Extending Bandwidth of Antennas With Coupling Theory for Characteristic Modes

Basing on the characteristic mode theory, this paper develops the concepts of source-mode (SM) couplings and mode-mode (MM) couplings, which characterized by the modal weighting coefficients and the inter-modal coupling coefficients, respectively. It is found that SM couplings could be adjusted by MM couplings when feeding configuration remains unchanged. Therefore, just by properly adjusting mode-mode couplings, we can reduce Q factor of an antenna at its center frequency and hence its bandwidth could be extended. A new time-saving optimization method based on controlling mode-mode couplings is also proposed. Two examples (one is lossless and the other one is lossy) are given and analyzed in detail to show how to control mode-mode couplings in order to reduce Q factor at the center frequency and hence enhance -10 dB bandwidth.