10dB. This

emphasises the detail of the pattern near the full-gain point and causes the lower level side-lobes to

be compressed toward the centre of the pattern without hiding them completely. The log-dB plot is

normalised so that the outer 0dB circle represents the maximum gain of the antenna in that plane.

The centre of the plot is minus infinity dB, but there isn't much area below -40 dB.


2.2 MAST POSITION

2.2.1 Omnidirectional

Mast positioning can affect the radiation pattern and care needs to be taken to ensure that the

appropriate antenna type is selected for specific masts and position on that mast. The main impact

is on the H-Plane pattern (also known as the Azimuth Pattern).

Chart 2 shows the ideal H-plane pattern represented by the circle in black. This scenario is realised

when the antenna is placed at the top of a mast, free from close obstructions. The radiation pattern

is allowed to develop its true radiation envelope. The other patterns are the result of placing the

same antenna at varying electrical distances from a one-metre mast. As you can see from the

radiation patterns, described by the coloured lines, when an omnidirectional antenna is placed at the

front of a triangular mast the pattern is distorted as the signal is reflected from the tower in an

irregular manner. As the distance increases, this effect is reduced.

The main problem is that a rigger will often place the antenna in the most convenient position

available and not necessarily the best position for optimum electrical performance.

O m n i S t a c k e d D i p o l e A r r a y

0 - 3 - 6 - 1 0

- 1 5- 2 0

- 3 0

d B

0

9 0

1 8 0

2 7 0

Chart 2 Omni Antenna on One Metre Mast at various Spacings

2.2.2 Offset Omnidirectional:

On the other hand, a different result is achieved for an ‘Offset antenna’ placed at the front of a

triangular mast. When the antenna is placed in front of the apex of the mast with the dipoles

arranged in the offset configuration the H-plane radiation pattern is less susceptible to the effects of

positioning at different distances from the mast. The H-plane is reasonably circular, but is offset


towards the front of the antenna. The chart shows how the pattern remains relatively consistent

even when mounted on the side of a mast, at varying electrical distances. As the distance increases,

this effect is reduced. The consistency of RF patterns makes network planning more reliable. A

positive side effect of the offset pattern is higher gain in one direction. This offset shape can easily

be incorporated into overall network planning by choosing sites with this in mind and directing the

main lobe in an appropriate direction to form a suitable total coverage pattern.

A separate paper is available during the third quarter of 2000 which looks at the effect of towers on

antenna radiation patterns and the use of multiple antennas to achieve omni pattern off a tower.

O f f s e t S t a c k e d D i p o l e A r r a y . N o D o w n t i l t .

0 - 3 - 6 - 1 0

- 1 5- 2 0

- 3 0

d B

0

9 0

1 8 0

2 7 0

Chart 3 Offset Antenna on One Metre Mast at various Spacings

2.2.3 Sectoral Arrays

Drawing from the cellular experience, capacity is increased by having sectored sites with three

panel antennas per site, each panel radiating on different RF channels.


- 3 d B

- 1 0 d B

- 3 d B

- 1 0 d B

Chart 4 Build up Of Sectored Site Coverage

Chart 4 shows how a sectored site is built up to optimise channel capacity. These sites are typically

used in urban environments where traffic density is higher than in rural areas. As vehicles or

people move from coverage of one antenna to another, the system takes care of the hand-over to the

appropriate available channel. The interleaving of site patterns ensures hand-over between sites

without causing interference between them.

2.2.3.1 Frequency Re-Use

As the number of sectors increases an antenna’s front-to-back ratio becomes important. The

spillage from the back of the antenna can interfere with cells some distance away, particularly if the

back lobes are directed at the horizon. Good design ensures that the back lobes are small and tilted

down. In Chart 5, if you imagine the two cells being separated by the cell reuse distance, you can

see that the energy radiated off the back of the bottom right blue cell could interfere with a mobile

in the coverage area of the top left blue cell. This is how front-to-back ratio affects the frequency

re-use of a system.

The TETRA standard sets limits on the maximum distance a mobile can access a site from (See

reference i, which states "This distance may be used to prevent MS from grossly exceeding the

planned cell boundaries"). However, this does not affect the levels of interference created by back

lobes, which still must be taken into consideration.


- 3 d B

- 1 0 d B

- 3 d B

- 1 0 d B

Chart 5 Illustration of Interference in Re-Using Frequencies

The frequency re-use policy dictates whether this is a major design concern or not. Networks that

have low frequency re-use, for example those which have a large separation between sites, will be

less concerned about RF spillage from the back of the panel antenna.

A method for controlling the back lobe is to use electrical down-tilt (See RF DOWNTILT on Page

12) with mechanical up-tilt (See Chart 6 on page 11 to see the effects of this arrangement on the

back lobe at the horizon).

One possible option for an antenna user is to purchase all sectored antennas with the maximum

amount of down tilt (15o). If another down-tilt value is required at a particular site, then all that

needs be done is to reverse the mounting brackets (bottom clamp at the top and vice versa) and the

antenna is tilted UP. So that for a 10o down tilt, we would tilt a 15o antenna up by 5o. Thus the

antennas to be delivered to all sites would be the same and the down tilt is decided at the

installation time and can easily be changed subsequently.

