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I have an old (~late 1920s) AM radio that I'm currently working on. As I don't want to modify any of the internals significantly, it would be nice to have a way to fake antenna input. It's not entirely clear to me how to safely do this, however (it has been a number of years since I dealt with anything involving RF).

Would it be sufficient/safe to just have a very low voltage (relative to a common ground) AM signal through the antenna input? Or would this be entirely wrong, and damage the radio? If this would work, what order of magnitude would one expect the required voltage to be?

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    \$\begingroup\$ The best way to FAKE an AM signal, is to produce an AM signal. It's not terribly difficult. \$\endgroup\$
    – Passerby
    Commented Mar 22, 2015 at 5:32
  • \$\begingroup\$ Can't you just tune in a station that is on the air? \$\endgroup\$
    – Dwayne Reid
    Commented Mar 22, 2015 at 5:52
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    \$\begingroup\$ What are you trying to achieve ultimately. High level answer is required to make any sence of this question. \$\endgroup\$
    – Andy aka
    Commented Mar 22, 2015 at 10:04
  • \$\begingroup\$ Per Andy's request: make your question clearer, and it may result in more upvotes. \$\endgroup\$
    – gbarry
    Commented Mar 22, 2015 at 21:25

2 Answers 2

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If you want to feed sound to an AM radio, you don't necessarily need to make any direct connection to it at all. You just need to have a coil nearby that magnetically couples a suitable AM signal into its antenna.

There are many examples of low-power analog AM transmitters on the web. One way to generate a very high-quality AM signal using modern digital technology would be to get one of the many inexpensive DDS (direct digital synthesis) modules (or just a bare chip) and feed digital audio from any source into its amplitude control register at a suitable sample rate.

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It's not clear what you are trying to accomplish. Normally you can connect an antenna and you'll pick up signals from AM stations in your area. If you are trying to repair the radio, perhaps you need a strong signal as a starting point. For that, you can use a signal generator. (Or make one...making a 1 MHz oscillator isn't that hard. Making a stable one is a little harder, but perhaps not necessary for testing).

I did a little Web research, to ballpark some numbers. These numbers apply to "shortwave" but that's close enough to an AM receiver (which is probably not even as sensitive as a shortwave receiver). A good signal (S9) represents 50 uV at the antenna input. Signal meters tend to go up to +60 dB over that. That would be 50 mV at the antenna. I'm sure receivers are subjected to even more than that. Your 1920's set will be using vacuum tubes, and they will take quite a bit more abuse than that.

I'll mention in passing that you should know if and how the chassis is grounded (or isolated by a transformer). Basic personal safety is important, especially if any of your test equipment is also grounded.

As a bonus comment, I'll mention that a radio with a 455 kHz IF will have an oscillator of its own, operating 455 kHz away from the receiving frequency. Sometimes you can use another radio to produce a signal in the one you are working on.

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