Frequency-Shifted Reflection of Electromagnetic Waves Using a Time-Modulated Active Tunable Frequency-Selective Surface

Abstract

An electromagnetic (EM) wave modulation technique using actively tuned frequency-selective surfaces (FSSs) to shift the frequency of EM waves reflected from the surface is presented. The reflected EM wave contains significant components at multiple sideband frequencies along with the original incident frequency. The time-modulated complex-valued reflection coefficient can be appropriately designed to highly suppress the original incident frequency component and significantly boost the sidebands. Such a time-modulated surface can help in evading Doppler radar where the object velocity is detected from the Doppler shifted frequency of received signal. The presence of multiple frequency sidebands and suppression of the original frequency component can make Doppler radar imaging very difficult or even impossible when the modulation frequencies are dynamically managed. Here, the reflected EM wave from an FSS is modulated by applying a time-varying periodic biasing voltage across varactor diodes in the FSS. Variation in the bias voltage changes the reflection coefficient of the FSS and thereby modulates the reflected EM wave. A prototype of the designed modulator surface is fabricated to verify the modulation performance experimentally. Here, two modulation schemes, sinusoidal and saw-tooth biasing, have been demonstrated to show symmetrical and asymmetrical power distributions in the sidebands.