137MHz Bandpass filter for NOAA / Meteor Satellites
Yesterday I saw a new blog post by Adam (9a4qv) in LNA4ALL. The post (here) talks about a band pass filter he did for Weather Satellites and I decided to try as well.
Unfortunately I don’t have a exact match for that components at home, so I tried to do something with the components I have. So the lower value I had for capacitors was 10pF, and the needed values for Adam’s Filter is 1pF, 4.7pF and 15pF. I decided then to use 10 in series to do the 1pF, 2 in series for the 4.7pF (that will be 5pF) and then one in parallel with two in series to give me the 15pF. Its a very close match, and I’m unsure about the effects of serialization of capacitores in the filter (increase inductance maybe?). So here is the results.
So my circuit was a little different from Adam’s circuit. The coils are 3 turn very spaced wire (I tuned for better performance at 137MHz) and here is my final circuit (the inductor values are approximated):
And here is how it looks like:
The first thing I did was to setup my “Spectrum Analyzer” (a.k.a. HackRF + RTLSDR) to test the filter response and tune the coils. My coils was wound very tight so I get the lowest possible response. Then I started stretching the coils until I reached the peak at 137MHz. Here is the response curve using QSpectrumAnalyzer and HackRF with sweep.
I did the sweep for the baseline as well, and the response of HackRF => RTLSDR is very flat. The peak is always at about -29.8dBm. So my baseline is -29.8dBm.
Analyzing the values, at 136.242Mhz I have a measure of -31.542dBm, so about 1.7dB Insertion Loss. Then for 109MHz I have -63dBm of signal, that is about −33,2 dB of rejection. Just for reference, this is the simulated filter response:
The simulation goes a little more far beyonde the edges, but if you see, the graph is pretty close to what we expected. So then I decided to give a try and receive NOAA signals. For a coecidence, NOAA18 was just passing by, with center frequency of 137.9125MHz. I also wanted to give a try using an LNA4ALL at the output of the filter (that otherwise would saturate due FM Signals).
They are much better than it was before, and I’m running the filter / LNA at SDR end. If it was on the Antenna End the performance would be probably better. I still need to check the performance for LRPT Signals, but even for 2m HAM Band (145MHz) the filtering is very good!
EDIT (15/11/2016) – My Meteor M N2 capture! The longest capture I ever did!