I tried the idea and I had LNB (used in satellite dishes) which outputs from 950-2160 mhz and also satellite receiver with loop out (power block). Hence I used it as signal source for the making Coaxial cable filter. Comparatively strong signals were on 940 Mhz and at 950 Mhz. So idea is to block it. Initially I tried band pass for 1090 but not able to succeed.
This is the noise source and it is not perfect but something better than nothing. Some what giving signal enough to make a filter.
Cut two coaxial cable for the lengths 940 and 950. Used two T joint adapters.
Putting this near antenna before the preamp worsens the reception as 1090 Mhz gets 2-3 db weaker and per amp fails to amplify it!
Hence I put it after preamp and reception seems to be better than earlier but hard to confirm exactly. Seeing more ground traffic.
Earlier if preamp voltage is increased above 6V then there won’t be any ADSB reception (may be 950mhz saturates). Now after the filter at 12V ADSB comes properly but at 6V no ADSB(may be preamp doesn’t send high DBs in the coaxial line and due to line loss with the filter it is affecting)!
AGC is constantly giving one ground message at -3.1 dBFS and some times even lower. Some aircrafts on the ground are coming around -7 to -10 and some at even -31 dBFS. But they are not consistent they just like that come suddenly and then vanish.
Will it be worth trying SAW filter and adding it near/after preamp to see whether it helps?
The present arrangement has around 3 db loss which will be same with SAW but quality of filter will be higher than now.