ADS-B In pre-filter, mounting plate and technical considerations

3 months ago

| Working on: Fuselage

I was alerted by Sling 2 builder Praveen of the need to filter out (troublesome) mobile carrier signals from bands adjacent to the 1090MHz ADS-B In. His proposal was the addition of a pre-filter at the input of the Dynon SV-ADSB-472 receiver. The 472 does include a bandpass filter after the input stage (verified with Dynon for 472s made in the last few years), however given that this unit has coverage also of the 978MHz UAT weather data band (not implemented in Australia) I would surmise (and am checking this with Dynon) that this filter is quite broad-band with rather a shallow rolloff. I would expect the rejection of the 850-900MHz mobile bands is not very great. If an external filter is added, it is important that it is centered on 1090MHz, narrow band enough to admit the ADS-B In data and not much more, low loss so as not to unduly degrade the system noise figure, and offer high rejection away from the passband.

Some candidate filters

1090 MHz cavity filter for Mode-S / ADS-B | cf1090-kt30 (sysmocom.de)