Date of Award
Doctor of Philosophy (PhD)
Dejan S. Filipović
Zoya B. Popović
Taylor W. Barton
W Neill. Kefauver
Mohamed A. Elmansouri
The thesis presents the design, and development of novel wideband Simultaneous Transmit And Receive (STAR) antenna systems. A STAR or in-band full-duplex system has the potential to double the throughput of a communication channel, which is highly important for the next-generation wireless networks. Similarly, these systems could increase the effectiveness of Electronic Warfare (EW) and Support (S) operation, by facilitating spectrum/channel sensing while jamming. A self-interference (SI) phenomenon, where the transmitter disrupts its own receiver is a major challenge in the practical realization of any radio frequency (RF) system. High isolation (>130 dB) is often needed to overcome this SI. A common approach to achieve this high isolation is some combination of cancellation levels such as antenna, analog, digital and signal processing layers. The thesis focuses on maximizing the isolation at the antenna layer, which is crucial for the system implementation. This is attained by researching bi-static, monostatic, and quasi-monostatic architectures that do not rely on polarization multiplexing.
Bi-static configurations use separate TX and RX antennas. Hence, the SI can be minimized by increasing the separation between apertures, embedding high impedance surfaces (HISs), or by recessing the RX antenna inside the absorber, as demonstrated in this thesis. The advantages and limitations of each of these techniques are analyzed through full-wave simulations and measurements. High power capable, wideband, metallic quad ridge horn (QRH) antennas are first developed and bi-static, dual polarized STAR system is realized with them. Measured isolation >60 dB is demonstrated between the TX and RX apertures operating over 6-19 GHz and separated by 4λ at the turn on frequency. Isolation >70 dB is obtained in the 18-45 GHz bi-static dual polarized in-band duplex antenna system. Further, the influence of scatterers on system isolation is discussed.
Bi-static configurations are robust, and system isolation is less sensitive to the asymmetries in the geometry. However, they require significant area particularly when highly directive apertures are needed. When a bi-static approach is applied to reflector-based systems, the overall size of the system is often prohibitively large. Hence, a monostatic configuration is highly desired for a high gain system. In this thesis, a monostatic STAR configuration, operating from 4-8 GHz, is developed by feeding the designed circularly polarized (CP) reflector antenna with all-analog beamforming network (BFN) consisting of two 90° and 180° hybrids and two circulators. The BFN is arranged to cancel the coupled/leaked signal from the antenna and circulators, by creating 180° phase difference between the TX and RX reflected signals. Theoretically, with ideal devices this approach can provide infinite isolation. In practice, the isolation is limited by the electrical and geometrical imbalances. Nonetheless, using COTS components with noticeable imbalances, average isolation >30 dB is achieved with the fabricated system, which is on average 15 dB higher than the isolation obtained with a conventional circulator approach.
Finally, a quasi-monostatic STAR approach is proposed to address the limitations of bi-static and monostatic configurations. The demonstrated configuration can achieve 30 dB (on average) higher isolation than the monostatic reflector architecture with the same BFN components. The quasi-monostatic STAR antenna system consists of a parabolic reflector antenna for transmission, and a receiving antenna mounted back-to-back with the reflector feed. To increase the system isolation both the TX feed and the RX antenna are CP. Further, to achieve the same TX and RX polarization the TX feed is LHCP, and the RX antenna is RHCP. The LHCP fields from the TX feed undergo polarization reversal after bouncing back from the reflector, thereby, the TX and RX operate in the same polarization. Th
Valale Prasannakumar, Prathap, "Wideband Bi-Static and Monostatic Star Antenna Systems" (2019). Electrical Engineering Graduate Theses & Dissertations. 36.