Absorption wavemeter

An absorption wavemeter is a simple electronic instrument used to measure the frequency of radio waves.

It is an older method of measuring frequency, widely used from the birth of radio in the early 20th century until the 1970s, when the development of inexpensive frequency counters, which have far greater accuracy, made it largely obsolete.

The most simple form of the device is a variable capacitor with a coil wired across its terminals.

The device will be sensitive to strong sources of radiowaves at the frequency at which the LC circuit is resonant.

When the device is exposed to an RF field which is at the resonant frequency a DC voltage will appear on the terminals on the left hand side.