Miniaturization techniques for cavity and MIMO antenna systems

Abstract

This thesis presents our novel design efforts for achieving compactness in antenna designs. In compact antenna designs, there are two major challenges we tried to solve in this thesis: isolation between antenna units and low-profile designs. Various isolation techniques have been reported in the literature to reduce mutual coupling between antenna elements in MIMO antenna systems. However, most isolation techniques only explored for linearly polarized antennas. Very limited researches were for circularly polarized (CP) antennas. The existing researches for CP antennas use either very large spacing between antenna elements or CP antenna elements with complex feedings. In this thesis, we have developed a new CP MIMO antenna with a very small spacing between elements. The antenna elements’ edge to edge spacing has been reduced to 0.06𝜆0 in this work. Two isolation techniques: defected ground slot and ground shorted stubs, have been employed to achieve over 20dB isolation with AR<3dB for the 2.5-2.55 GHz band. A simple off-center feeding scheme has been used to avoid complex feeding system and save the design space. The proposed novel antenna has achieved a maximum gain of 6.1 dBic. Several isolation techniques have been explored for MIMO dielectric resonator antennas (DRA) but most of them were only useful for the single band. Some dual-band isolation techniques were reported to use large spacing between antenna elements. The isolation techniques that use the metasurface as a superstrate usually place them far away from antennas. To overcome this drawback, in this work, a dual-band metasurface has been proposed. It is placed only 0.08λ0 away from the antennas with the center frequency at a lower frequency band. The dual-band metasurface unit cell consists of two copper traces of different lengths printed on both sides of a PCB. High isolation of more than 20 dB has been achieved for 3.4 GHz and 4.9 GHz 5G bands. Compact Fabry-Pérot cavity (FPC) antennas that employ artificial magnetic conductor (AMC) and electromagnetic bandgap (EBG) structures have a weakness in its low gain. Furthermore, conventional FPC antennas are based on energy standing waves and employ isotropic metasurfaces. To overcome these drawbacks, a new resonance condition (polarization standing waves) has been used to design an FPC antenna. A 16.27 dBi boresight gain has been achieved, and the height of the cavity is reduced to 𝜆/8. Frequency reconfigurable FPC antennas reported in literature used switching diodes with complex biasing circuits. The biasing circuits make antenna bulky and costly. To address these issues, a novel FPC antenna is proposed that uses the polarization standing wave (PSW) based new resonance condition for frequency tuning. This technique makes it possible to tune the frequency of an FPC antenna by simply rotating the top plate of the cavity. Using PSW based resonance condition, a frequency reconfigurable FPC antenna has been proposed that covers 60-66 GHz band. The antenna achieves maximum 20 dBi simulated boresight gain with limited variations over the whole tuning range.