— indispensable test gear for microwave mavens
The need to know the operating frequency of an amateur signal in the X-band assignment can be met with an adjustable cavity wavemeter and a detector. The cavity can be adjusted so that it accepts a small amount of power from the transmitter, and the detector, which is coupled loosely to the cavity, "sees'" this energy and converts it to a small current that will operate a micro-ammeter, which serves as a resonance indicator.
The wavemeter must
have some sort of dial which can be calibrated in frequency. The accuracy of this calibration will depend upon many factors, such as the temperature stability of the cavity, the resettability of the adjusting system, and the exactness of the calibration reference. There are others, but these are the three which will concern us while constructing a suitable unit. The other problems will become evident as we move along.
A cavity wavemeter is a parallel resonant circuit (see Fig. 1). You have used these devices as traps in the output of your transmitters on the lower frequencies and in many other circuits of your equipment. The operation of a microwave unit isn't different from those. The Q of the circuit in
Phofo A. Cavity wavemeter for X-band.
Fig, 7. Parallel resonant equivalent circuit of wavemeter.
which it is used and the Q of the wavemeter itself make the difference for a eali-brated frequency reference. A microwave cavity such as the device we are about to construct can have a working Q of nearly 3000, The working Q will depend upon the loading of the cavity and several other factors.
An important limitation to recognize is that the device we are making will put you within the band imits, but it is not an absolute frequency meter,
A cavity wavemeter circuit diagram might look like the circuit shown in Fig. 2. A difference in the circuit when using waveguide for the transmission line will be, for example, that the coupling loop in Fig. 1 is a
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