Analysis on the Dips of the Radiation Efficiency in the Reflection Method Using the Transmission Line Model

For measuring the radiation efficiency of the small antenna, the Wheeler method can be used because of its simplicity and accuracy [1]. It makes possible a wideband measurement of the radiation efficiency except resonant frequencies of its shielding. At these frequencies, dips of the radiation efficiency can be observed because the loss of the shielding is dominant over the loss of the antenna [2]. In the reflection method [3], the shielding is replaced by a straight waveguide and two sliding shorts so that the dips of the radiation efficiency can be observed as well as the Wheeler method for the same reason. To facilitate the mechanism by terms of simple expressions, we consider the waveguide and two shorts as a transmission line which is terminated by resistive loads in place of the shorts. That is, the loss of the shielding is expressed by the resistance at the terminations [4]. The transmission line model can predict not only the center frequency of the dip but also the frequency response of the estimated efficiency if the normalized resistance is well adjusted [4]. However, our previous paper assumes that the antenna be ideal. That is, when the S parameters of the corresponding two-port network are introduced, the relation of |S11| = |S22| = 0 and |S21| = |S12| = 1 is assumed. In this paper, the mechanism of the dips will be revealed in terms of the transmission line model with no assumption of the ideal antenna. According to our result, the estimated efficiency can be decomposed into the true antenna efficiency and the efficiency in the part of the waveguide and the shorts. The total estimated efficiency has possibility to have its dip in conjunction with the dip in the latter efficiency. This is the reason why the dips are observed in the total estimated efficiency. Of course, the antenna efficiency is independent of the dips in the latter efficiency. Moreover, this paper clarifies the fact that dips are observed when the center of the circle on the latter efficiency is extremely offset from the origin as well as its radius approaches to zero. This means that the value of the standing wave in the waveguide is null at the position of the antenna so that the loss of the shielding is dominant.