Can an audio amplifier be used to transmit or receive radio signals?

Question

Is it possible to make an audio amplifier transmit radio signals when connected to its output with a ferrite coil or any other loop antenna and make the signal receive with another audio amplifier?

I have this question in my mind because of these reasons:

1. An audio amplifier can respond to frequencies from 20 Hz to 20 kHz and when connected with a speaker, the coil in the speaker resonates as per the output frequency and if that can be connected to a loop antenna instead, it should radiate radio waves too.

2. While experimenting with a tape recorder many years ago, I encountered one strange thing. While I was adjusting the screw of the tape head, the screwdriver accidentally touched one of the head connectors that was connected with the audio amplifier. And I was able to hear an AM radio station. I guess the screwdriver was working like a whip antenna and the tape head was working like a loading coil.

So, based on these facts, I am assuming that a audio amplifier is able to transmit or receive radio frequencies without the need of any carrier wave. This would be done using direct induction just like how a matching transformer works between an audio amplifier and a speaker.

Answer 1

Short answer: not without a carrier wave:

• Transmitting an audio-modulated radio frequency is fairly easy
• Receiving that signal to the exclusion of other signals, and driving a speaker with recovered audio is much harder.

If you transmit audio with light (from a red LED for example) the circuitry is more straightforward. One should be wary of saying "a terahertz carrier is AM-modulated with audio" because a photodiode detector does not put out a signal proportional to electric field or magnetic field of the carrier - a photodiode is inherently a power detector: its output current is audio, which must be amplified greatly.
Nevertheless, transmitting audio by amplitude-modulating a LED's current can transmit over considerable distance - https://www.modulatedlight.org/
Pointing is a pain, and it is best done at night.

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If the process must be done with a carrier using radio frequency, one should look for a radio receiver first, because that's the hard part.
AM or FM radio receivers might be a good choice. All that's left to do is build a transmitter.

Transmitting AM signals on the AM broadcast band (550 kHz to 1600 kHz in North America) is made a bit more difficult because efficient antennas are large. FM broadcast band allows much smaller-size efficient antennas.

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And I was able to hear an AM radio station. I guess the screwdriver was working like a whip antenna and the tape head was working like a loading coil.

YES on the antenna, NOT on the tape-head inductance. The tape head's very small audio signal requires considerable amplification. If you over-drive the audio amplifier with a large radio frequency, it can act as an AM detector, which is amplified by following stages.
Audio designers attempt to discourage such things from happening. You'd need to attach a very efficient antenna to the tape-head's audio output. And you'd probably have to modify the audio amplifier's input to defeat the designer's attempt to reduce radio frequency disturbances.

Answer 2

You can transmit audio this way, but this is not strictly transmitting a radio signal. It works with transformer coupling, as you say; there's no need for the Ground and Antenna symbols on your diagram.

I've tried this and it works quite well.

• Make a large loop of wire, a few metres across, with a couple of turns, and connect it to the loudspeaker output of the amplifier. Keep the volume low so as not to damage it, or add a series resistor.

• Then make a smaller loop, maybe 10 cm diameter, 100 turns, and connect it to the microphone input of an audio amplifier.

• Place the small loop within the large loop and you will hear the sound from the large loop amplifier.

This method of coupling is still used in "Hearing Induction Loop" to Telecoil telephones, which have a coil in the earpiece of the handset to couple directly into a special coil in the hearing aid. When the hearing aid is set to "T" mode it amplifies only the magnetic signal, turning off its microphone, so the user can hear only the telephone, not the street noise etc. T-coils are required in all telephones in the US.
is the T-coil symbol.

It was also used for Through-The-Earth communications in mines, where a coil would be used to send baseband audio into mines, and even a few hundred metres between users. This was quickly replaced by SSB with a carrier of 80 kHz.

Disadvantages of transformer coupling as a means of transmitting audio:

1. Short range. The fields from the loop decay as $1/r^2$ so the power density decays at $1/r^4$. Travelling electromagnetic waves leave the source forever so their power density decays only by $1/r$ (as you'd expect from something spread out over the surface of a sphere).

2. Limited band. There's only one audio baseband, so you will share it with every other user. Not usually a problem because of the limited range.

3. Noise! The atmosphere is quite noisy at these frequencies, so you'll hear lightning and other things, but also tons of artificial noise from electronics, motors, and a strong 50/60 Hz signal from the magnetic fields from the mains wiring in your house. The last one may not be such a problem as your receive loop is much more effective at higher frequencies, and you can filter it out.

Answer 3

A short answer is not really.

