Mode-J Filter

  • If you have a hard time hearing your own downlink but have no trouble hearing other stations, you may be experiencing “desense.”

    Desense is where your transmitter signal overloads your preamp (or receiver) causing it to become temporarily less sensitive.

    This problem mainly occurs on Mode-J satellite operation with 2m-Uplink and 70cm-Downlink because of 3rd harmonics of your transmitted signal!

    Example: 145.900 MHz (TX) x 3 = 437.700 MHz (RX)

    Unfortunately our original AMSAT-DL Mode-J Filter Kit from DL1CF is no more available since many years.

    But there is a simple work-around:

    More in the article from Anthony Monteiro, AA2TX.

    I have been just asked and thought other's might have similiar problems..

    73s Peter

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG

  • For LEO SATs I'm using a LPF on the 2m side with 2x FT817 and the arrow antenna. Simple to make. Got the schematic from the PA1IVO website:…able_satellite_setup.html

    Works great both in mode J and B with no noticable desense on RX part. 1st time when tried to work LEOs without the LPF had interferences in both modes when using the arrow and couldn't hear my transmission back at all.

    Never measured the response of the filter but PA1IVO claims should have -50dB on 70cm. Seems that for FT817 and 5W it provides enough attenuation. Probably by using more power and a pre-amp on RX this one wouldn't cut enough and the one Peter posted would be more suitable.

    73 de S57RW