The simplest of these uses an inductor and capacitor to form a tank circuit. In a tank circuit energy sloshes back and forth between an inductor and capacitor.

Figure 1: The system and the operation principles. We have carried out two types of measurement. In the first, a response analysis, the LC tank circuit is driven near-resonantly by a low-frequency ...

There are more details about how the tank circuits work in those videos. You can also see what the output looks like on a spectrum analyzer. You can hear what that transmitter sounds like, too.