The benefits for this type of arrangement is that the purchaser will have all the logistical advantages

of having only one sectored antenna type with maximum front-to-back ratio.


Chart 6 - An Electrically Down-Tilted Panel Antenna Mechanically Up-Tilted

2.2.3.2 Combining Radiation Patterns from Multiple Antennas

It is not normally practical to combine the radiation patterns of multiple antennas to give an omni

pattern from, for example, three panel antennas. This is because there is a single source of signal

(the transmitter) and the relative phases of the radiated signals from each antenna will determine

the final radiation pattern of the antenna system. Both the relative lengths of the feeder cables and

the distance between the antenna centres (i.e. where each antenna is mounted) control these phases.

This is in contrast to diversity (See DIVERSITY on Page 13) which is used for the receive path

only and uses up to three separate receivers to achieve the gain. In this case, the phases of the RF

signals do not matter, as they are processed in the receivers before the gain is achieved by

aggregating the demodulated outputs using an additive or selective process.

A separate paper is available during the third quarter of 2000 which looks at the effect of towers on

antenna radiation patterns and the use of multiple antennas to achieve omni pattern off a tower.

2.2.4 Directional Arrays

Directional antenna patterns are used to establish point to point communication or up/down traffic

corridors. The beamwidth is much narrower than panel antennas and consequently yield a higher

gain. Yagi antennas or two sectored panel arrays are used for this purpose.

The pattern shown below in Chart 7 shows the resulting pattern using two Yagis (one pointing

“East” and the other pointing “West”). As the front to back ratio on these antenna types is high,

there is little interference between the back of one pattern and the front of the other.


The second illustration is a possible mounting arrangement for these types of antennas. Note that it

shows vertical separation for diversity, this will require 10 meters (See section 2.4).

0 - 3 - 6 - 1 0

- 1 5- 2 0

- 3 0

d B

0

9 0

1 8 0

2 7 0

Chart 7 Illustration of the Use of a Directional Array.

2.3 RF DOWNTILT

The effect of down-tilt applies to both Sectored and Non-Sectored antenna arrays. Omnidirectional

and sectoral antennas use pattern tilting to regulate the size of cells and control the signal strength

in overlap areas. The tilt may be provided using either mechanical or electrical tilt and in some

cases a combination of both. Chart 8 shows the difference between mechanical and electrical down-

tilt. The blue pattern is a cross section of the radiation pattern for our SPA Series panel antenna,

with no electrical down-tilt, but which has been mechanically tilted down. The red pattern shows

the same antenna type but with 15 degrees of electrical down-tilt. As you can see the electrically

down-tilted pattern has two larger secondary lobes, but more importantly, the back lobe is also

down-tilted. This provides a powerful and positive means of preventing unwanted spillover into

cells some distance away. (See also 2.2.3.1 Frequency Re-Use above and Chart 6 - An Electrically

Down-Tilted Panel Antenna Mechanically Up-Tilted above)


Chart 8 Illustration Of difference between Electrical and Mechanical Down-Tilt

2.4 DIVERSITY

Chart 9 illustrates the variability of the strength of a received signal coming from a mobile

transmitter over time, in to both polarisations of a dual slant polarised antenna. Signals usually

arrive at the receiver via multiple paths (see below). This receiver diversity can be used to enhance

systems performance. This is particularly useful when the system requires talkback from low

powered handheld devices. This technique ensures that the network receives the same signal at

least twice (dual receiver mode) which is then manipulated either by an additive or a selective

process to ensure a better net received signal to noise ratio.

Signal Strength In Dual Polar Antenna with Distance Travelled

-120

-100

-80

-60

-40

-20

0

Time Travelling

Sig

na

l S

tre

ng

th

Left Polar

Right Polar

Chart 9 Variability of the Signal Strength coming from a mobile transmitter over time


Quoting from reference ii may help to understand the complexities of propagation in the mobile

radio environment: - "Radio wave propagation in the mobile radio environment is described by

dispersive multi-path caused by reflection, diffraction and scattering. Different paths may exist

between a BS and a MS due to large distant reflectors and/or scatterers and due to scattering in the

vicinity of the mobile, giving rise to a number of partial waves arriving with different amplitudes

and delays. Since the mobile will be moving, a Doppler shift is associated with each partial wave,

depending on the mobile's velocity and the angle of incidence. The delayed and Doppler shifted

partial waves interfere at the receiver causing frequency and time selective fading on the

transmitted signal."

The available antenna diversity options are: - Space -Vertical.

Space - Horizontal

Polarisation (Usually dual polarisation)

The principle is the same for each, in that the receiving base station has a choice of two signals on

the incoming path. The process on average yields a ‘gain’ on the receive path.

A separate paper which looks at diversity gain versus antenna spacing is available during the

second quarter of 2000.

2.4.1 Diversity Gain Explained

Diversity gain only operates on the up-link (Mobile Station to Base Station). It is required because

portables usually have one watt transmit power towards the base, but bases can be up to 40-Watts

back to the mobile. The measurement test involves a mobile and a base station with special test

software in it.

