Date of Award

Spring 5-15-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering and Computer Science

Advisor(s)

Lee, Jay K.

Keywords

Base Station Antenna, Characteristic Mode Analysis, Common Mode, Fabry-Perot Cavity Antenna, Mutual Coupling, Shared Aperture Antenna Array

Subject Categories

Electrical and Computer Engineering | Electromagnetics and Photonics | Engineering

Abstract

This dissertation proposes antenna design techniques which suppress mutual coupling in densely populated dual-polarized broadband multi-band base station antenna (BSA) arrays for improved radiation characteristics. Array face densification without the proposed techniques leads to undesirable interactions between arrays and radome cover. This research is supported by extensive full-wave electromagnetic simulations, characteristic mode analysis, equivalent circuit models, and array theory. The results are validated through measurements of the radiation patterns and scattering parameters of fabricated prototypes.Upon excitation of a mid-band (MB) array, an electrically long low-band (LB) parasitic, necessary for broadband matching of the baseline LB dipole, exhibits dipole-like induced currents which degrade the MB radiation patterns. The currents are suppressed through parallel plate series capacitance integrated along the length of the LB dipole arms to lower the dipole self-impedance, enabling broadband matching using an electrically short LB parasitic. The proposed technique improves MB radiation pattern symmetry, cross-polar radiation (CPR), sidelobe level, and gain. Upon excitation of the proposed LB dipole, the baseline MB dipole balun feed stems exhibit induced common mode currents, which radiate as electrically short monopoles over a conducting ground plane, that degrade the LB radiation characteristics. These currents are suppressed by integrating parallel plate series capacitance along the length of the balanced ports of the MB balun. The proposed balun improves the LB beamwidth stability, CPR, and gain stability. The proposed serially loaded LB dipole and MB dipole balun feed stem are implemented simultaneously in a six-foot BSA in which the system performance improvements are verified. Upon excitation of a high-band (HB) array in a tri-band array environment, the dielectric radome behaves as a partially reflecting surface. The reflected waves interfere with the HB excitation, resulting in significant boresight radiation attenuation in the upper part of the HB operating band. A Fabry-Perot cavity antenna (FPCA) operating in the second resonance mode (N=1) is proposed to improve this attenuation through increasing the boresight directivity. Upon HB excitation of the proposed FPCA, circular-disk MB parasitics, necessary for broadband matching of the neighboring MB array, exhibit dipole-like induced currents. The induced currents radiate as dipoles which reduce the HB half power beamwidth (HPBW) in the lower and middle part of the HB operating band. These currents are suppressed by introducing electrically thin radial slots in the baseline MB parasitic. The radial slots change the characteristic modes and lower their modal significance, which eliminates the secondary radiation sources. The proposed technique increases the HPBW in the lower and middle part of the HB operating band. These proposed design techniques provide new tools to antenna designers which allow for the densification of BSA arrays for use in next generation cellular networks, while minimizing radiation pattern degradation otherwise present in baseline BSA designs using existing techniques.

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