A tunable EBG absorber for radio-frequency power imaging

Absorption characteristics of a tunable electromagnetic band-gap (EBG) absorber are analyzed, which is designed to capture 2d radio-frequency (RF) power distributions incident on the absorber surface. The EBG absorber has lumped resistors interconnecting the mushroom-type surface patches to absorb the incident RF power at the resonance frequency where the EBG structure exhibits a high-impedance feature. The absorbed RF power distribution is measured by directly detecting the amounts of RF power consumed by the individual resistors. Varactor diodes are inserted in parallel with the resistors for tuning the resonance frequency of narrowband absorption. The absorption characteristics at normal incidence are evaluated in detail based on an equivalent circuit model which exactly explains the frequency behavior of the surface impedance of the tunable EBG absorber observed in EM simulation. The small resistance existing in the varactor diode makes it difficult for the surface impedance to be matched with the incident wave impedance (i.e., for a high absorption to be achieved) over a wide range of resonance frequency. A means to improve the absorption performance of the tunable EBG absorber is examined.