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Isolation of antennas

2Kathrein/Kretzschmar 05/04

Isolation General

Different mobile communication systems require a min. totalisolation of 70 - 80 dB

This isolation may not be achieved by antennas only, butmust be generated together with filters

Required antenna isolation > 30 dB

The coupling between two antennas is based on

radiation induced currents via the common mast

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Isolation General

Factors influencing the isolation value:

Electrical items: Frequency Polarisation Half-power beam width Electrical tilt

Mechanical settings: Vertical / Horizontal separation Angle Pole-/ Wall-Mounting Design

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Isolation General

Electrical itemsMechanical settings fixed

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Isolation Frequency

Antennas are not filters Also out-of-band frequencies are received but with worse VSWR Resulting mismatch creates attenuation called mismatch loss Mismatch loss increases the isolation

Antennas operated in different frequency bands have higher isolation values than antennas operated in the same frequency band

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Isolation Polarisation

Lowest isolation when antennas have same polarisation for different polarisation, the isolation values increase For comparing vertical and slanted polarisation, only the vertical

component of the slanted polarisation is responsible for the isolation Amplitude of this vertical component is 3dB smaller compared to the

complete signal

Antennas with different polarisations show higher isolation values than antennas with the same polarisations

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Isolation Half-power beam width

With two antennas side-by-side facing into the same direction, theradiation against each other (orthogonal to the main beam) determines theisolation

The broader the half-power beam width, the higher the radiation level at+/- 90

The isolation decreases with a growing half-power beam width

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Isolation Electrical Tilt

Achieved by feeding the dipoles with unequal phases of a signal The different phases lower the coupling between two antennas

Higher isolation values for antennas with electrical tilt (fixed or adjustable) rather than for antennas without

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Isolation General

Mechanical settingsElectrical specifications fixed

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Kathrein/Kretzschmar 05/04

Isolation vertical/horizontal separation

Antennas have dedicated radiation patterns with nulls above and belowmain beam

Therefore the radiation level towards an antenna above or below is small

Vertical separated antennas show higher isolation values than two horizontally seperated antennas at same distance

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Kathrein/Kretzschmar 05/04

Isolation angle separation

Signal level behind the antenna is much smaller than in front or at +/-90. Therefore if antennas are separated by an angle, the mutual level of

radiation becomes less.

The isolation grows with the azimuth angle between them

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Kathrein/Kretzschmar 05/04

Isolation Pole-/Wall-Mounting

Antennas have high front-to-back ratio But still, the influence of a large plane behind the antenna (e.g. building

fascade) can not be neglected The reflections from the surface result in a slightly smaller radiation pattern Therefore the level of radiation towards the neighbouring antenna is

reduced. Two same antennas mounted on a wall show higher isolation

values, than if being mounted on a pole

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Kathrein/Kretzschmar 05/04

Isolation Design

Current at the edges of the reflector significantly influences the isolationbetween two antennas

The currents depend on the construction and the kind of radiatingelements used (e.g. dipole, patch)

Therefore one of the biggest influencing factors is the design of theantennas

Isolation values from one manufacturer may not be used for antennas from another.

Kathrein antennas are designed

for high isolation values

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Kathrein/Kretzschmar 05/04

Isolation General

Save distance

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Kathrein/Kretzschmar 05/04

Isolation Save distance

Standard question of network planners:What is the required minimum save distance for isolation values >30dB?

Vertical separation:

The isolation values are always quite good

Only the minimum possible distance is needed

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Kathrein/Kretzschmar 05/04

Isolation Save distance

Horizontal separation:The minimum save distance depends on the wavelength and on thehorizontal half-power beamwidth(same frequency, polarisation and no tilt are used)

Please note:

The stateddistances are onlyvalid for the shownsituation!

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Isolation Save distance

Angle separation:

Taking a 120 angle, 30dB of isolation are already achieved with theminimum mechanical distance

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Kathrein/Kretzschmar 05/04

Isolation General

Optimised minimum distance

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Kathrein/Kretzschmar 05/04

Isolation Optimised minimum distance

Optimised minimum distance may only be found with measurements Kathrein has done a number of isolation measurements for typical site

configurations, that are available for our customers Measured values are up to 50dB Values of more than 50 dB depend also on the special site and are therefore no

longer typical.

Measurement ofdecoupling values

Isolation of antennasIsolationGeneralIsolationGeneralIsolationGeneralIsolationFrequencyIsolationPolarisationIsolationHalf-power beam widthIsolationElectrical TiltIsolationGeneralIsolationvertical/horizontal separationIsolationangle separationIsolationPole-/Wall-MountingIsolationDesignIsolationGeneralIsolationSave distanceIsolationSave distanceIsolationSave distanceIsolationGeneralIsolationOptimised minimum distance