Conference Paper

A tunable EBG absorber for radio-frequency power imaging

Grad. Sch. of Natural Sci. & Technol., Kanazawa Univ., Kanazawa, Japan
DOI: 10.1109/URSIGASS.2011.6050776 Conference: General Assembly and Scientific Symposium, 2011 XXXth URSI
Source: IEEE Xplore


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.

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    ABSTRACT: The thin radio-frequency (RF) absorber constructed with an artificial magnetic conductor (AMC) surface is used as a sensor array to measure incident 2-d RF field (amplitude and phase) distributions. The AMC surface employs a 2-d dense array of mushroom-type square metal patches on a dielectric substrate. Incident waves are absorbed by the lumped resistors interconnecting the metal patches on the surface, when they are matched with the incident wave impedance at the resonance frequency of the mushroom structure. A 2-d distribution of the amplitude and phase of the incident RF field is obtained by directly measuring those of the voltages induced on the individual resistors. The validity of this new technique of RF field measurement is evaluated using electromagnetic simulation. It is confirmed that the voltage induced on the resistor can be used to monitor the incident (absorbed) RF electric field. For a finite-sized absorber the measurement accuracy is degraded near the outer edge due to edge reflections. This technique is expected to be useful for capturing the snapshots of RF field distributions in situ, while the electromagnetic environment is almost undisturbed by the AMC absorber.
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