Can an audio amplifier be used to transmit or receive radio signals? - Page 1
Forum section: electronics / rf-microwave

author: prabhatkarpe4
author_rank: Contributor
timestamp: November 15, 2024, 03:57:13 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 04:28:23 pm
content: What frequency do you intend to transmit?The US Navy operates a 2 MW transmitter at 18 to 24 kHz out of Cutler, Maine, to allow penetration below the surface of salt-water oceans.https://www.navy-radio.com/xmtr-vlf.htmYou seem to be confusing induction (where one coil induces a current in a nearby coil, in the "near field" of the coils) with wave propagation (in the far field of an antenna).Please go to your public library and read about these topics.

author: djacobow
author_rank: Super Contributor
timestamp: November 15, 2024, 04:35:37 pm
content: There is a 2200m (136kHz) ham band. This is well out of the range that most audio amps can deliver, but some hams do in fact use modified audio power amplifiers to operate in this band.It's a difficult band to participate in given the antenna requirements.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 05:07:36 pm
content: With respect to your experience touching a tape head connection:Tape heads, magnetic phono cartridges, and "dynamic" microphones inherently generate low audio voltages, and require high-gain amplifiers.A common problem with such audio preamplifiers is that if an RF voltage (picked up by your screwdriver if an AM transmitting station is not far away) is conducted to the input, non-linearity (bad connection or semiconductor action in the input device) can rectify this RF voltage and the demodulated signal is like any other audio voltage and will be amplified.With microphones and phono cartridges, where the cable between source and amplifier is not short, careful filtering to reject RF voltages is necessary, but your tape head runs through a short wire inside the box, and may not have RF filtering.Loudspeaker coils do not resonate at the audio frequency, although the electromechanical system (voice coil, magnet, and moving cone) will have a resonance (usually near the low end of its useful frequency response).Radiation is not the same as induction. If you make a huge loop antenna, you could drive it at 20 kHz and get radiation at 20 kHz for Morse-code transmission.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 15, 2024, 11:27:11 pm
content: Quote from: prabhatkarpe4 on November 15, 2024, 03:57:13 pmYour assumption is incorrect.The carrier wave is being demodulated by the diodes within the amplifier circuitry.Also your screwdriver may be creating a diode junction as well, which is amplified by the amplifier (time index @4:04)

author: jh15
author_rank: Frequent Contributor
timestamp: November 16, 2024, 02:15:56 am
content: Not radio, but fun with "transmitting" with a pa amplifier in the 60's. I had read where soldiers in enemy territory could stick a couple bayonets in the ground near phone lines and hear conversations.I connected the tube amp through some navy surplus xfmr and connected one lead to the house cold water pipe, and another to a metal rod or something stuck in the ground. I had those black high impedance headphones of the day and sticking the leads of them in the ground and hearing my music all over the yard.

author: LM21
author_rank: Regular Contributor
timestamp: November 16, 2024, 09:54:41 pm
content: First GSM phones could cause strong banging noise in amplifiers. Imagine being in a concert and one of the listeners got a call.It is possible to hear strong AM transmissions from a low quality amplifiers. But it is of very little use, you cant change the station, you can't listen to something else and it is not always there. I think FCC and CE certificates tell that amplifiers and radios are protected.

author: Whales
author_rank: Super Contributor
timestamp: November 16, 2024, 10:00:53 pm
content: Receive:yes, just needs long unshielded or unpaired wires. RF gets picked up on these wires, enters the amplifier, gets rectified and turned into audio band. I've had this happen before a few times, including on the mic input of a laptop. Many modern audio amps and opamps are designed to be "RF immune" specifically to avoid this problem.Transmit:yes, but less likely, very weak and almost certainly much lower than 1MHz.Scenario 1: If the output amplifier stage is oscillating due to instability at some high freq (eg 100KHz) then playing audio through the system might modulate that carrier waveform. I built an audio amp once that oscillated above hearing frequency, but I don't remember exactly what frequency it was.Scenario 2: Baseband audio radio gets transmitted by either your speaker coils or unshielded or unpaired wires going to the speakers. Hearing aids are designed to pick up baseband audio (this is how they receive announcements at train stations).

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2024, 10:11:08 pm
content: Scenario 2 is "near field", by "induction", not "radiation".

author: nali
author_rank: Frequent Contributor
timestamp: November 16, 2024, 10:16:18 pm
content: Yes, audio is regularly transmitted in this way although magnetically, not by radio.https://en.wikipedia.org/wiki/Audio_induction_loop