Dual-Band Dual-Polarized Ultra-Thin Flexible Antenna Array for mmWave Wearable Applications

Abstract

An ultra-thin flexible antenna array based on dualband, dual-polarized antenna elements is presented in this paper. In ultra-thin millimeter-wave antennas, maintaining high isolation between orthogonal polarizations across two frequency bands is challenging due to strong coupling through the patch structure and limited substrate thickness. To address this issue, a partial differential feeding technique is used, in which only one port is differentially excited using a wideband coupled-line phase shifter integrated with a T-junction. Additionally, the thin substrate limits the achievable bandwidth; therefore, parasitic elements are added around the main patch antennas to improve the bandwidth, which results in relatively large antenna elements with a size of 5.18 mm. This size is large compared to the free-space wavelengths at the operating frequencies of 28 GHz and 38 GHz, which are approximately 10.7 mm and 8.1 mm, respectively. As a result, forming an antenna array capable of beam scanning becomes challenging, especially at the higher frequency band. To overcome this limitation, an offset (staggered) arrangement of the antenna elements is employed to reduce the effective inter-element spacing. This configuration enables practical beam scanning performance, achieving scan angles of $\pm 45^{\circ}$ at the lower band and ±30° at the upper band. The array is implemented on a ${248}-\mu \mathrm{m}$ thick flexible printed circuit (FPC) substrate. The dual-polarized antenna operates simultaneously in the 27.5-28.7 GHz and 37-38.5 GHz bands.