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.

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.

FM/TV Diplexer

So here is the result:

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

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, 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)

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!

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