A typical test route is driven; the mean bit-error rate is measured at the base, using a vertically

polarised antenna of equivalent gain to the antenna under test. The route is then driven again, using

either two vertically polarised antennas spaced apart, or the two halves of a cross polarised antenna

(as two separate tests) each being fed into separate receivers. The mobile transmit power is

reduced in steps until the same bit error rate is achieved at the base as was measured in the

reference drive. The amount by which the power is reduced is the equivalent Diversity Gain of the

base antenna configuration chosen for the test.


2.4.2 Experimental Results

During Sigma’s initial TETRA antenna development work, tests were performed to determine if

diversity gain existed in the 400MHz band. From these experimental results, we know that the

diversity gain of a cross-polarised antenna in a suburban environment is about 4dB. The diversity

gain of two vertically polarised antennas horizontally spaced at 5.5 metres is about 4.5dB. We

also know that in a high-density urban environment the gains are increased by a further 1dB. In

open countryside, there is some small gain improvement (over a single antenna) for both

configurations. For three antenna diversity, it is possible to assume that there is at least a 1.5dB

improvement over two antenna diversity. Thus, the diversity gain of a particular antenna

configuration will also depend on the type of environment in which it is being used.

Most cellular operators have, over the years, done experiments to assess the gain obtained with

different diversity schemes, and some have published the results. The findings are generally

similar, but never identical. One set of results is given here iii.

Area Type Estimated Diversity Gain with 45

Slanted Antenna

Estimated Diversity Gain with Space

Diversity

Urban, Indoor 3.7 dB 5.0 dB

Urban, Outdoor 4.7 dB 3.3 dB

Suburban, Indoor 4.0 dB 3.7 dB

Suburban, Outdoor 5.7 dB 4.7 dB

Rural 2.7 dB 5.3 dB

Table 1 - Diversity Gain as a Function of Operating Environment

2.4.3 Different Diversity Schemes Described

• Horizontal space diversity requires that approximately 5.5 meters should horizontally

separate two antennas. Reducing this space reduces the gain and the final gain obtained

depends on the antenna height above surrounding terrain as well as the spacing between the

antennas. This is the optimum situation electrically, but in reality access to the required space

is limited. The greater the antenna separation, the less likely that fades will occur in both

antennas simultaneously. If optimum diversity techniques are used in the base station, expect a

minimum of 3dB diversity gain for two antennas and 4.7dB gain from three antennas.

• Vertical space diversity can be easier to implement, but again the requirement is for

approximately 10 metres vertical separation between two antennas to give the best


improvement over a single antenna (similar to that given by horizontal spacing). Most of the

diversity advantage is lost at 4 metres. One reason for this failure is that the coverage area of

the two antenna systems is very different at this spacing. This will cause many problems trying

to balance the signal quality received at the base with that received at the mobile / portable.

Another disadvantage of this type of diversity is that the two received signal are not the same

strength at the antenna, causing a reduction in diversity gain.

• Dual-polar diversity is achieved using a single antenna structure with two sets of dipoles

positioned at +/- 45 degrees to each other. The dipoles positioned in this way produce of

typically of 3 to 4.5dB better than a single vertically polarised antenna of similar dimensions.

The gain of these antennas is usually specified as Co-Polar gain i.e. the gain measured at +/-45

Degrees. The 2 to 4dB gain improvement is relative to this gain. If, however, you measure the

antenna gain vertically polarised, it will be 3dB less than that measured at +/-45o. The vertical

space occupied by a dual polarised antenna of a given Co-Polar gain is the same as for a

vertically polarised antenna of the same gain.

RedFeed

BlueFeed

Chart 10 Illustration of Dual Polarisation Diversity

2.4.4 How does Space Diversity work?

To achieve diversity at least two receivers are required. These will receive signals from diverse

sources - two antennas. These antennas will provide a separate signal to each receiver, this signal

comes from the same original source - the portable / mobile (called a mobile in the following

discussion) but via different paths. These antennas will need to be positioned on a mast in a

suitable position to allow them to appear as two separate diverse sources of the same signal. The


greater the distance between the antennas horizontally, the less likely that a signal fade (received

from a moving mobile) from one antenna will occur at the same time as a signal fade from the other

antenna. Thus, the diversity gain (reducing the effect of these fades) increases as the separation

increases and relies on the concept that the strength of the two signals should be nearly equal on

average. On average, if the two signal strengths are not equal, then the full diversity gain cannot be

achieved.

At 900 MHz, antennas are generally regarded as being at optimum separation at 2.75 metres. At

400 MHz, this optimum is generally regarded as occurring at five and a half Metres, which is often

difficult to achieve in practical situations. The required separation is in fact a function of effective

antenna height. A separate paper which looks at diversity gain versus antenna spacing is available

during the second quarter of 2000.

The correlation coefficient between the amplitude envelope of the received signals depends on the

antenna spacing. To give an adequately low coefficient (0.7), the antennas should be at the same

height and spaced at least 5.5 metres apart. In other words, the long-term correlation between the

amplitude of the received signals should be high, but the instantaneous value of the correlation

should be very low. (The lowest short term correlation coefficient achievable with two antennas is

approximately 0.5, which is adequate to achieve expected diversity gain. The lower the short-term

correlation coefficient, the better the diversity gain). If these criteria are met by the antenna

system, (and the receivers receive equal amplitude signals on average in the long term), the gain

achieved by two receivers over one can be up to 5dB and by three receivers is up to 7dB, dpending

on the surrounding propagation environment.

