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Sunday, March 6, 2011

Crystal Radio Sleuthing

As part of my effort to stamp out broadcast interference to the SolderSmoke podcast, this weekend I reassembled the crystal radio that Billy and I had built in London. It is REAL simple: Just a parallel LC circuit with a germanium diode detector and some high impedance phones. (I also put a chunk of galena and a cat's whisker on the board -- that's for when I get the urge to form my own PN junctions.) As expected, I immediately heard two AM broadcast stations: WFAX 5 kw 1220 kHz (religious) and WUST 20 kw 1120 kHz AM (mostly foreign language). I found out the hard way that these stations reduce power at night: I was bragging to my wife about the EXCELLENT reception I'd been getting on the crystal set, but when, after dinner, I brought her into the shack for a demonstration, she could barely hear anything. Oh well...

But here's a surprise: These are NOT the stations that are getting into the podcast! With the crystal radio in operation, I did some audacity recording and then quickly checked to see if the breakthrough sounded like what they were playing on WFAX and WUST. NO! The breakthrough was ZZ Top! I'm guessing that the breakthrough was from an FM broadcaster. I note that the length of the cord to the microphone would seems like it would be a nice antenna for the FM broadcast band... What do you guys think?

Whatever the source, I think I have taken care of the problem. I got big ferrite toroid core and wrapped about ten turns of the mic cable through it. No more broadcast breakthrough.

2 comments:

  1. Very nice troubleshooting, using the crystal set! You seem to be bathed in RF at Soldersmoke HQ, "in the wilds of NoVa".

    Maybe you can take the toroid out for a bit and grab some call letters off the offending station's signal (assuming it is an FM broadcaster and not some other form of wireless re-broadcast of music).

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  2. "I note that the length of the cord to the microphone would seems like it would be a nice antenna for the FM broadcast band..."

    It must have a reasonable high Q also, else it's hard to explain, how the FM is turned into an audible AM - neat circuit you
    built :-)

    73


    Peter/DL3PB

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