To rephrase and to apply my question to an example:
Why is it popularly understood that when two radios are close together (e.g. like in two cars during a traffic jam) they interfere with one another? Since a receiver is only passively intercepting radio waves, and not actively sending them, then why does the radio of one car interfere with the radio in the car that pulls along side of it during traffic? Thanks for your lengthy replies.
If this does happen it’s probably because the radios aren’t in compliance with FCC standards for stray emissions from electronic devices.
Radio receivers aren’t strictly passive. All of them have an oscillator that generates a radio frequency. However this should never result in emissions that exceed the standards.
My guess is that it is much more likely to be the ignition systems of the cars that creates the interference.
I’ve never noticed that happening. You can get weird effects from multipath as your antenna moves, or nearby objects move.
I’m guessing that you’re next to somebody who is using a small transmitter to connect his iPod to his radio and it’s broadcasting on the frequency you’re tuned to.
Also some satellite radios have an RF modulator that transmits a weak signal to the car’s FM receiver in the same way.
Most radios today are what are called superheterodyne receivers. The math might be a bit much for some, but wikipedia has a decent article describing how they work:
These type of receivers generate a radio signal, but as David said, it shouldn’t be strong enough to interfere with another radio in a nearby car. I have however seen a very cheap home stereo interfere with a radio in the next room that was trying to receive a weak signal.
Interference in cars comes typically from one of two sources. Either someone has an RF modulator for their Ipod, cd player, satellite radio, or whatever, or the car has certain types of high performance spark plugs installed which generate a large amount of radio frequency noise.
It’s not popularly understood, but radio receivers are not a passive device. Let’s use AM as an example for simplicity.
“Superheterodyne” often radios have an intermediate frequency of 455kHz (the AM band is generally 600kHz to 1600kHz); that is all radio signals are converted to this frequency at one point via the converter and oscillator stages of the radio. Other types of superheterodyne radios have different intermediate frequencies but all of them “transmit” as such. This interference you note is a good test to see if a radio is working or not - as it is sure evidence the oscillator is oscillating.
It’s additive though - say you are listening to WSM at 650kc - add 650 + 455 = 1105. In this instance, two radio receivers in close proximity - a radio tuned to 1100 may well be affected by the radio tuned to 650. A similar effect can be observed with FM, though the IF or intermediate frequency is 10.7mHz. All of the devices that convert MP3 or other audio files for use in a car radio will exhibit this same principle.