I think of a varactor as a dynamically varying capacitor: as the AC applied to it changes, the capacitance changes – so if this varactor is in a circuit with tuned circuits for input, output and idler, it can double or triple the input at the output.
I needed to double the 121.5 MHz emergency locator transmitter frequency to 243 MHz so I used the 2N3904 collector junction as the varactor diode.
