A Spherically Conformal Transmit-Array Antenna With High Gain and 2-D Steerable Beam for Air-to-Ground Emergency Communication Applications

This article proposes, for the first time, a spherically conformal transmit-array (SCTA) with high gain and 2-D steerable directional beams for unmanned aerial vehicle (UAV)-based air-to-ground emergency communication applications. The multifaced conformal antenna architecture consists of 5 × 5 identical-sized planar transmit-subarrays (TSAs), each integrating 6 × 6 cross-dipole-shaped elements of metasurface. When illuminated by a feed source, each TSA generates a distinctively oriented subbeam. By continuously rotating the feed source to illuminate the selected TSAs, the proposed SCTA synthesizes a high-directivity beam and achieves continuous 2-D beam steering capability and achieves continuous 2-D beam steering capability. A terrestrial-inspired spherical topology is proposed with strategic array arrangement, mapping all TSAs onto predefined latitude-longitude coordinates across the doubly curved surface. A vector synthesis method is proposed to obtain the beam direction distribution for all TSAs. Due to the distinct beam directions of the TSAs and their discontinuous spatial locations, abrupt compensation phases calculated at the inter-TSA boundaries cause severe performance degradation. To tackle this issue, a phase smoothing method is proposed to enhance the gain and suppress the sidelobe levels (SLLs). To validate the design, a prototype of the whole SCTA architecture was fabricated and measured. Experimental results have indicated the proposed SCTA exhibits a peak gain of 20.6 dBi at the 30-GHz band and a continuous 2-D beam steering range of ±15° with a 1.7-dB average steering loss, which will be very attractive for UAV-based emergency communication applications.