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Near-field phase transformation for radiation performance enhancement and beam steering of resonant cavity antennas
thesisposted on 2022-03-29, 01:08 authored by Muhammad Usman Afzal
Resonant-cavity antennas (RCAs) are promising candidates in advanced wireless applications because of their moderate-to-high gain, simple conﬁguration,low and planar proﬁle, and lack of feed networks. This dissertation presentsnovel mechanisms to enhance directivity and other radiation characteristics andto steer beam of RCAs. Solutions to both challenges are based on near-ﬁeldphase transformation and have been demonstrated by designing metasurfacesfor this transformation. A classical RCA comprising of a patch antenna and anunprinted all-dielectric superstructure is considered as the base antenna to assessthe eﬀectiveness of the proposed solutions.To enhance directive radiation characteristics, a near-ﬁeld phase transformation methodology is developed to improve the uniformity of electric near-ﬁeldphase distribution. This is realized by designing several phase-correcting structures/surfaces (PCSs). This novel method employs electromagnetic (EM) simulators to accurately obtain a non-uniform phase distribution on the apertureof RCAs. Two diﬀerent types of PCSs prototypes were developed. The ﬁrstproof-of-concept prototype is an all-dielectric varying-height PCS with a steppedgeometry, which corrects the phase at discrete points on the RCA aperture. Simulation and measurement results of the classical RCA with the PCS indicatethat the PCS increases the peak directivity of the RCA by 9 dB (from 12.5 dBito 21.2 dBi) and enhances its uniform aperture area by 178%. As an extensionto this, a quasi-analytical approach is developed and is utilised to synthesise a continuous PCS for a circularly polarized (CP) RCA. The performance of thecontinuous PCS is evaluated through numerical simulations, and predicted resultsindicate that the improvement in radiation performance is comparable to whatwas achieved with the former dielectric PCS. The second PCS prototype was acompact, lightweight design, which was implemented using printed phase-shiftingelements. The planar PCS prototype is 57% lighter and is only 9% of the maximumheight of the all-dielectric PCS prototype. The RCA with the planar PCS has thepeak directivity of 20.5 dBi, which is 8 dB higher than that of without PCS.To address beam steering, a mechanism based on the rotation of a sawtoothtime-delay (STD) metasurface pair is presented. The metasurface pair is physicallylocated on the aperture of the RCA, in the near-ﬁeld region for the purpose ofphase transformation. Each metasurface was designed to create a STD in thepropagating electric ﬁeld so that the beam is tilted by an angle of 20o. Themetasurface pair is rotated concurrently and independently, to move the beamto any direction in a conical region with a 92o vertex angle. As a technicaldemonstration, only a few angular positions were tested, however, the system iscapable of scanning the whole angular space. The peak realized gain of 19.4 dBiwas achieved in the broadside direction and the gain was 17.5 dBi when the beamwas moved to the farthest elevation angle of 46o. The conspicuous attributes ofthe proposed beam-steering system are its extremely low-proﬁle, which is 34.6 mmor 1.3λ0, its passive nature, and its ability to maintain planar geometry duringscanning.The proposed solution of near-ﬁeld phase transformation successfully demonstrates radiation performance enhancements and beam steering of RCAs. Althoughthe methodology was demonstrate for RCAs, this approach is applicable to otherplanar antennas and arrays.