FM/TV Diplexer Test

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 the idea behing is a diplexer is that it have a common input (or output depending on the direction that is used) that have FM + TV Signals. To separate (or combine) there is a low pass for the FM Band (Usually the lowpass cuts at something arround 115MHz, and there is a High Pass to the TV Band that cuts on the same freq. So I wanted to use the TV Band and strip the FM Band. So I hooked up the input as my QFH antenna , the RTL-SDR on one of the outputs and started qspectrumanalyzer to analyze the filter response and put a 75 Ohm terminator on the other output.

So here is the result:

Without the Diplexer (Diret to antenna) – Spectrum from 50 to 300MHz

With the Diplexer, with terminator on the FM Side (RTL on the TV Side)

With the Diplexer. Terminator on the TV Side (RTL-SDR on the FM Side)

So the filter looks amazing. It have a base atenuation arround 20dB. This should give me enough room for a LNA. Also check that the other band remains basically inaltered.

So soon as possible I will post new results of APT Signals capture. Keep noticed!