Why does an AM radio channel require any bandwidth at all? Why can’t it just transmit on a single, precise frequency?
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The station frequency is what the tuning circuit in your radio locks on to and that’s called the carrier wave. The audio information ends up being broadcast on one or both sides of that carrier wave frequency because of how it is modulated (basically mixing the audio wave into/onto/with the carrier wave).
Depending on the quality of the broadcast, the bandwidth required to transmit the audio might be higher or lower. If there was no audio information at all modulated onto the carrier wave there would be a single narrow precise frequency with nothing either side.
If you have a good enough radio you can tune to the carrier frequency and adjust the bandwidth to hear what difference it makes. A narrow bandwidth reduces the higher frequency audio information, again because of how it’s modulated onto the carrier wave.
You can try it out here : http://websdr.ewi.utwente.nl:8901/
Scroll in and out and left and right on the waterfall view. It’s showing you a very wide spectrum of frequencies that the radio can tune into. Frequency is horizontal axis and time is vertical axis. Strong signals will show up as orange lines. Drag the yellow tuning thingy over to one of those lines. Then choose USB for upper side band (bandwidth to the right of the station frequency), or LSB for lower side band (to the left), or AM (both sides), or CW for just the carrier wave. Then play with the width of the filter.
You’ll see that some stations are broadcasting using different bandwidths. Ones that need a lot of audio fidelity like music stations use more bandwidth than something like a CB radio where it’s just speech that needs to be understood.
And that is the key thing to understand about bandwidth. Our hearing goes up to about 20khz, but to encode that wide of a frequency range onto a carrier wave would require 20khz of room either side of the carrier wave. And there’s only so much of the radio spectrum to go around. If each station required 40khz of bandwidth, plus some separation, we’d have far fewer stations.
The way things work now, AM stations get half of that to themselves. They get exclusive use of whatever frequency they’re transmitting their carrier wave on, plus 10khz either side. That’s the standard and law that we have settled on.
That might not be answering your specific question but I think it’s beyond an ELi5 answer to explain how radio wave modulation works. The thing to know is that the more audio information you want to broadcast – the more bandwidth you need – because it’s more information.
And AM radio is analog. The information is contained in constantly varying electrical voltages. It’s not broken up into packets that can be re-assembled at your leisure. It has to all be broadcast at the same time, received at the same time, and demodulated at the same time. So it always needs the full amount of bandwidth, whatever that might be.
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