Determination of Antenna Q from the Reflection-Coefficient Data [Antenna Designer's Notebook]

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 first 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 reflection coefficient. The antenna's Q obtained this way accurately predicts the maximum obtainable bandwidth for either a single- or double-tuned matched antenna. At frequencies above the first resonance, the Q is evaluated by approximating the impedance or the reflection coefficient with polynomials within a narrow frequency range.