No abstract

SUMMARYA thin electromagnetic band-gap (EBG) absorber is employed to capture the 2-d image of radio-frequency (RF) power distribution. The EBG absorber consists of an array of mushroom unit cells formed on a thin dielectric substrate with a metal backplane, and lumped resistors interconnecting the surface patches of the mushrooms. Around the resonance frequency at which the EBG structure acts as a high-impedance surface, the RF power incident on the surface is absorbed by the lumped resistors which are matched with the incident wave impedance. By detecting directly the amounts of power consumed by the individual resistors, an "RF power imager" can be constructed which captures the 2-d distribution of the RF power illuminating the EBG surface, where polarization discrimination is possible. The resonance (i.e., absorption) frequency is made tunable by adding varactor diodes in parallel with the lumped resistors. The EBG absorber tunable in the frequency range of 700 MHz-2.7 GHz is designed and fabricated, and its performance is evaluated by an equivalent-circuit analysis, simulation and measurement. It is shown that the small resistance of the varactors have a considerable effect on the absorption performance. RF power distributions radiated from a dipole antenna are successfully measured by a matrix of sensitive power detectors installed on the backside of the absorber. Using such an RF power imager, the power distributions of even impulsive RF signals and/or noises can be captured and visualized in situ and in real-time, while the electromagnetic environment is almost undisturbed by the EBG absorber.

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.