Why does an AM radio channel require any bandwidth at all? Why can’t it just transmit on a single, precise frequency?
In: 95
AM is something you can [experience in real life](https://youtu.be/b9UO9tn4MpI?si=PWaLv98qpzs38T95).
All sounds can be represented as a set of frequencies and intensities that vary over time. When you speak and make sound, the lowest frequency is centered at 0 Hz. We call this “baseband”, as it’s the fundamental signal itself.
AM is one of the simplest ways to represent sound, where it is essentially a way to shift baseband up to a very high frequency. Instead of 0 Hz, we instead have a powerful reference point, called a carrier. That’s all it really is, you mix your voice with some powerful pure tone RF signal, and that sends it up to effectively be transmitted by an antenna.
AM works off a phenomenon called heterodyning. Despite the complicated name, you experience it all the time. When you hear two tones that are really close (say, 445 Hz and 440 Hz), you can hear the difference as a 5 Hz wobble. That’s how you recover the signal from the carrier.
About that video, the cool thing about it is that the electric arcs themselves produce a carrier, and a sidebar. Imagine them as a ton of constituent waves with pure frequencies, each being decoded back to baseband by mixing with the strongest component: the carrier wave.
Now, we still haven’t answered your question. Why does it take bandwidth? If you look at the waveform itself, it “looks” like it’s a constant frequency, right? But! The caveat is that to change the intensity of that carrier, you need other frequencies. Adjust the volume means adding more constituent waves in a precise manner. Or, more simply put, “modulating with the baseband”. Those frequencies just so happen to shape the wave so that it’s overall appearance (called the “envelope”) looks like baseband. In reality, you can think of AM as just disguised baseband, which already requires bandwidth (as you know).
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