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General requirements for antenna isolation in mobile communicaiotns systems
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1Kathrein/Kretzschmar 05/04
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
3Kathrein/Kretzschmar 05/04
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
4Kathrein/Kretzschmar 05/04
Isolation General
Electrical itemsMechanical settings fixed
5Kathrein/Kretzschmar 05/04
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
6Kathrein/Kretzschmar 05/04
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
7Kathrein/Kretzschmar 05/04
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
8Kathrein/Kretzschmar 05/04
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
9Kathrein/Kretzschmar 05/04
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
14
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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Kathrein/Kretzschmar 05/04
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
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