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

[ofosot69]

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.

 

[mmckenna]

There are some guys that do this on —extremely— low frequencies. Like down in the tens of kilohertz range. I'll have to dig around and see if I can find the article on it. I believe they were using some sort of earth antenna, where the feedline was essentially connected to a deep ground rod.

 

[nd5y]

I heard that somebody used a PA amplifier connected to two ground rods, I don't know long or how far apart, and another person about 1/4 mile away was able to receive audio with some type of mic preamp connected to ground rods.

 

[jwt873]

There are audio frequencies, and radio frequencies. But there's no precise frequency that separates them.

I know of a a few stations that have transmitted in the 17 kHz range. NAA in Cutler Maine at (17.8 kHz) and SAQ in Grimeton. Sweden at (17.2 kHz). (SAQ is off the air and NAA is up at 24 kHz now running at 1.8 MegaWatts).

In the past, SAQ would occasionally fire up their transmitter for special events. You could connect an antenna to the input of a computer sound card and pick it up. There was Windows software available for this. (I tried but unfortunately I was never successful).

Edit.. Did a bit of digging after I posted this and found the windows software 'SAQrx' to use with a sound card. RX from 0 to 22 kHz. SM6LKM - SAQrx VLF Receiver

 

[RFI-EMI-GUY]

Yes you can do it the only difference is the transducer/antenna. For audio the frequency is converted to mechanical motion in the speaker. For radio the frequency is converted into electro magnetic waves . You can probably find some amplifier like DC coupled that will go well beyond 20 KHz with some modifications.

 

[spongella]

What an interesting question and excellent comments. I know this is in the weeds but it brings to mind optical transmitters/receivers such as those using infrared waves to transmit audio. However these are much more complicated circuitry-wise.

 

[RFI-EMI-GUY]

Various articles on cave communications using audio and VLF/ULF techniques.

HeyPhone Articles

 

[dlwtrunked]

There are audio frequencies, and radio frequencies. But there's no precise frequency that separates them.

I know of a a few stations that have transmitted in the 17 kHz range. NAA in Cutler Maine at (17.8 kHz) and SAQ in Grimeton. Sweden at (17.2 kHz). (SAQ is off the air and NAA is up at 24 kHz now running at 1.8 MegaWatts).

In the past, SAQ would occasionally fire up their transmitter for special events. You could connect an antenna to the input of a computer sound card and pick it up. There was Windows software available for this. (I tried but unfortunately I was never successful).

Edit.. Did a bit of digging after I posted this and found the windows software 'SAQrx' to use with a sound card. RX from 0 to 22 kHz. SM6LKM - SAQrx VLF Receiver

To be clear: Audio is audio and radio is radio. The frequency is not what distinguishes-this is why there is "no precise frequency that separates them". The spectrum of both is just measured as frequencies. NAA in Cutler is radio which is why no microphone will receive them. And your voice is audio which is why no radio will directly receive that. Hooking an antenna to the sound card to an antenna makes it a radio receiver (hooking a microphone there instead makes it an audio receiver), and the NAA transmission is not audio, it is radio. right at NA, you will not hear it with your ear despite the high power (I have been there several times). (By the way, SAQ did its traditional cw broadcast in December 2024.)

Saying "audio frequencies" means simply "frequencies of audio" (air vibrations) and saying "radio frequencies" means "frequencies of radio" (electromagnetic radiation). Similarly, I can say inches of wire and inches of rope, but it is meaningless to talk of where some number of inches turns rope into wire. The length of both can just be measured in inches.

 

[popnokick]

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.

The likely explanation for what turned your tape recorder into an AM broadcast receiver was something known as "direct rectification". It's the same thing that makes crystal radio sets able to receive strong AM broadcasts. And also has caused people to be able to receive a radio station from the fillings in their teeth when just the right conditions exist (filling acting as diode and in presence of strong radio signal).