# Polarization mismatch between a LEO satellite and ground terminal

Cross-polarization loss between a circularly polarized antenna and vertical or horizontal polarized antenna is known to be -3dB. This is one of the reasons why Circular Polarized antennas are preferred for satellite based communications. However, practical mismatch loss between a RHCP and LHCP antenna can be -20 to -30 dB (theoretically $$-\infty$$).

I am trying to understand what happens between a RHCP satellite and a LHCP ground emitter as the satellite crosses from horizon to horizon as the LoS geometry changes. Is there any rule of thumb in terms how the losses will look like as satellite rises, culminates, and then finally sets? If I know the azimuth and elevation at each rise, culminate, and set instance can I say something about the mismatch loss?

• Not implying this is off-topic, because I think this is a fine question, but the Amateur Radio StackExchange also exists and might be helpful for this kind of question. I don't read/post there myself though so I might be totally off-base Oct 26, 2022 at 4:50
• @ErinAnne, thanks for the suggestion, I will wait for a few more days and if no answers are given I might cross-post it to the Amateur Radio stackexchange.
– jkt
Oct 26, 2022 at 17:24
• To double check, the satellite antenna is RHCP and the ground antenna pointed back at it is LHCP? The condition is such that theoretically the loss would be infinite except for antenna imperfections and ionospheric/atmospheric effects? Is the ground antenna tracking the satellite? Is the satellite's antenna also tracking the ground station? The reason I ask is that once you start looking away from an antennas preferred or bore sight direction the polarization state will be very different, and it depends a lot on the specific antennta.
– uhoh
Oct 29, 2022 at 11:19
• @uhoh, consider a nadir pointing satellite with RHCP antenna and the ground antenna is LHCP. Ground emitter is static so no tracking involved. As you mentioned if you are looking away from the boresight; the polarization will be different, so that is basically what I am trying to figure out. For instance, if the satellite was passing directly overhead at 90 degree elevation the theoretical -$\infty$ dB loss would be incurred but usually for LEO the satellite will passing at a certain azimuth and elevation and I am trying to understand certain scenarios where the loss would be minimum.
– jkt
Oct 29, 2022 at 18:01
• @jkt So I guess that the ground station will have a low-gain antenna so it can receive from much of the sky. Will the spacecraft have a similar antenna?
– uhoh
Oct 29, 2022 at 21:39