Antenna-Cavity Coupling Effects on Measurement of Antenna Efficiency

The coupling between the cavity and the antenna is analyzed by placing a small antenna inside the cavity. It is seen that at the resonant frequencies of the cavity, the input resistances of the antenna attains values two or three orders of magnitude higher than that at frequencies removed from resonance. A field-circuit equivalence of this coupling system is derived using full wave analysis. We show that the input resistance of the antenna measured at resonant frequencies of the cavity is not merely the loss resistance desired in determining the antenna efficiency but is actually the sum of the loss resistance of the antenna and the coupling resistance of the cavity. The results of numerical computations are in reasonable agreement with measurements for a short wire antenna.

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