Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Antenna Isolation Calculations

Antenna Isolation Calculations

Antenna Isolation Calculations

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?
Replies continue below

Recommended for you

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

Suggestion: It is not clear from the original posting where things stand, whether they are on the paper or computer or in the making/experimenting.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close