Electrically Small Spheroidal Loops Wound on Hollow Ferrite Cores

The complete electrical properties of a small loop wound on a hollow spheroidal ferrite core are calculated using quasistatic theory. Closed-form expressions are derived for all of the elements of an equivalent circuit. It is shown that not only may the bandwidth be increased by the use of a ferrite core, but also that by redistributing the same volume of ferrite over a slightly larger spheroid as a thin layer, the bandwidth may be increased even more. Core losses are computed for a typical ferrite. Although the primary objective is to simplify the design of a transmitting antenna for use in the lower part of the HF band, 2 to 8 MHz, the theory presented is equally valid for the design of receiving antennas and for other frequencies.