2.4.5 How Does Polarisation Diversity Work?

As a RF signal travels from a moving mobile towards the base antenna, it will follow multiple

paths. The obvious one is directly from the mobile antenna to the base antenna. However this path

is often obstructed, a more indirect path may give a better signal. There will be many paths and

each will be due to reflections. These reflections will change the polarisation of the signal. The

amount by which the signal's polarisation is changed depends on the angle of incidence at the

reflection point. In most instances the signal will be partly reflected and partly refracted at the

surface of the 'reflecting' material. There will be multiple signals propagating from the mobile to

the base and, as it moves, the points at which the signals are reflected will be constantly changing.

Thus, the polarisation and strength of the incident signals at a single base antenna will be constantly

changing in a similar manner.


If two receiving base antennas occupy the same space, but receive signals at different polarisations

(+/- 45o), the RF signals coming from these antennas will be diverse or different (see Chart 9 on

page 13) as the signals incident on the two base antennas from the mobile are of different

polarisations. One antenna characteristic of importance in this regard is Cross-Polar

Discrimination, which quantifies the ability of the antenna to discriminate between the polarisations

of the signals impinging on them. If this is at 15dB or better the correlation coefficient is at 0.58

and it falls rapidly below this value. Thus an antenna with the ability to discriminate between

opposite polarisations at better than 15dB across the field-of-view of the antenna, will have an

adequately low correlation coefficient to achieve the 3 to 5dB diversity gain improvement.

3 Antenna Characteristics for Optimum Performance

The role of an antenna system is driven by the need to balance the conflicting requirements of

electrical performance (gain, pattern, tilt), physical dimensions (restricted space available on masts)

and product costs.

• Having wide coverage from sites reduces network costs, but also reduces capacity. Traffic

density will steer you either to Omni, Offset Omni or sectored panels.

• Reduced Physical dimensions make it easy to get lower cost mast space, but reduces electrical

performance. Gain, Front to Back ratio and bandwidth are affected by the dimensions

• Sector planning will influence the choice of tilt. The cell size and frequency re-use plan will

dictate the level and type of tilt required (mechanical or electrical). In addition, the ‘pattern’

control ensures predictable coverage in a range of environments.

Chart 11 below shows the process for selecting the optimum antenna system depending on the

application:


Sectored

• Gain• Receiver Diversity• Front to Back Ratio• Bandwidth

Omni

• Gain• Diversity arrays• Bandwidth

Low powerdevices

Mast space/costs

Yagi

Traffic Density

Coverage

Chart 11 Process for Selecting Optimum Antenna System

3.1 High Traffic Density

Dense traffic systems typically will be required in urban environments where simultaneous

communication is the important issue. The use of multi-frequency sector arrays ensures more

capacity in the area covered by the antenna array.

Taking the sectored approach will require an array of antennas at each site and as mast space is at a

premium, care needs to be taken to ensure that the panel antennas have the smallest dimensions

possible while delivering good electrical performance. Key parameters at risk as you try to reduce

the overall dimensions include, gain (length), front to back ratio (width), bandwidth (depth) and

horizontal beamwidth.

On the inbound side, receiver diversity is used to balance the system. Cell sizes can be maximised

using dual polarised antennas or arrays of space diversity antennas. Space diversity may be used

with Omni as well as panel antenna arrays and offers the maximum electrical performance possible.

However, from a practical point of view the current practice is to use dual polar panel antennas in

high traffic density sites and use arrays where space is more readily available.


3.2 Medium / Low Traffic Density

Omnidirectional antennas can be set up to work in an omni or an offset mode. The choice depends

on the type of mast on which it is placed. If a ‘top of mast’ position is available, the result is

optimum. The main issue is the gain of the antenna and where the inbound signals are low then

either the outbound power needs to be restricted (small cells) or receiver arrays need to be used in

conjunction with a separate Tx antenna.

4 Good Tetra Antenna System Design Practice

Good design practice ensures that the antenna system is compatible with the infrastructure and

offers additional benefits through delivery of: -

1. Lower Costs

2. Higher redundancy

3. Optimum channel usage (capacity)

These benefits can be derived using a combination of good mounting practices and infrastructure

enhancement. The following outlines some possible antenna configurations, along with possible

explanations for choice of configuration. These are only examples and the final choices taken will

be determined by the system designer. The diversity gain shown in the examples is 3.5 for

illustration purposes only. It is up to the system designer to avail of the currently available

information on diversity and the radio environment to decide on the value to apply in a particular

circumstance. The examples are given primarily to stimulate thought and not to be final solutions.

4.1 Receiver Isolation from Transmitters

In section 6.5.1 of Reference i, the level of blocking for a base station is -25dBm. With a

transmitter level of +47 dBm, the isolation between two antennas with transmitter into one antenna

and receiver into the other will need to exceed 72dB. This does not take into account any additional

filtering, or the fact that some manufacturer's equipment will exceed minimum TETRA

requirements. In fact many system designers use a band-pass duplexer on the transmit / receive

antenna and use a separate band-pass filter for each receiver. (Some even use half of the duplexer

for this purpose, as this reduces the number of different filter types on and individual site). In all

antenna configurations given below this requirement for isolation must be taken into account as

must any additional insertion losses in the receiver and transmitter paths.


