High-Efficiency, Broadside-Radiating Parasitic Arrays Implemented With Gap-Coupled Periodic Structures Based on Seamlessly Integrated Inverters

Microstrip parasitic arrays typically generate tilted-beam radiation due to the intrinsic phase shift between adjacent elements through coupling. In this work, we propose a novel design method for broadside-radiating parasitic arrays, implemented using quantitatively designed gap-coupled periodic structures. These arrays effectively counteract the inherent phase shifts by integrating +90° K-inverters, ensuring in-phase excitation for each parasitic element. Moreover, the current magnitude of each element can be controlled by adjusting the impedance value of the K-inverter, enabling effective regulation of current distribution across the array and facilitating customizable sidelobe level (SLL) design. As a proof-of-concept, a 3.90λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>-length uniform linear array was designed at the target frequency of 4.91 GHz, achieving fan-beam radiation with a wide half-power beamwidth (HPBW) of 80.26° in the H-plane and a narrow HPBW of 12.93° (close to the theoretical 12.72° for a uniform current source of the same length) in the E-plane. Furthermore, a low-SLL parasitic array operating at 4.91 GHz was realized by customizing the K-inverter to achieve tapered current distributions, yielding a measured SLL of -22.92 dB. The array topology, operational principles, design guidelines, and detailed quantitative design processes are elaborated. Experimental results agree well with simulations, demonstrating highly efficient fan-beam radiation in a compact form factor, with a narrow radiator width of 0.065λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and a low profile of 0.026λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>.