OK, for the Ku Band 12 to 16GHz, I can
definitely recommend the octagonal horn patented in
1978 by UK ERA, Dr.Alan Rudge. The details are in
the UK Patent (now expired) 26226/78, and an article
in the UK IEE (now IET) conference publication 169.
I have dimensions for 13GHz, and I made a model for
a test range (for study purposes only of course) and
it worked very well. Dr Rudge claimed to achieve
-50dB cross-polarisation over part of the band.
The key to obtaining low CP levels is to ensure
(somehow) that the E and H plane radiation patterns
are as nearly identical as possible. This is
possible to achieve with a horn because the currents
are constrained by the walls.
At 1.8-2GHz it
might be possible to construct a similar horn scaled
in frequency, but the dimensions would be large. It
is rare to find a design, for example a patch
antenna, that has a CP level any better than about
-15dB relative to the co-polar beam on boresight. It
might be better to contact a company like
data sheets on the web do not quote the CP
performance, usually a warning sign they are not
good. If something is good, marketeers are never
slow to blow the trumpet.