At lower frequencies the cavity becomes excessively long and at higher frequencies the dimensions become very small.
The arrangement illustrated is of the transmission type. It has two coupling loops, one for feeding power into the line through a co-axial cable, and one for coupling a crystal rectifier indicator to the oscillators in the cavity.
Tuning is accomplished by varying the position of the short-circuit plunger, using a lead screw. Provision is made for accurately measuring the resulting displacement of the plunger.
At shorter wavelengths, where the total line length available is one wavelength or more, frequency can be measured by determining the displacement Δl between adjacent maxima using the equation
where Δl is in metres.
The accuracy of co-axial wavemeters can be quite high (values of 0.05%). This accuracy is a result of the fact that the Q of a co-axial resonator is very high.