A Circuit Model for Spherical Wheeler Cap Measurements

The efficiency of an electrically-small antenna may be measured conveniently by placing it in a conducting Wheeler Cap. This paper presents a composite equivalent circuit that models an antenna both inside a spherical cap and also radiating into free space. The circuit is used to show how the measurements are affected by the antenna size, efficiency, and polarization (TE or TM), the cap radius, and experimental errors. Methods for using a single cap or multiple caps are analyzed and compared. Antennas with typical properties are represented in a general form rather than by either series or parallel circuits as is commonly done. Loops and spherical dipoles are used as examples.