4.2 Omnidirectional Diversity Applications

4.2.1 Introduction

GSM networks give priority to site capacity. Thus, the use of GSM sectored sites using panel

antennas is widespread and has led to the use of polarisation diversity, in preference to space

diversity.

In contrast to GSM, TETRA networks will place a lower emphasis on capacity and will seek to

maximise cell size (and minimise the use of frequencies) using omnidirectional antenna arrays.

Most of the work in the area of diversity was originally done for GSM frequencies, where the

horizontal diversity spacing is only 3m.

The examples given below are presented to show that there are many different ways to achieve

omni coverage from towers. They will give the designer some idea of how to go about coming up

with the design that is optimum for his own network.

A separate paper which looks at diversity gain versus antenna spacing is available during the

second quarter of 2000.

4.2.2 Sample Power Balance Calculation

Table 2 below shows an example non diversity power balance calculation for a system with a

single antenna of 5dB gain connected to a base station with a duplexer of 1dB insertion loss and a

combiner and filtering with a loss of 4dB. There is no diversity gain. The figures are only

representative and should only be used as a guide. It assumes a 3-Watt portable. The base station

transmit power is adjusted to balance the outbound path with the inbound path. This adjustment of

the base station power applies in all examples in this section.

In the table below BS is the Base Station and MS is the Mobile Station.


BS -> MS MS->BSTx Power 42 dBm 35 dBmCombiner/Filter Losses -5 dB -1 dBMobile Antenna Gain 1 dBi 1 dBiBase Antenna Gain 5 dB 5 dB BaseCable Losses -2 dB -2 dBDiversity Gain 0 dB 0 dBRx Sensitivity -112 dBm -115 dBmResultant System Gain 153 dB 153 dB

Tx

Combiner / Filter

Duplexer

Rx

Main Antenna

Table 2- System Gain in Reference Example

4.2.3 ‘Top of Mast’ Omni plus Two Offsets

This method uses a single omni at the top of the mast and two offset four stack arrays positioned so

that their tops are positioned below the omni and are mounted about three metres off each side of

the mast. Table 2 shows the resultant increase in system gain. Setting the Transmitter Power in the

Base Station (BS) four and a half dB higher than in the reference example (section 4.1)

counterbalances this extra gain and the reduced loss.

This configuration will suit triangular masts of up to around three metres. Above this size, the

pattern of the offset antennas will become increasingly distorted. The exact amount of distortion

depends on how far away from the tower the antennas are mounted and the exact nature of the

tower's construction. (See MAST POSITION on Page 7).

BS -> MS MS->BSTx Power 45.5 dBm 35 dBmCombiner/Filter Losses -4 dB 0 dBMobile Antenna Gain 1 dBi 1 dBiBase Antenna Gain 5 dB 5 dBBase Cable Losses -2 dB -2 dBDiversity Gain 0 dB 3.5 dBRx Sensitivity -112 dBm -115 dBmResultant System Gain 157.5 dB 157.5 dB

Tx Rx1 Rx2

Main Antenna Other Two Antennas

Table 3 - System Gain for Two Offsets plus an omni.

4.2.3.1 Diversity Gain in this configuration

The pattern of the omni antenna at the top of the mast is shown in black in the drawing below. This

is 3.5dB less gain than peak gain of the offset antennas. At the 90 and the 270-degree areas, of the


pattern shown below, the gain is 3.5 dB higher than that of the Transmit antenna. As we sweep

around towards the 0o and the 180o positions, we see two antennas with nearly equal gains.

The offset antennas are spaced at 5.5 metres from each other and would give a diversity gain of

3.5dB, in the direction where their gains are equal (at the 0o and the 180o positions). As we rotate

away from this the gains of the two antennas become markedly different, so the diversity gain will

be reduced in these directions but the gain of individual antennas becomes closer to 8.5dB. Thus,

overall we will get an apparent improvement of about 3.5dB over a single 5dB antenna mounted on

the top of the mast.

In such a configuration, consideration should be given to providing sufficient isolation between the

transmit antenna at the top of the tower and the receive antennas on the side of the tower. (See 4.1

Receiver Isolation from Transmitters on page 20)

0 - 3 - 6 - 1 0

- 1 5- 2 0

d B

0

9 0

1 8 0

2 7 0

Chart 12 'Top of Mast' Omni Plus Two Offsets.

4.2.4 ‘Top of Mast’ Omni plus Three Panels

This method uses a single omni at the top of the mast and three panel antennas set at 1200 to each

other around the mast. The transmitter feeds the omni at the top and the three receivers operate

from the three panel antennas around the mast. Setting the Transmitter Power in the Base Station

(BS) 3.1dB higher than in the reference example above counterbalances this extra gain. This


arrangement best suits triangular masts over two to three metres per side. It can be used on any

size tower, as the system works better as the distance between the panels increases above 5.5

metres.

