Measuring FM Filter Performance in Downtown Houston

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Joined: Mon Sep 09, 2019 6:28 pm

Measuring FM Filter Performance in Downtown Houston

Post by moeses » Mon Sep 09, 2019 6:46 pm

I'm new to all of this and trying to learn antennas and SDR and the like by trying to get some weather satellite imagery. I built a double cross antenna ( to get the imagery off the top of my parking garage in downtown Houston. I've had marginal success. In order to assist I got a RTL-SDR branded 88-108 MHZ Bandstop Broadcast FM Reject filter. I did some testing inside my apartment to see how well the filter worked with the antenna and the results were great. I had a 29.7 dB reduction in the SNR with no change in any settings.

I took the antenna up to the roof where there is no overhead obstructions at all and upon plugging in the filter (I also had a Low Noise Amplifier connected inline) the FM signal was still quite strong. This time I removed the filter and LNA from the system to check what the signal was like without anything and the SNR actually got better.

These radio stations broadcast within a few miles of me so i expect the signal to come in really strong. My question is, what is a good metric to look for to determine that the filter is working ok? Is the SNR not a good measure? Why would the SNR decrease without the filter and increase with the filter?

I wrote up a short paper detailing the tests, results, with some SDR# screenshots @ ... r.pdf?dl=0

Thanks All!

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Re: Measuring FM Filter Performance in Downtown Houston

Post by rtlsdrblog » Wed Sep 11, 2019 6:55 am

I don't know why LNA's are so popular for 137 MHz satellites. At this frequency LNA's have minimal benefits unless you have a lot of cable loss. They're actually more likely to cause overloading issues.

Try without the LNA, but with the filter in place. SNR is the best metric. I can only guess that when the antenna is on the roof, the signals are so strong that the filter can't fully block it. Probably there are other signals causing overload too.

The RF gain on the RTL-SDR also needs to be reduced when an LNA is being used.

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