ORTHOGONAL MODES IN A RECTANGULAR CAVITY
摘要
An approximate theory of the operation of a rectangular cavity with a symmetrical aperture is presented. With conducting bars on this aperture the two orthogonal modes inside the cavity are excited so that one mode called the working mode is to be perturbed by an external body near the aperture, while the other called the reference modeis not affected. By measuring the difference of the resonant frequencies of these two modes △f=f_1-f_2 and that of the external Q's of these two modes △Q=Q_(L1)-Q_(L2)we can determine two parameters of the property of the perturbing external body, for instance, the moisture content and the thickness of the external body. This method for measurement of the parameters characterizing the external body benefits from that we can utilize two modes so that we can have the self-compensation on the effect of external environment such as the changes in humidity and temperature (see Fig. 8 for the self-compensation). Thus an instrument manufactured on this principle will work in adverse industrial environment. We have actually built one in our laboratory, which works well. We have calculated the discontinuity impedance of the aperture with metallic grid bars to set up the complete equivalent circuit of the cavity resonator with an symmetrical aperture lined with metallic grid bars.