Absorption wavemeter
The Absorption wavemeter is a simple device for the measurement of radio frequency energy at different frequencies.
The device can be used by the users of radio equipment to check the approximate frequency of a strong signal source, and also to check the output of a transmitter for harmonics. Many radio amateurs use one to check the purity and frequency of their transmitters [1][2][3]. Also devices can be made which work in the same way that are able to detect mobile phones [4]. As an alternative a dip meter can be used.
HF and VHF
The most simple form of the device is a variable capacitor with a coil wired across its terminals. Attached to one the terminals of the LC circuit is a diode , then between the end of the diode not wired to the LC circuit and the terminal of the LC circuit not bearing the diode is wired a ceramic decoupling capacitor. Finally a galvanometer is wired to the terminals of the decoupling capacitor.
The device will be sensitive to strong sources of radiowaves at the frequency at which the LC circuit is resonant.
This is given by 
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. The coil is often outside the case of the unit so it can be brought close to the object being probed.
UHF and SHF
At the higher frequencies it is not possible to use lumped components for the tuned circuit. Instead methods such as stripline are used. One design for UHF and SHF is a resonant λ/4 (quarter wave) rod which can vary in length. Another design for X-band (10 GHz) is a resonator cavity which can be changed in length.
As an alternative for UHF, Lecher transmission lines can be used[5]. It is possible to measure roughly the frequency of a transmitter using Lecher lines.