Designing a Tightly Coupled Ultra-Wideband Millimeter-Wave Phased Array Antenna

Abstract

The advent of 5G and 6G wireless communication networks has significantly increased interest in Ultra-Wideband (UWB) phased array antennas operating in the millimeter-wave (mm-wave) spectrum, particularly within the 24-71 GHz range. Tightly Coupled Arrays (TCAs) have emerged as a promising candidate for planar, low-profile UWB phased array implementations.

Despite the wideband capabilities of Tightly Coupled Dipole Arrays (TCDA), a primary challenge remains designing compact, low-profile baluns suitable for integration within small unit cells at high mm-wave frequencies. This thesis addresses this challenge by intro- ducing a novel, planar Marchand balun-based antenna element that achieves an acceptable Voltage Standing Wave Ratio (VSWR) across the 22.6 to 72.6 GHz band while maintaining a thin profile suitable for mobile devices. In addition, a wideband bowtie element, utilizing the planar Marchand balun, is developed to cover two key 5G mm-wave frequency bands (24-29 GHz and 37-43 GHz). The performance of these elements is evaluated through the design, fabrication, and measurement of a small 2x2 antenna array.

In this project, in order to provide more space for accommodating the complex feeding network and reduce the cost of the array by reducing the number of required transceiver (T/R) modules, a thinned 8x8 TCDA is proposed in which only 22 elements are excited. By strategically terminating a part of unexcited elements (20 elements) and leaving the rest as open-circuit (22 elements), the array leverages mutual coupling to induce appropriate currents and maintain acceptable radiation characteristics. The scheme of excitation is obtained by investigating the current distribution among elements and with the help of optimization. Genetic Algorithm (GA) optimization is employed to determine the optimal excitation voltages and terminating impedances for broadside radiation and scanning up to 45 degrees in the E-plane.

An innovative, low-profile distributed circuit, comprising a cascade of striplines ter- minated to a resistance, is designed to realize the required terminating impedances. Ad- ditionally, a wideband matching network is developed for each excited port using the Real Frequency Technique (RFT) to ensure impedance matching across the operating fre- quency range and steering angles. The thinned 8x8 TCA, including the antenna array, terminating impedances, matching networks, and feedlines, is simulated using Ansys High Frequency Structure Simulator (HFSS). Simulation results demonstrate the effectiveness of the thinned TCDA concept, achieving acceptable input reflection coefficients at all ex- cited ports and maintaining desired Side Lobe Level (SLL) and directivity across the entire frequency range and steering angles.