Advanced radial line slot array antennas for wireless and satellite communications

Radial line slot arrays (RLSA) antennas are low-profile, thin, planar, highly efficient antennas with the advantage of having a simple and single feed. Despite their attractive features, conventional RLSAs suffer from extremely narrow band- width and high cost, which make them undesirable for modern wireless communication systems. This dissertation introduces several novel RLSA design strategies to significantly reduce the production cost, enhance gain and gain-bandwidth, increase aperture efficiency of beam-tilted antennas, and reduce side lobe levels. Conventional RLSAs use expensive commercial dielectrics to slow the waves in radial waveguides that increase production costs. These dielectrics additionally limit the use of RLSAs in high-power and space-borne applications, which need materials with high dielectric breakdown and radiation hardness. The thesis ad- dresses this issue by choosing a radically different design principle to make the antenna fully metallic in an air-filled radial waveguide. This design prevents the onset of grating lobes without having to insert any dielectrics or other wave- slowing structures, thus significantly reducing the manufacturing complexity, cost and weight of the antenna. The new antenna needs only two at metal sheets, one slotted and the other is the ground plane, resulting in a significantly lower prototyping cost. A circularly polarized (CP) RLSA was designed and proto- typed at Ka-band. The total antenna thickness was only 0.43λ0. It has a gain of 36 dBic, an aperture efficiency of 54%, and a large 3 dB axial ratio bandwidth of 22.9%.

The thesis also presents a novel method to significantly increase the 3 dB gain bandwidth of RLSAs from less than 10% to over 30%. In this new approach, the conventional uniform radial transverse electromagnetic (TEM) waveguide is replaced with a non-uniform radial TEM waveguide in such a way that the condition for the coherence of broadside radiation is satisfied over a much wider range of frequencies by different sections of the RLSA, leading to a multi-fold increase in the 3 dB gain bandwidth of the antenna. At the same time, good axial ratio and 10 dB impedance bandwidth were demonstrated together with an extremely high gain-bandwidth product (GBP) per unit area (A). The predictions have been validated with experimental results of a prototype with a measured 3 dB gain bandwidth of 27.6%, a measured gain of 27.3 dBic, 3 dB axial ratio bandwidth greater than 31.1% and a 10 dB return loss bandwidth greater than 34.8%. The area of the antenna is much smaller than previous RLSAs with similar gains and an extremely high GBP/A of 88.

Conventional beam-tilted RLSAs do not effectively use antenna aperture due to very sparsely arrayed slots, resulting in low gain and poor efficiency. The third major contribution of the thesis is to substantially increase the aperture efficiency of beam-tilted RLSAs. The method is realized by truncating the slot layout in the RLSA aperture and removing the aperture where slots are sparsely distributed. Consequently, a reflector is placed to force the propagation of the TEM wave in the direction of slots and feed is moved to the edge of the TEM waveguide. Compared with the classic design strategy where more than half of the physical aperture has sparsely distributed radiating slots, the proposed method achieves more than 5.5 dB gain and 20% more aperture efficiency from the same physical area. The new concept and predicted results are validated using a prototype antenna with the beam tilted to an angle of 25o.

In order to improve the radiation characteristics and address typical issues of side lobes in RLSAs, several CP RLSA antennas were investigated and analyzed. Optimal design parameters were recommended to achieve tapered amplitude distribution in the near-field region while maintaining a nearly uniform phase distribution to reduce side lobe levels in the far-field. The amplitude tapering is implemented by monotonic variation of slot length with radius and a slot coupling analysis. A prototype was fabricated, which suggested an excellent agreement between the predicted and measured results. The sidelobe levels of the measured prototypes are less than -20 dB with directivity of 34.3 dBic and an aperture efficiency of 48% at 20 GHz. The thesis also proposes the concept of using extremely narrow reflection cancelling slots (RCSs) in linearly polarized (LP) RLSAs to improve the impedance matching at the antenna input. It was found that the aperture of LP RLSAs is densely packed with radiating slots (RS) and reflection cancelling slots (RCS) to improve inherently lower return loss, which results in the physical overlap and design complexity. A new LP RLSA design was presented and it addresses this issue using narrower RCS, which provided excellent return loss and substantially high impedance bandwidth. The measured result of a prototype has achieved an impedance bandwidth of 74.3%, which is 50% more than the conventional LP RLSAs.