A wide-beam microstrip patch antenna and antenna array ranging from 77 to 81 GHz are presented in this study for millimeter-wave applications. A substrate-integrated-waveguide probe-fed patch was developed. For beamwidth enhancement, two I-shaped parasitic elements were placed next to the main patch to establish a three-element subarray. The current directions of the parasitic elements were opposite to that of the main patch, leading to beamwidth enhancement in the E -plane radiation pattern. The proposed I-shaped parasitic element has the advantages of coplanar structure, compact size, and easy adjustment of the induced current. An array-factor method was employed to analyze the effect of the parasitic elements on the beamwidth of the main patch. Three amplitude distributions of the subarray were compared. The distribution of -0.1:1:-0.1 was proved to present the most beamwidth, which can be realized easily by the proposed I-shaped parasitic elements. To achieve both wide beamwidth and high gain, a 1 × 8 patch antenna array with the I-shaped parasitic elements was proposed. The measured results showed that the design can offer a gain of 10.74 dBi and a wide beamwidth of 138° at 79 GHz.
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