Equivalent circuits for electrically small antennas

A method for finding the equivalent circuit of an electrically small antenna (ESA) is introduced. The equivalent circuit is based on the ESA´s frequency dependent polarizability. Using this equivalent circuit, the input impedance, radiation quality factor (Q), and radiation efficiency (η_eff) can be found. The circuit also provides physical insight into the fundamental operation of a small antenna. Equivalent circuits for a negative permittivity sphere and a negative permittivity shell are reported.