Re: RF Clipper Diode Design


jdow
 

You probably want multiple different antenna connections and SDRC configurations. The configuration in the first image is probably your best setup conditions unless you filter out commercial FM while receiving other things. I doubt there is much you can do to tame the super power FM stations. Walls don't do much to attenuate FM broadcast. A hill is what it usually takes to make a big difference. A lot of buildings might bring in 20 dB of attenuation which would protect the front end from those signals. An off frequency antenna could buy another 10 dB or more. (Be careful if you install a good FM antenna. +81 dBm with 80 dB of attenuation from basic path loss is still more than 0 dBm. If your radio has not died already it likely can stand well more than 0 dBm on its input. 1V peak is where I'd expect things to get dicey. That is about +14 dBm.)

K9YC, Jim Brown, has some nice articles to read about surviving in a theatrical environment with worse signal levels than you are suffering. http://k9yc.com/publish.htm The theater he worked in until he retired was in Chicago in a tall building with a large number of very powerful broadcast stations. He made the theater's audio systems work in that environment.

To sum it up, I believe you will find FM DX with SDRs a rather unrewarding experience at your present location. But, you can make ham radio work nicely with an FM band filter - or two or three. The BSF-108+ from Mini-Circuits can provide 60 dB of attenuation. So one of them should allow you to run with full sensitivity on most ham bands. And it appears for FM reception you can personally redefine "DX" and run without the filter with reduced front end gain. That spectrum splash is what happens when A/D converters become overloaded. It is a basic limitation on the high power end.

{^_^}

On 20200203 02:50:22, goscickiw wrote:
I think it's sort of solved with the 100.9MHz signal. I had automatic IF gain set, the signal became better after I changed it to manual.
In case of other weak signals, I tried with a 4-20dB variable attenuator and they became much worse or disappeared, while the strongest signals stayed just as good. Same situation when I use a short piece of wire or a smaller antenna.
These signals come from different transmitters so a directional antenna would help, but the problem here is space. I can't put a 5 element FM Yagi on a wall.
Also something I should have mentioned, my antenna doesn't have a direct line of sight to the transmitters because of location - surrounded by higher buildings. Putting it on the roof would probably help, but would require much convincing of other residents. It's a vertical folded dipole.
Here is how the signals look like on the spectrum in normal conditions - no attenuator.
When I increase the RF gain by 1, the spectrum becomes this mess:
With an attenuator:
I'm not sure why this subject expanded in this direction... Originally I wanted to protect my HackRF One, that I use just for testing stuff like modulators and usually don't connect an antenna to it, from accidentally giving it signal stronger than its nominal -5dBm... Then I started talking about receiving FM. WTF brain.

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