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Antenna Isolation Calculations

Antenna Isolation Calculations

Antenna Isolation Calculations

(OP)
What are the most realistic calculations one can do in order to determine the required vertical or horizontal (or both)separation to fulfill a given isolation need? Is there a way to incorporate the antenna radiation pattern of both antennas so as to have a better estimate?

RE: Antenna Isolation Calculations

There are two basic analytical approaches depending upon the complexities of the structure as compared to the wavelength.

GTD/UTD theory can be used on large 'simple' structures where each structural element can be approximated by one of the canonical shapes of the theory and each structural element is at least about ten times larger than the wavelength.  This is ideal for most aircraft and VHF and up.  One nice product is ALDAS from ASL in the UK.

GTD/UTD theory has the advantage that the problem becomes more tractable as the frequency increase.  It is possible to do dozens of modeling runs in an afternoon using a PC(once everything is set up).

For problems where the structure is too complicated or the frequency is too low, then you'll have to use a finite-element approach like good old NEC.

The problem with any finite element approach is that the problem becomes more intractable (at a cube - ^3 - rate) as the frequency increases.  Modeling VHF/UHF and higher frequencies on a larger aircraft would require significant processing time.

Other options include partial scale model, full scale model testing, or real world tests.

Finally, you could ask someone with experience to look at the situation and say, "Oh, about 50dB or so..."

RE: Antenna Isolation Calculations

There are numerous antenna manufacturer websites that provide graphs of isolation vs distance.  We find these very reliable.

RE: Antenna Isolation Calculations

Your question depends on the Tx/Rx frequencies.  Isolation is measured by the return loss of the OMT of the feed in both H & V and is usually conducted in a range test.  I'm speaking of microwave frequencies way above UHF (Ku-Band for example) using parabolic antennas.  

The isolation is provided in part by the orthogonal design of the OMT (Ortho-Mode Transducer) itself, and can range from 30 to 35 dB between V & H (Ku-Band).  But this isolation is greatly increased by the use of the transmit reject filter.  So together, the isolation of 70-75 dB can be realized.  

The V can be used for Tx and the H for Rx, or vice versa.  The uplink & downlink frequencies themselves provide a measure of isolation. But what you don't want is an OMT that passes both Tx and Rx through the same port.  Hence, the transmit reject filter.  

I don't personally bother with formulas or calculations, but use instead range tests of randomly selected parabolic antennas.  This involves return loss tests of the OMT on both ports.  This is more practical because it's based on actual test data.  The transmit reject filter simply adds to the isolation by rejecting Tx signals at the Rx side of the OMT.  For the Tx side, HPAs usually have Rx reject filters already incorporated inside them.

Sometimes even manufacturers of such parabolic antennas, especially those that include feeds, include isolation data in their specifications.  


RE: Antenna Isolation Calculations

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