All readers of this thread may be aware that since I use my SUN SDR on 2m, I tested quite a number of homebrew filters as well as various retuned commercial surplus filters and dedicated ham band filters.
So far I never got all filter parameters in line: when I reached the wanted selectivity, I had too high insertion losses. most low loss filters had too wide bandwith and did not cure all problems.
I also tested the insertion of a 2m bandpass filter in between RX-out and DAC-in, but noted that the input MMIC in my heavily polluted area (JO64) contributed a good part to my problems as the MMIC was overloaded by strong out of band signals and mirror images .
As last resource I tested a large size helical filter manufactured by Juri, IZ4OSG.
This Filter Its more than half the size of my sun, but the plots Juri provided tempted me to purchase one of his helical filters.
In brief: the spects Juri publishes are met - insertion loss is below 0,4dB and the out of band selectivity is very good.
All strong out-of-band signals are attenuated to a level where they do not harm anymore.
No SW break through. I cannot barely measure any mirror signal around 175 MHz mixing into the 2m band.
There is still a 10dB increase of noisefloor (in other directions more than 15 dB), polluting the 2m band and all adjacent frequency bands.
This pollution is caused by high level leakeage from a local Vodafone cable net, carrying high level 256 Quam signals (Internet downlink - Eurodis) in the whoile VHF band polluting also airband frequencies.
The presence of this large band pollution was confirmed by the German authorities (Bundesnetzagentur) but it appears they lack manpower to take action.
The screenplots show worst case situation Antenne turned to highest out of band signals.
Setup: Sun LNA on, #### internal sun wideband filter DEACTIVATED (!!!) ####
IZ4OSG Helical Filter between 2m Yagi and RX intput.
I use an additional 2m Bandbass between RX-out and DAC in (The helix input filter is activated/deactivated via a transferswitch for this test setup)
The signals visible at 90-100-110 120 MHz etc are internal or external leakeage from the 10MHz GPS timebase (or its distribution amplifier).
I propose the use of cheap surplus ferrite cores as choke on the antenna cables and recommend good DC filters in in the 15 V DC supply.
Wishing you good reception
73, Charly, DF5VAE