Any rectifier will become an envelope detector for AM, all that's needed is the proper resonant circuit to tune to the frequency, but crappy unshielded speaker cable can & will work for this handily, particularly if you're near a transmitter. Around what times does this occur?
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Around 4pm-ish I think, though I haven't noted the exact time. Short duration suggests the transmitter is moving. Maybe CB or police?
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CB would be exactly my guess too. Police would be FM (or a digital mode) on UHF, which is a type that can't be accidentally demodulated like this.
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CB seems very plausible living ~100 feet from a highway truckers frequent.
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CB fits everything else: modulation is either DSB-LC (AM) or SSB-LC so it can be detected with an envelope detector (i.e. a simple diode), it's ~27MHz so your wavelength is ~11m & a quarter-wave resonant antenna would be ~2.75m or so, i.e. a common length for speaker input wire.
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Where is the diode acting as the envelope detector? In the amp?
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Any PN junction in the wrong spot in the initial amplifier stage, yes!
End of conversation
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Equipment included: the sound card of my computer, corded landline phones, tape decks that did not have builtin radios.
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There could be an AM broadcast transmitter nearby. I used to get interference from a local station on random equipment.
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