Wavemeters are calibrated resonant circuits used to measure frequency. Although the accuracy of
wavemeters is not as high as that of heterodyne frequency meters, they have the advantage of being
comparatively simple and can be easily carried about.
Any type of resonant circuit may be used in wavemeter applications. The exact kind of circuit
employed depends on the frequency range for which the meter is intended. Resonant circuits consisting of
coils and capacitors are used for low-frequency wavemeters. Butterfly circuits, adjustable transmission
line sections, and resonant cavities are used in vhf and microwave instruments.
There are three basic kinds of wavemeters: the absorption, the reaction, and the transmission types.
Absorption wavemeters are composed of the basic resonant circuit, a rectifier, and a meter for indicating
the amount of current induced into the wavemeter. In use, this type of wavemeter is loosely coupled to the
circuit to be measured. The resonant circuit of the wavemeter is then adjusted until the current meter
shows a maximum deflection. The frequency of the circuit under test is then determined from the
calibrated dial of the wavemeter.
The reaction type derives its name from the fact that it is adjusted until a marked reaction occurs in
the circuit being measured. For example, the wavemeter is loosely coupled to an oscillator, and the
resonant circuit of the meter is adjusted until it is in resonance with the oscillator frequency. The setting
of the wavemeter dial is made by observing the output current of the oscillator. At resonance, the
wavemeter circuit takes energy from the oscillator, causing the current to dip sharply. The frequency of
the oscillator is then determined from the calibrated dial of the wavemeter.
The transmission wavemeter is an adjustable coupling link. When it is inserted between a source of
rf energy and an indicator, energy is transmitted to the indicator only when the wavemeter is tuned to the
frequency of the source. Transmission wavemeters are widely used in measuring microwave frequencies.
In figure 3-27, a typical cavity wavemeter is illustrated.