Radarspotting
Mode-S Receivers => DVB-T Dongles and Self-Assembly Receivers => Topic started by: Clubba on July 30, 2023, 02:02:14 PM
Can anyone recommend a good air-band filter?
I'm currently using a NOELEC Flamingo FM Broadcast filter but Classic FM is getting past it for some reason.
How far away are you from the transmitter.
I see that the filter you have shown is an AM filter. To stop you would need this filter by the same company.
https://www.amazon.co.uk/Flamingo-FM-Applications-Problematic-SMA-Connected/dp/B07XKY8YKB
The filter you link to is the one I am using :) and it is not too good TBH
What radio are you using and how for away from the transmitter are you. Classic FM only broadcasts on 99.00 - 101.9 around the UK on 15 transmitters.
Nearest Tx as far as I know is probably 10 miles direct. Using RSP1a 133.600Mhz.
I think Classic FMs signal is a bit of a nightmare for SDR in general, it always increases noise level on any signal that is sited on their harmonics/splatter.
I've tested the Flamingo @ 88mhz it has -52db rejection and it is roughly -19db rejection at 99-102mhz and thats why classic FM can bleed through. It appears the filter is most effective at the lower end of the FM band. This is only a rough estimate as I followed a couple of youtube vids on how to test a bandpass filter using a VNA.
Well I found a good airband filter!
https://www.ebay.co.uk/itm/385872577744
This sellers filters are mailed in UK, mine arrived within 2 working days of dispatch.
On my VNA I measured the filter from 65mhz-165mhz: @70mhz rejection was -94db (more or less the same on the higher side of the frequencies tested) and @100mhz -61db. Between 118-136 the loss is a mere -1.3/2.4db, the filter peak covers 114mhz to 141mhz so should be good for 137mhz satellite band too.
SDR Noise floor is about 25db less and interference from Classic FM has been reduced a lot and is barely visible on the Waterfall (I won't fully know how effective this filter is until it rains again as that's when Classic FM RFI peaks).
No noticeable loss in incoming signals either and my squelch is so low it's almost off, I'm well chuffed with it.
I can highly recommend this bandpass filter over an FM broadcast blocker for VHF Airband.