||Frequency selective surface (FSS) is able to modify the state of polarization of continuous waves, for example, the phase and polarization of the electric and magnetic fields. In this dissertation, design and analysis of FSSs are first introduced. Two applications of FSSs are then demonstrated. The first application is the design of a wearable radar in which the clutter from the environment is suppressed by a single-layered FSS. The second application involves a transmit-array design whose transmission loss is reduced by employing a multi-layered FSS along with a novel discrete Jones matrix.|
In the first application, the frequency selective surface was used as a polarization converter that converted linear polarization to circular polarization, thus achieved the polarization diversity. In this work, a radar tag enabling wearable vital sign sensors with high sensitivity, low power consumption, and mobility is designed. The proposed radar system is designed to be bi-static: first with a self-injection-locked oscillator (SILO) tag with frequency selective surface attached to a subject’s chest, and second with a handheld receiver at a remote location. Embedded in the tag is a linearly polarized (L.P) antenna for the SILO and with a frequency selective surface to facilitate the linear-to-circular polarization conversion. The circular polarization becomes the tag’s output signals to the receiver for Doppler detection and achieving the polarization diversity. Owing to polarization diversity, the tag exhibits strong immunity to the moving clutter caused by the receiver. In the experiment with a 2.4-GHz ISM band prototype, the tag delivered reliable cardiopulmonary information when there was a relative movement between the tag and the receiver.
In the second application, frequency selective surface is used in a transmit-array antenna as a phase controller. Phase corrections are needed when a planar FSS is used to receive spherical waves in the space. Since the FSS-based phase controller does not use transmission lines, including conducted and radiated losses, for phase corrections, the approach effectively improves the transmit-array antenna gain. Further, a high-gain horn antenna as a source of radiation was first designed. The bandwidth was about 18% in Ku band, and a gain of more than 10 dB was achieved. And the phase controller consisting of a large number of small unit cells, about 0.39 λ×0.39 λ, was designed and realized. The phase controller achieves phase corrections by analyzing the waves as a sum of two orthogonal components. The backpack sized phase controller employs more elements in the same area, as a result of using smaller unit cells. The denser elements achieves a higher gain since the phase corrections were more accurately compensated. In FSS design, novel discrete Jones matrix calculation was formulated, which allows us to obtain accurately the phase corrections for each unit cell location. The improved transmit-array design achieved a gain of about 26.1 dB, and aperture efficiency of about 61%, which are better by 30% to 40% compared to other reported results.