Electromagnetic and Structural Comparison of Ultra-Wideband Antenna Radomes

Abstract

This work examines the electrical performance of a series of dielectric radome panels whose thickness is determined from a set of common loads. Specifically, existing radome designs on NASA’s Gulfstream V (GV) aircraft are used for sizing to derive radomes sized with fiberglass and quartz using both monolithic and sandwich designs. The radome fairings currently on NASA’s GV are primarily fabricated from S-2 glass/epoxy and include a small vent panel for mounting a 2 – 18 GHz Snow Radar antenna and a new outer moldline fairing for two MCoRDS antennas next to the Snow Radar antenna. A 2 – 18 GHz Ultra-Wideband (UWB) Snow Radar is particularly sensitive to the design of its protective radome due to its microwave operating frequencies. Though the Snow Radar horns are only located over the small vent covers, the MCoRDS panels are also analyzed over the 2 – 18 GHz range to provide a wider sampling of thicknesses. The goal of this work is to examine the existing and alternative radome designs for this UWB radar system to examine the system’s electrical performance in the 2 – 18 GHz band and determine if the electrical performance improvements justify the added cost of designing radomes with quartz over S2-glass fibers. Structural finite element analysis is performed to determine new laminate designs for each of the panel solutions. While the mechanical properties of the S2-glass/epoxy material are widely available, information regarding the quartz material is limited to manufacturer provided properties. Given that these quartz properties are not equally conservative to those used for S2-glass, a mechanical testing program is conducted. A small amount of quartz material was donated for this project; however, the material was beyond its shelf-life. Regardless, the material testing provides a possible lower bound on the mechanical properties, acknowledging that the amount of material provided allowed for an insufficient sample size for a proper statistical analysis of potential process variables. The tested coupon tensile strength and Young’s modulus is ~70% and ~75%, respectively, of the provided manufacturer data. Given the high levels of delamination found in the failed coupons (indicating poor resin flow), it is assumed that the tested mechanical properties are, in fact, conservative. The tested and manufacturer properties are both used in the sizing of the panels to provide bounds for the quartz panel solutions. Following the structural sizing the electrical performance of each new radome design is determined through theoretical analysis using the Boundary Value Problem (BVP) approach. The theoretical transmissivity calculated using the BVP is verified through anechoic chamber measurements. The electrical testing performed for this analysis demonstrates the validity of using the BVP method for preliminary analysis for radome designs. Data collected from anechoic chamber testing shows good agreement with the overall transmissivity response determined by the BVP solution. In general, the BVP is capable of determining the electrical performance of a potential radome structure within 1 dB. Discrepancies between the BVP solution and the measurements are primarily attributed to the small size of the test panels and their close proximity to the antenna. For the thinner vent cover design, the sandwich S2-glass exhibits the best performance with near unity transmissivity over the 2 – 12 GHz range. This represents the best UWB performance across all panels. As compared to the monolithic S2-glass and quartz designs, the sandwich S2-glass has upwards of ~2.5 dB and ~1.5 dB, respectively, improvement at some frequencies. The sizing of the MCoRDS radomes showed a thicker laminate is better suited for narrow band applications, for this instance in the Ku – band (8 – 12 GHz). The sandwich quartz panel has comparable transmissivity to the S2-glass sandwich over the 8 – 12 GHz band, however, the frequency band is shifted downward by about 1 GHz. Outside of the 8 – 12 GHz band, the quartz panel has ~ 1 dB improvement over the S2-glass panels (-1.5 dB vs. -2.5 dB). Given the significantly higher cost for the quartz and the relatively good electrical performance of the S2-glass sandwich design, the fiberglass sandwich design is recommended for 2 – 18 GHz radomes. The thin sandwich design has near unity transmissivity in the 2 – 12 GHz band, and is the closest to providing UWB performance over the band of interest. In general, the quartz sandwich design only shows modest improvements over a S2-glass sandwich design. For 2 – 18 GHz applications, these results suggest it would be better to design a smaller footprint with a thinner sandwich structure that is bound by stiffeners than to design a thicker sandwich without stiffeners. More specifically a symmetric sandwich made from four surface layers of S2-glass and core is recommended for the 2 – 18 GHz band.