BS -> MS MS->BSTx Power 45 dBm 35 dBm

Combiner/Filter Losses -4 dB 0 dBAntenna Gain 5 dB 5 dBDiversity Gain 0 dB 3 dBRx Sensitivity -112 dBm -115 dBm

Resultant System Gain 158 dB 158 dB

Tx

Combiner / Filter

Rx1 Rx2 Rx3

Transmit Antenna

Three Panel Antennas

Table 4- System Gain for Omni plus Three Panels.


The pattern of the omni antenna at the top of the mast is shown in black in the drawing below. At

the zero point, the first panel antenna (red pattern) has a gain that is 3.1dB higher than the omni.

As we sweep clockwise around towards the 60-degree point, the gain of this antenna drops by

about 7 dB. If the panels are spaced at centres of 5.5 metres or more, the resultant diversity gain is

about 3 to 4dB (counting both the red and the blue pattern). This leaves the net resultant gain in

this direction about 3 to 4dB down on the peak of the red pattern (-7dB + 3 and -7dB + 4). In a

similar manner, the net gain will also be reduced at 1800 and 3000 for the other coloured patterns.

Thus, overall we will get an apparent improvement of about 3.1dB over a single 5dB antenna

mounted on the top of the mast, with a reduction of approximately 3dB in net gain at the overlap of

the patterns.


transmit antenna at the top of the tower and the receive antennas on the side of the tower. (See 4.1

Receiver Isolation from Transmitters on page 20).


0 -3 -6 -10

-15-20

dB

0

90

180

270

Chart 13 'Top of Mast' Omni plus Three Panels

If we assume that the panels are mounted at the end of an extension pole on each side of a

triangular mast, Table 5 below gives an indication

of the length of the extension poles required to give

a total of 6 metres between the centres of the

panels. Note that this is end to end of the extension

poles and takes into account the fact that the

electrical centres of these antennas are about

100mm forward of the backplane and the brackets

mount the antenna about 200mm further away from

the vertical mounting pole. Thus, the extension

poles net length could be reduced by up to 400mm,

without affecting the spacing too adversely.


Chart 14 Extension Pole for Omni Plus 3

Panels

Tower Side Size (Metres)

Pole Extension length (Metres)

1 32 2.43 1.94 1.35 0.656 0

Table 5 - Extension Pole Length Vs Tower Size

4.2.5 Side Mount ‘Omnidirectional’ Diversity Array

This antenna array comprises a pair of four stack antennas, one of which is used for Tx while both

are used for receiving to achieve space diversity gain. The antenna system functions identically to

and has the advantages of the ‘Offset omni’ (See page 7).

The antenna configuration has the radiation pattern described below. Network planners would

direct the pattern peak in the appropriate direction.

First Antenna

Rx Multicoupler A

Duplex Filter

Tx Combiner

1A 2A nA1 2 n

N by 4 Voice ChannelsN by RF ChannelsTwo Diverse antennas shown

Second Antenna

Rx Multicoupler B

1B 2B nB

Chart 15 Side Mount ‘Omnidirectional’ Diversity

Array


The radiation pattern is shown in Magenta as both the

Tx and Rx patterns overlap completely. The dipole

arrays of both antennas need to be pointed in the same


0 - 3 - 6 - 1 0

- 1 5- 2 0

d B

0

9 0

1 8 0

2 7 0

Chart 16 - Side Mount ‘Omnidirectional’

Diversity Array Gain

direction (note that they are shown in the centre of the radiation pattern and it is worth noting their

directions). The diversity gain of the antennas will yield a 3dB improvement in all directions

around the mast compared with a single offset antenna and the magenta represents the combination

of the red and blue radiation patterns.


transmit antenna and the receive antennas on the other side of the tower, especially receiver 'B'.

(See 4.1 Receiver Isolation from Transmitters on page 20).

4.2.6 Two Sector Hybrid Sector System

In some circumstances, it may be appropriate to install a hybrid sector array which operates as two

discretely separate sectors. The first part of the sector uses a cross-polarised panel antenna in one

direction and the second part of the sector uses two separate stacked dipole arrays using space

diversity. The pair of stacked dipoles is used in the classic way, one antenna Tx/Rx with a

duplexer and the other antenna providing the second Rx path. The stacked dipole in its offset

configuration is Omnidirectional but skewed in one direction. This should be pointed in the

opposite direction to the panel antenna. The resulting coverage is egg shaped and slightly offset in

the direction of the panel. It has been used where a high traffic density is required and where this

shape of coverage is not a disadvantage (Such as at Motorway Junctions, with the heavier traffic in

the direction of the Panel).

Second Antenna

1C 2C 4C

1A 2A 4A1 2 43

Half of First AntennaSecond Half not Illustrated

for Clarity

Rx Multicoupler ATx Combiner A

Example shows two sector cell – One Offset andone panel system

Rx Multicoupler B

1B 2B 4B

Tx Combiner B

1B 2B 4B

Duplex Filter Duplex Filter

3B

Third Antenna

Rx Multicoupler C

First Sector - Panel Second Sector – Two Offset

Chart 17 Two-Sector Hybrid Sector System



The radiation pattern for the stacked array is shown in Magenta as both the Tx and Rx patterns

overlap completely, as in the ‘two offset’ mode.

