Efficiency characterization of small antennas

Electromagnetic coupling effects on the antenna in a conducting cavity are studied theoretically and experimentally. An equivalent circuit is derived for an antenna coupled to a shielding cavity via field analysis. A proof is provided for the validity of Wheeler\u27s assumption that the loss resistance of the antenna placed in the Wheeler\u27s cap is not changed by the presence of the cap if the dimension of the cap is appropriately large. This proof extends the validation of Wheeler\u27s assumption to a hollow cavity of arbitrary shape. It is observed in experiments that at the resonant frequencies of the cavity, the input resistance of the antenna attains values two or three orders of magnitude higher than that at frequencies away from resonance. It is shown via theoretical analysis that the input resistance of the antenna measured at resonant frequencies of the cavity is not merely the loss resistance desired in computing the antenna efficiency, but is actually the sum of the loss resistance of the antenna and the coupling resistance between the antenna and cavity. This coupling effect is demonstrated quantitatively by numerical computations for dipole and monopole antennas. The computational results for the input resistance are in agreement with the measured data. A method is proposed to avoid the cavity-antenna antiresonance in the measurement