Application of the Perturbation Method in Some Microwave Measurements

The perturbation method is most suitable for the experimental determination of electromagnetic field patterns in closed regions. The analysis of Slater´s theorem proves that systematic errors as well as method errors committed while determining relative field distributions are of the order of 1 percent or less if the volume of the perturbing element, for instance a thin metal needle, is of the order of 10-6 - 10-7 of the resonator volume. In this case the corresponding change in the resonant frequency is of the order 10-4 - 10-5. In order to achieve high precision of measurements, a sensitive phase-equilibrium bridge and a digital frequency counter were used. Two problems have been discussed in this paper. The first concerns the experimental-mathematical method for the determination of the field pattern, which consists of measuring the field distribution in the plane of the iris and obtaining the field pattern in the whole resonator through analysis of the measured distribution. Using this method the field distributions of the H1,0,1 mode rectangular resonator with an iris were investigated and the congruity of the calculated results with the experimental ones was within a few percent. The other problem concerns the design of equivalent circuits for resonators with heterogeneities. On the basis of the known field distribution an equivalent circuit of the rectangular resonator in the H1,0,1 mode with a capacitive post was developed. With given geometrical dimensions, it was possible to calculate the resonant frequency with an accuracy of a few percent.