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Parabolic Reflector Cross Polarization levels - RF Cafe Forums

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nubbage
 Post subject: Parabolic Reflector Cross Polarization levels
Posted: Wed Jul 05, 2006 7:25 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
I confess to being rather mystified as to how a parabolic reflector and feed horn radiator combination can produce a cross polar isolation of -28dB or better (some firms claim -34dB) across a wide angle around boresight axis.

The theoretical level for a perfect uni-polar feedhorn at the focus and an edge taper of -10dB is two "rabbit ears" at about -27dB relative to main beam boresight level, at an angle corrsponding to the first zero in the copolar level.

So how do the manufacturers beat the physics?


 
   
 
The Doctor
 Post subject:
Posted: Fri Jan 12, 2007 8:31 am 
 
Lieutenant

Joined: Fri Jan 12, 2007 8:22 am
Posts: 2
The way I design systems is as follows:

Introduce higher order modes into the feed, such as TE21. These change the the feed X-pol pattern to produce two X-pol lobes. The feed X-pol is set up to be in anti phase to the X-pol lobes from the dish. Using this method I can get get better than 32dB X-pol off a reflector in the 13.75-14.5 frequency range.

The Doctor


 
   
 
nubbage
 Post subject:
Posted: Fri Jan 12, 2007 9:46 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Thanks very much for that solution. But how does one determine the relative level of TE21 to main dominant mode?
Or is it by trial-and-error?


 
   
 
The Doctor
 Post subject:
Posted: Thu Feb 01, 2007 7:09 am 
 
Lieutenant

Joined: Fri Jan 12, 2007 8:22 am
Posts: 2
I am sure that it would be possible to generate a set of design curves for TE21/TE11 mode content and I have, in the past, run series of parameter studies looking at this. In all the designs I have done at Ku band, which, for what I am involved in is the biggest market for this type of feed, the higher order mode generating section is a fairly standard design and dimensions tend to be similar.

The Doctor






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