Radios are made up from four basic building blocks:
filters, amplifiers, and , sometimes, oscillators.
If one connects a sensitive headset to an antenna-ground system he may
hear some AC hum and atmospheric noise, but he won't hear any radio
frequency signals. However, if he adds a nonlinear
device, such as a diode, to the circuit he can hear, at least, the
stronger AM stations. The diode functions as a detector or
changer", as our British cousins call them, thaking advantage of the heterodyne principal
to mix the
signal carrier and
sidebands to produce signals in the audio range.