The dipole arrays of these two antennas need to

be pointed in the same direction (note that they

are shown in the centre of the radiation pattern

and it is worth noting their directions). The

diversity gain of the antennas will yield a 3dB

improvement in all directions, for this antenna

pair over a single antenna of the same type. The

magenta represents the combination of the red

and blue radiation patterns. On the opposite

side of the mast a single cross-polarised panel

antenna is mounted on a different set of

channels. This also has a diversity gain of 3dB

relative to the nominal gain of the antenna. This

antennas radiation pattern is shown in green.

5 Mounting Criteria for Optimum Performance

5.1 Electrical

5.1.1 Background

The following testing was carried out by Dolphin Telecommunications. Where several services

share a radio site there will inevitably be some potential for interference between the various

signals present. This section examines the possibility of signals radiated from a TETRA 80º panel

being received by a 900 MHz base station through coupling between the two panels. The TETRA

band is not harmonically related to either of the TACS or GSM frequency bands and so

interference caused by the direct impact of harmonics of the TETRA carrier has been discounted.

It is therefore assumed that the primary mechanism for concern would be blocking of the 900 MHz

receiver by the TETRA transmitter.

GSM interference level recommendation 05.05 requires that a GSM base station is able to operate

normally in the presence of an interfering out of band signal at a power level of 0 dBm at the GSM


0 - 3 - 6 - 1 0

- 1 5- 2 0

d B

0

9 0

1 8 0

2 7 0

Chart 18 - Two Sector Hybrid Sector System

Gain

receiver antenna terminals. It is therefore desirable to keep any TETRA signal presented to the

receiver terminals of the GSM receiver below this level. The TETRA EIRP is limited by licensing

requirements to less than +47 dBm. A minimum acceptable loss in the coupling between the two

panels is therefore 47 dB.

5.1.2 Dolphin Measurement

While the antenna characteristic data can be obtained for each antenna, the published data refers to

the performance within the intended band of operation. Since coupling between the 400 MHz and

900 MHz systems includes reception of the potential interference outside the intended band of

operation of the antenna, it is difficult to predict the resulting coupling theoretically.

5.1.3 Practical measurement

In the absence of a theoretical calculation the coupling between two example systems was

measured to indicate the levels involved. Two GSM panel antennas, both cross-polar units with

different H-plane beamwidths, were examined in close proximity to a TETRA 80º panel antenna to

simulate the situation on a shared mast. The measurements were made using a tracking

generator/analyser combination to establish the isolation between the two antennas. The

measurement sweep encompassed all anticipated TETRA frequencies in using the range 380 - 430

MHz. To identify the worst-case coupling between the two panels, each measurement was

repeated using the second polarisation on the antenna so that the cases corresponding to a co-polar

and cross-polar coupling were both taken into account.

T E T R Ap a n e l

8 0d e g r e e s

H B W

G S Mp a n e l

8 5d e g .

H B W

4 2 t o 6 2 d B l o s s

4 7 t o 6 8 d B l o s s

a t T E T R Af r e q u e n c i e s

a t G S Mf r e q u e n c i e s

1 . 0 - 1 . 5 5 m e t r e s

Chart 19 - Measurement configuration for horizontal separation


For comparison purposes, a measurement of the coupling between the two panels in the GSM

frequency range 870 – 960 MHz, with the test generator connected to the GSM panel, was also

taken.

Tests were carried out with the panels on a common azimuth in each case. Two horizontal

separations (1m and 1.55m) were tested to examine the effect of increasing the separation between

the two panels. To investigate the possibility of mast sharing where the TETRA panel is installed

below a GSM installation, a separate measurement was made of the coupling when the two panels

are positioned end-to-end. The measurement was made with a single vertical separation of 1m

TETRApanel

80degrees

HBW

GSMpanel

85deg.

HBW

47 to 68 dB

loss

1.0

m

etre

s

at T

ET

RA

fre

qu

en

cie

s

Chart 20- Measurement configuration for vertical separation

5.1.4 Summary of results

TETRA to GSM, 1m horizontal separation

Radiating panel Polarisation TX band RX panel Polarisation Minimum

isolation

Maximum

isolation

TETRA 80º +45º TETRA GSM 60º +45º 42 dB 50 dB

TETRA 80º +45º TETRA GSM 60º -45º 48 dB 51 dB

TETRA 80º +45º TETRA GSM 85º -45º 45 dB 51 dB

TETRA 80º -45º TETRA GSM 85º -45º 43 dB 49 dB


TETRA to GSM, 1.55m horizontal separation


isolation

Maximum

isolation


TETRA to GSM, 1m vertical separation, TETRA below


isolation

Maximum

isolation


TETRA 80º -45º TETRA GSM 85º +45º 54 dB 76 dB

GSM to TETRA, 1m horizontal separation


isolation

Maximum

isolation

GSM 60º -45º GSM TETRA 80º -45º 51 dB 61 dB

GSM 60º -45º GSM TETRA 80º +45º 47 dB 68 dB

5.1.5 Analysis of results

The coupling between the two panels is affected by the polarisation relationship, despite the fact

that the receiving panel is not tuned for the receiving frequency and that the ‘side-on’ orientation

does not obviously suggest any correlation. The results show that the isolation is reduced when the

two elements in use are on the same polarisation.

