High Q Resonant Cavities for Microwave Testing

TUNABLE high Q resonant cavity is a particularly useful tool for determining the over-all performance of a radar quickly and easily1.* Further, since it uses the radar transmitter as its only source of power, it can be made quite portable. When a high Q cavity is provided with two couplings, one for the radar pickup and the other to an attenuating device, crystal rectifier and meter which serve for tuning the cavity not only can an indication of over-all performance be obtained but other useful information as well. For example, the transmitter frequency can be measured; calibration of the crystal affords a rough measure of the transmitter power; and an analysis of the spectrum can be made by plotting frequency versus crystal current. This information is of particular importance in radar maintenance. The Q required for this purpose is quite high, comparable to that obtained from quartz crystals in the video range. For this reason, such cavities have many additional possibilities for use in microwave testing equipment and microwave systems. For example, they may form component parts of a narrow band filter, or be used as discriminators for an oscillator frequency control. Resonant cavities are of two general types--tuned and untuned. A tuned cavity is designed to resonate in a single mode adjustable over the radar frequency range. An untuned cavity is of a size sufficient to support a very large number of modes within the working range. Both are useful, but the tuned variety can give more information about the radar and hence has been more widely used.