An audio amplifier can respond to frequencies from 20 Hz to 20 kHz and when connected with a speaker, the coil in the speaker resonates as per the output frequency and if that can be connected to a loop antenna instead, it should radiate radio waves too.

The coil in the loudspeaker is not resonating with anything, at least that is not the intended or desired behavior, to produce acoustic vibrations. Resonance is often a "magic" word, which leads to a lot of specious statements, so please be careful with that word. If you can't explain the behavior without using that word, it deserves a critical review.

If you use a large enough loop, you can produce a magnetic field of the audio signal. However, that is just that, the magnetic field. The field dies off inversely proportional to the distance squared. That will not lead to a radio signal or electromagnetic wave that propagates which requires both magnetic and electric fields.

Those local magnetic fields were used in the past in wireless guidance devices (used in museums, etc.). Each room had different loops to induce the magnetic field. I'm not sure if those are still in use somewhere, but most likely, virtually all have been replaced by networked or storage-based devices by now.

I said "not really," and not "exactly no." If you study WWVB transmitter site in Fort Collins, CO and its antenna, you know what it takes to transmit 60kHz signal. Some other standard time broadcasts (e.g., JJY of Japan) use 40kHz in addition to 60kHz. So, 40 kHz is the lowest radio frequency commonly used (and I know of). German DCF77 in Mainflingen uses 77.5kHz despite having a bigger land than Japan. (I think there was 20-30kHz broadcast station in Sweden that went QRT fairly recently. Submarines also use very low frequencies with towed antennae in the water.) Anyway, those are the lowest commonly used frequencies, and you can study their antenna to appreciate what it takes to transmit at those frequencies. If you have a lot of land, you could presumably transmit at the top of the audio amplifier's frequency range, but you would still require a license, but that certainly won't be an amateur radio license.

Your tape recorder experience is actually something else. You were receiving some 1MHz radio signal, probably on the order of 100uV to 10mV range depending on the field strength, which was AM detected (rectified) by the transistor used to amplify and equalize very weak signals coming out of the playback magnetic head.

Now, this is also something else. If you used a class-D amplifier, especially if the amp omits the output LPF or you modified/removed it and fed a large enough antenna, you can actually transmit long/medium/short wave radio. That is because class-D operates by switching mode, and the switching frequency and its harmonics serve as carriers. The receiving end requires a detector to demodulate the audio signal. I would expect the demodulated signal to be heavily distorted because of the way class-D works. Caveat: You'll need a high-pass filter and impedance matching so that you don't blow the amplifier. Also, this is illegal. Just keep it to your conceptual simulation.

This is also something else, but peripherally related. There are JJY/WWVB emulator apps that run on tablets and smartphones. Those produce an audio frequency carrier one of whose harmonics come to 40kHz or 60kHz, modulated by the time code. The magnetic field (not radio wave) created by the loudspeaker is picked up by the radio-synched clock placed right next to it to automatically set the date/time where the JJY/WWVB signal is weak (such as US northeast).

Answer 4

SAQ Grimeton in was built 1922-1924 in Sweden and operates on 17.2 kHz. It still operates occasionally and has been heard all over Europe, and in Canada, USA, Brazil and Alaska. So yes, radio transmissions can be accomplished at audio frequencies.

In this case the carrier is 17.2 kHz and is mechanically generated, using a 500 HP motor driving a high speed generator running at about 80kW, although it is capable of 200 kW. The speed of rotation is modified slightly for keying which puts the very, very, large antenna out of tune which stops it from radiating. Other very long wave stations have been shut down in the past few decades.

There was a station in the UK "GBR" which operated on 16 kHz. In the original CD (not remastered version) of Mike Oldfield's "Tubular Bells" album, the 16 kHz CW signal is audible (not to me, but to someone younger with better hearing) as the GBR transmitter was close to the recording studio. Using appropriate SDR software like SDR#, the signal can be filtered and brought down to an audio frequency that is easier to hear.

So yes, radio waves can be transmitted at low frequencies and can travel a long way, but it requires a lot of power and massive antennas and so is not going to be an easy project. Using the SAQ example, even a very large antenna is going to have miniscule bandwidth, so transmission of audio is not going to happen. CW is probably the best you could do, and even then would probably need to be keyed very slowly on a short antenna with a big loading coil.

One interesting project I know has been done successfully is to use speakers and microphones capable of ultrasonic frequencies (up to 50kHz or so) and modulating a carrier with audio or data and sending it through a speaker to a microphone, even when the frequency is above human hearing range, although it would drive my dog crazy. That kind of thing is within reach of the average person who doesn't have a huge budget and a few hundred acres to play with. It has been done in GNU radio, for instance.