The isolation between the two panels is noticeably increasing with separation – in the test carried

out the minimum isolation improved by 5 dB as the distance was increased from 1m to 1.55m

without changing any other parameters. The results suggest that the required TETRA to GSM

isolation is achieved with an inter-panel horizontal separation of as little as 1.55 metres.

For vertical separation, the test results suggested that the required isolation was achieved with a

separation of 1m in all polarisation combinations.


5.1.6 Test limitations

1. The test antennas shared a common azimuth in all configurations. While the data sheet polar

pattern will probably not be applicable in such close proximity to the antenna, it would be

reasonable to assume that the isolation would be reduced if the TETRA panel were turned

towards the GSM panel.

2. The tests were carried out with the two panels mounted horizontally on a non-conductive

support. While the forward beam patterns should have been into free space, it is possible that

building reflections may be responsible for some of the minor frequency-dependant

perturbations observed. Further work to establish the limitations of the test set-up would be

appropriate, however it would seem likely that the isolation figure indicated would be increased

if reflective paths between the two panels are reduced or eliminated.

3. The tests were carried out with two GSM panel antenna types. For more general applicability,

it would be appropriate to repeat the exercise with other products.

4. The figures indicated assumed that there is no additional bandpass frequency filtering in front

of the GSM receiver, providing additional attenuation at the TETRA frequencies.

5.1.7 Conclusions

From the data collected so far, it would be appropriate to use an inter-panel spacing guideline

requiring horizontal separation of at least 2m between TETRA and GSM, if the sector orientations

are on a similar azimuth. It is suggested that the vertical separation should be at least 1m.


5.2 Physical Mounting Criteria

The antenna system most appropriate for a given application is governed by:

• Mast type and position.

• Traffic Density

• Radio system configuration

5.2.1 Sectored Panel array

1. Pole mount

This approach involves the three antennas mounted at the same height, each arranged at 120

degrees to each other. Either the antennas may be vertically polarised or dual polarised where

diversity gain is required.

2. Mast mount

The panel may be mounted on each leg of a triangular tower, again at the same level as before.


3. Building mount

The panel antennas may be mounted at the edge of a building giving sufficient clearance from the

rooftop and held in position by a steel structure. The positioning of the panels should be as with a

mast, in terms of level and orientation. Free space in front of the antenna should be provided for at

least 20M extending down at an angle of 30 degrees. Avoid roof edge obstructions.

6 Antenna / System Integration

6.1 Low Density System.

This type of system uses a simple approach to radio coverage and shows how a single antenna may

be used to allow two-way communication with four RF channels. The duplex filter allows

simultaneous Tx/Rx operation and is only restricted by the RF power requirements dictated by the

number of channels in use. Below is a representation of how a non-diversity system functions,

using one antenna. This antenna could be Omnidirectional, sectored panel or directional. The use

of one antenna in this arrangement reduces the cost of antennas, plus the cost of mast space.


Antenna

Rx Multicoupler

Duplex Filter

Tx Combiner

1 2 n1 2 n

N by 4 Voice ChannelsN by RF Channels

6.2 Medium Density

The schematic below shows a system using antenna diversity. The Transmitters and the first set of

receivers are connected to one antenna and the second set of receivers is connected to the second

antenna. Therefore, there are diverse sources for the signals being fed into the receivers. Note

there is a practical limit to the number of transmitters that can be fed into the duplexer and the

antenna. This is determined by the PEAK power of the transmitters (the peak power of a

transmitter in TETRA is 6 dB above nominal power and therefore, there is a limitation set by the

voltage as well as the power capabilities of these elements). See also 4.1 Receiver Isolation from

Transmitters for more information on protection of Rx multi-coupler.

The use of an antenna system, in this way enables the use of large cells with the benefit of diversity

gain on the receive path. This could be a single antenna with dual slant polarisation or two

antennas.

First Antenna

Rx Multicoupler A

Duplex Filter

Tx Combiner

1A 2A nA1 2 n

N by 4 Voice ChannelsN by RF ChannelsTwo Diverse antennas shown

Second Antenna

Rx Multicoupler B

1B 2B nB


6.3 High Density

The illustration below shows how a large site might be configured. Half of the transmitters would

be fed into one antenna system and the other half would be fed into the second antenna. All the

receivers would be fed from each antenna to give diversity.

First Antenna

Rx Multicoupler ATx Combiner

1A 2A 8A1 2 4

Example Shows 8 Tx and 16 RxRedundancy in Duplexers and in Tx Combiners

Second Antenna

Rx Multicoupler B

1B 2B 8B

Tx Combiner

5 6 8

Duplex Filter Duplex Filter

3 7

The key benefit of this configuration is to spread the power load between the two antennas and in

that way to give greater redundancy in case of antenna failure, it offers diversity gain as before.

Receiver protection for the second one is also increased by such a configuration.


i ETS 300-392-2 First Edition, March 1996 Section 10.3.3

ii ETS 300-392-2 First Edition, March 1996 Section 6.6.3.1

iii Jaana Laiho Steffens, Jukka Lempeiainen et al., “Experimental Evaluation of Polarisation Diversity Gain at Base

Station End in GSM900 Network”, IEE Transactions, Vehicular Technology 0-7803-4320-4/98 Pages 16-20.

sigma antenna system design

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