Frequency of an RF channel is best understood as the frequency of a carrier wave.
A carrier wave is a pure wave of constant frequency, a bit like a sine wave. By itself it doesn’t carry much information that we can relate to (such as speech or data).
To include speech information or data information, another wave needs to be imposed, called an input signal, on top of the carrier wave. This process of imposing an input signal onto a carrier wave is called modulation. In other words, modulation changes the shape of a carrier wave to somehow encode the speech or data information that we were interested in carrying. Modulation is like hiding a code inside the carrier wave.
Recall that any wave has three basic properties:
1) Amplitude – the height of the wave
2) Frequency – a number of waves passing through in a given second
3) Phase – where the phase is at any given moment.
There are different strategies for modulating the carrier wave. First, a user can tweak the height of the carrier. If an input signal’s height varies with the loudness of a user’s voice and then adds this to the carrier, then the carrier’s amplitude will change corresponding to the input signal that’s been fed into it. This is called amplitude modulation or AM.