Determination of Antenna Q from the Refl ection - Coeffi cient Data

Equivalent circuits containing frequency-variable radiation resistances are postulated for small monopole and small loop antennas above a ground plane. The equivalent circuits are valid below the fi rst natural resonance frequency. By adding an external series inductance or a parallel capacitance, the antenna’s Q is evaluated with the use of an invariant form for the refl ection coeffi cient. The antenna’s Q obtained this way accurately predicts the maximum obtainable bandwidth for either a singleor double-tuned matched antenna. At frequencies above the fi rst resonance, the Q is evaluated by approximating the impedance or the refl ection coeffi cient with polynomials within a narrow frequency range.