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Crystal radio

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picaflor

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I've built a crystal radio type receiver to see what I could pick up. It's the simple type, nothing more an antenna and ground connected to each end of an 1n4148 diode, and audio tapped across each end of the diode as well.

Having no luck with the radio and thinking that the diode was too insensitive to the weak signal, I connected a spent AA in series with a 5k ohm linear taper potentiometer (radioshack catalog # 271 1714) to give the diode additional bias. Success! I finally could hear transmissions, but when I caught a station id, it turned out to be FM.

This "shouldn't be possible" to my understanding so what is going on?
 

OCO

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Replace the 1N4148 with a germanium diode..1N34A diodes (Radio Shack part number 276-1123) (or get a rusty razor blade and a "cats whisker"
See my sig... (I've still got the case, gutted the interior of mine for an antenna tuner - doh!)
 
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picaflor

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Germanium diode

I wish Radioshack still sold them as discretes... The only way I could get one was to buy one of the science kits they carry and I wasn't really up to spending thirty bucks just to get the diode out of the science kit.
 

OCO

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This is stupid - a 30 cent diode with 5.00 shipping. Scrounge around for a junk radio..The most common silicon diodes have a forward voltage drop of about 0.65 volts. Germanium diodes have a forward voltage drop of about 0.1 volt. Or there always is the rusty razor blade...... No idea on the FM question - IMHO it would have to be near field - maybe if you were near an FM station.
 
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K9WG

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...snip...
Success! I finally could hear transmissions, but when I caught a station id, it turned out to be FM.

This "shouldn't be possible" to my understanding so what is going on?
This is known as "slope detection". The edge of the FM signal varies in frequency and an AM detector will rectify this variance as an audio signal.

Frequency and phase modulation detectors

AM detectors cannot demodulate FM and PM signals because both have a constant amplitude. However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.
Detector (radio) - Wikipedia, the free encyclopedia
 

OCO

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For some reason I missed the detail concerning the lack of a tuned circuit....Picaflor - you need an oatmeal carton and some enameled copper wire and a couple of variable caps...
 
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