What is harmonic desense in LEO satellite operations?

Many amateur radio Low Earth Satellite operators have difficulty in hearing their own signals on the downlink. Hearing one’s own signal on the downlink tells you for sure that your signal is reaching the satellite. It also helps us to avoid interference to other stations by ‘blind’ calling when another station is having a contact. Sometimes it could be due to what we call ‘harmonic desense’. UHF amateur radio band comes as third harmonic of VHF band. Hence a full duplex radio which can receive on UHF while transmitting on VHF, can receive the third harmonic of your own transmitted signal, causing overloading of the receiver front end. Then you will not be able to hear the signal relayed by the satellite transponder properly. Instead you may note white noise or intermodulation distortion on UHF. You will be able to hear other stations when you are not transmitting on VHF. That is because at that time there is no ‘third harmonic desense’. Harmonic desense can be confirmed by reducing transmit power which removes the problem.

But reducing transmit power may reduce your ability to make contacts during low signal levels as near AOS (Acquisition of Signal – when satellite is just rising) and LOS (Loss of Signal – when satellite is setting. Due to FM capture effect, lower power stations may not be picked up by the satellite transponder during busy passes. Harmonic desense can also be reduced by increasing the physical separation of VHF and UHF antennas as far as possible, though it may not be possible for all. Another solution is to install a band pass filter like a diplexer on the UHF line, to reduce the intensity of harmonic pick up. An article by AA2TX on AMSAT website gives more detailed coverage of these solutions.