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Following on from this question, does the volume of the receiver increase the closer it is to the transmitter because it's receiving more of the radio signal? This question specifically is for a crystal/diode based AM receiver. I would expect most commercial AM radio receivers would have some kind of automatic volume/gain control to ensure the average volume stays consistent.

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2 Answers 2

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If you detuned from a strongly received AM radio station transmission, you would immediately hear a lot of background noise. You can even hear the background noise when you receive a weak station that is a large distance away. That doesn't mean that the background noise varies within the part of the spectrum you are tuned to; rather it tells you that an AM radio uses an automatic gain control (AGC) mechanism to try and keep all radio stations at around the same loudness level.

does the volume of the receiver increase the closer it is to the transmitter because it's receiving more of the radio signal?

If it didn't have an AGC circuit it would vary a lot. Think about that background noise level - for a good receive signal you can barely hear it and, for a totally detuned and isolated part of the spectrum it might be 50 dB higher (about 32 times louder). Note that the bel (the "B" in "dB") is approximately equivalent to a doubling of the psychoacoustic perceived loudness of the ear.

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    \$\begingroup\$ Ah right, I remember when I played with a crystal radio over a decade ago that if tuned to a station, I would hear it clearly, but when tuned off of a station, I wouldn't hear any static like you would expect with a commercial radio. \$\endgroup\$
    – horta
    Feb 17 at 19:39
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    \$\begingroup\$ Crystal radios don't use AGC @horta hence, what you remember is highly likely. \$\endgroup\$
    – Andy aka
    Feb 17 at 19:45
  • \$\begingroup\$ I had thought that AM used a DC offset on the signal to be modulated, so in periods of silence the carrier was still there. This then acts as a baseline for gain control. So you don't need an AGC in the sense of looking at the volume of the encoded signal (or quiet and loud sections of music would be the same), just one that amplifies the signal prior to modulation so the DC offset is at a known level. Not an answer as this is just a vague memory. \$\endgroup\$
    – abligh
    Feb 18 at 7:49
  • \$\begingroup\$ @abligh it's still an AGC whatever method. \$\endgroup\$
    – Andy aka
    Feb 18 at 8:18
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Yes, a crystal AM radio will receive stronger signals louder than weaker one.

You are also correct that nearly all commercially-made AM radios have AGC or Automatic Gain Control to help alleviate loud vs. not-so-loud reception as you tune across the range.

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