Technical Background

Radios are made up from four basic building blocks:  detectors, 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 "frequency 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.