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Parabolic Reflector Cross Polarization levels - RF Cafe Forums
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| nubbage
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Post subject: Parabolic Reflector Cross Polarization levels
Posted: Wed Jul 05, 2006 7:25 am
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| General |
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Joined: Fri Feb 17, 2006
12:07 pm
Posts: 218
Location: London UK
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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?
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The Doctor |
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Post subject:
Posted: Fri Jan 12, 2007 8:31 am
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Joined: Fri Jan 12, 2007
8:22 am
Posts: 2 |
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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
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nubbage |
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Post subject:
Posted: Fri Jan 12, 2007 9:46 am
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Joined: Fri Feb 17, 2006
12:07 pm
Posts: 218
Location: London UK
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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?
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The Doctor |
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Post subject:
Posted: Thu Feb 01, 2007 7:09 am
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Joined: Fri Jan 12, 2007
8:22 am
Posts: 2 |
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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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Posted 11/12/2012
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