The level difference between the wanted co-pol signal and unwanted cross-pol signal is likely to be in the order of 20 to 35 dB.
Detecting (and measuring) the cross-pol signal is difficult because of its normally low level.
If you tried using a big TV carrier you might get a 15 dB high hump (measured difference) on your spectrum analyser. This measured difference is C/(C+N) and means the C/N is 14.86dB. See
https://www.satsig.net/lnb/ebno-calculator.htm to convert the measured difference to C/N.
If you then turn your feed 90 deg (linear pol) or switch to the other polarisation (circular) then the cross-pol signal will be near noise floor. If the new hump was exacyly 3 dB high that would correspond to cross pol of 14.86 dB, since noise floor + signal at noise floor level adds up to a 3 dB hump.
If you have a spectrum analyser with a narrow filter, like less than 10 kHz, preferable 10 Hz and very stable PLL type LNB, then you can get very high co-pol C/N like 30 dB using a satellite beacon which makes it far easier to see and measure the cross-pol signal.
When measuring near the displayed noise floor make sure the displayed noise floor corresponds to the real noise floor of your LNA. Pull out the spectrum analyser cable and check the noise floor displayed goes down at least 10 dB, preferably more.
On a linear polarisation system, once you can see the cross-pol signal, misadjust the feed rotation a few degrees until you can see a clear level. Note the level and mark the angle on the feed throat. Turn the feed to the other side of the null and get to exact same level again. Then mark the throat and rotate physically to the middle.
Checking your transmit cross pol level is important as it ,
may cause severe interference to others. The NOC will ask you to transmit a CW carrier, while they watch their spectrum analyser at the hub. Doing full receive x-pol tests privately first will set you in good stead.
For safety, don't get in front of the feed or antenna when transmitting.
Best regards, Eric
