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Parabolic Reflector Cross Polarization levels - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


 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?


 
   
 
 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


 
   
 
 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?


 
   
 
 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






Posted  11/12/2012
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