Design of Planar and Curved Spiral Antennas Covered by 2-D Conformal Thermal Protection System

The conformal thermal protection system (TPS) is widely used to protect embedded antennas of hypersonic vehicles in high-temperature environments. An equivalent analytical equation for the radiation performance of a general circularly polarized (CP) antenna loaded with the conformal TPS is presented. Compared with planar TPS, conformal TPS leads to more complicated impacts on the antenna's radiation performance, mainly manifested by main lobe splitting, gain reduction, and axial ratio (AR) deterioration. The extent of this effect is related to the curvature radius of the conformal TPS. A method to design a nonuniform impedance ground (IG) replacing the PEC ground to repair the degradation of the radiation performance by the conformal TPS is proposed. A broadband Archimedean spiral antenna loaded with a conformal TPS and a nonuniform IG is fabricated and measured to verify the proposed theoretical method. According to the simulation and measurement results, the antenna has an AR of less than 3 dB from 3 to 12 GHz and an AR beamwidth of 60° on average by loading the nonuniform IG. The main lobe splitting at 4 GHz is repaired, and the gain is improved by ~2.5 dB. A fully conformal model is proposed and fabricated with a conformal Archimedean spiral antenna and a conformal IG, which significantly reduces the antenna profile while using an asymmetric Archimedean spiral antenna resulting in a gain improvement of ~1 dB from 10 to 12 GHz.