RADIATION EFFICIENCY MEASUREMENT OF A SMALL ANTENNA USING THE WHEELERMETHOD

The antennas for mobile communication handsets have become smaller in shape, which has degraded their electrical performance significantly. To obtain a small but high-performance handset, it is important to inc rease the antenna efficiency. The important parameter for such a purpo se is the radiation efficiency which can be measured easily by the Whe eler method. This method is carried out based on the assumption that t he input power is equal to the power loss if the antenna is covered wi th a cap of a size on the order of a radian sphere. However, unless th is assumption holds, the computed Wheeler efficiency does not give acc urate radiation efficiency. When the radiation efficiency is actually measured by the Wheeler method, the measured values often have a dip f rom the theoretical value. In this paper, the cap is approximated by w ire grids and the Wheeler method is simulated on a computer. It is con firmed that the dip is not caused by the measurement method. It is sho wn that the cause derives from the invalidness of the presumption of t he Wheeler method that the internal reactive electro-magnetic field do es not change when a cap is placed. It is shown also that the Wheeler method can be used sufficiently even if the size of the cap is larger than the radian sphere. The limit of application is studied.