So I’m still mad about the FM Spectrum (88-108MHz) noise I get when using a LNA or something else. So I’m still looking for a nice and easy way to filter out these signals.
Last time I posted about a TV/FM Diplexer, that works great, but doesn’t attenuate enough the signals. So the solution would be cascading several of them, but I found that is very hard to find one Diplexer or FM Trap filter those days.
So I started looking out how to make my own filters. So I reopened Adam’s Website (LNA4ALL) that have a small filter he did for FM Trap and tried (again) to make my own.
So it has been a time since my last post (again). Today I’m writting about a TV/FM Diplexer that I bought to address a issue that I’m having here with my NOAA stuff.
So the biggest problem is that in São Paulo the FM Radio (88-110MHz) are VERY strong. I can receive a -40dBm signal with a RTL-SDR with no gains in almost all channels. This is a big issue since the RTL-SDR does not have a input filter (actually it has, see my patches at https://github.com/librtlsdr/librtlsdr ) the LNA gets very easily saturated when getting gains over 25dB (usually needed by APT Signals). So I start to search for a FM Band Stop Filter. But it turned that it was not so simple to do a good FM Band Stop Filter.
So I started searching for a commercial filter, and I noticed that most of the FM Filters were discontinued a few years ago and the only thing I could find was a FM/TV Diplexer.
So some people already saw in my facebook that I started playing with SDRs (Software Defined Radio).
I always wanted to do my own radio receiver, and I did some in the past. But it’s very hard to adapt the radio for anything new you want do to, and also when you want to process data in your computer things become harder.
So a few months ago I found a nice tutorial of how to get NOAA Satellite Images using a cheap DVB-T (Digital Video Broadcast – Terrestrial) dongle that can be used as SDR. It costs about R$70 (roughly US$10) and the model I got (with R820T2 tuner) can tune from 24MHz to 1.74GHz!!!
What is inside this spectrum?
Actually a lot:
FM Audio Radio Broadcasts
VHF / UHF Television (Both Digital and Analog)
Weather Satellites (APT, LRPT, HRPT)
ADB-S (Air Traffic Telemetry)
FM Air Traffic Radio
So my goal was to receive NOAA APT Signals (I even made a decoder!) but I don’t have a good enough antenna (yet).
So I made up a piece of antenna with two copper pipes (I call a piece, because its a dipole from a Double Cross Antenna) (I will make a tutorial later how to do it) to have better reception for the 2m band (~135Mhz) but every time a satellite was in range, I would need to go outdoor and turn on my laptop and start capturing. This was annoying.