Radome Design and Curvature Effects on Antenna Performance, with Application to Radar Array Installation in Transonic Aircraft

Abstract

Significant and fast changes in the cryosphere have been observed and monitored using radioglaciology and remote sensing technology. A fuselage mounted Ultrawideband Multichannel Coherent Radar Depth Sounder (UWB MCoRDS) with eight Tx/Rx channels was proposed for measuring polar ice sheets to depths up to six kilometers. This radar system operates in the 150-600 MHz frequency range and would be integrated onto the DC-8 aircraft platform for remote sensing. The high speed of the aircraft (M=0.8) results in high design loads that restrict the size of the radome and can thus limit the capability of the radar system. The desire to increase the size of the array along the cross-track direction to improve system performance resulted in an increase in radome size of 73% spanwise and 52.5% chordwise compared to systems previously installed on the DC-8. This significant increase in size and mass required investigating methods to reduce the aerodynamic loading while minimizing negative effects on the performance of the UWB MCoRDS antenna. These methods include reducing the radome depth by decreasing the antenna ground plane distance, and conforming the antenna or ground plane to more aerodynamically efficient profiles. Two dimensional CFD analyses show that conforming the radome to a supercritical airfoil shape results in lower aerodynamic loads compared to only reducing the radome depth. Single antenna electromagnetic simulations indicate that variations in ground plane distance or the addition of curvature to the radiating/reflecting components affect the antenna performance differently depending on the frequency range and antenna parameter of interest. Changes of 0.59 in. (15 mm) or less in effective ground plane distance minimally impacts the return loss for the UWB MCoRDS antenna addressed herein. Smaller ground plane distances reduce the gain for f≤450 MHz (improves it for f450 MHz), but have negligible effects on the cross-track half power beam width (HPBW) in the 200-400 MHz range. Larger ground plane distances at the feed generally widen the main beam in both cross-track and along-track directions. Similar results are observed for either adding curvature to the ground plane or the antenna (in both cases the ground plane distance at the feed location is increased). Conforming the UWB MCoRDS antenna to a SC0010 airfoil curvature results in minimal changes to the simulated return loss, less than 0.5dB change in gain, and less than 8 degrees change in cross-track HPBW. These results were experimentally verified using a bare UWB MCoRDS antenna. However, a 1.93dB reduction in gain at 550 MHz was observed instead, and the main beam does not have a dip at nadir for frequencies higher than 475 MHz. Deviations from the simulation results, in these instances, are attributed to the use of ideal properties in simulations and possible errors introduced by the test set-up structure among other reasons discussed herein. By considering the antenna array performance rather than the single antenna element performance, conforming the antennas does not have noticeable effects in the normalized cross-track pattern and HPBW, but the single antenna effects are still evident in the array gain. The radar range equation for extended targets shows that higher frequencies are mainly affected by conforming the antenna due to large gain reductions in the 350-575 MHz frequency range. However, conforming the antennas to a supercritical airfoil shape is preferred instead of reducing the number of elements as the latter limits the radar system performance much more significantly. Negative effects on antenna performance caused by curving the antenna can be compensated, if needed, with variations in flight altitude, transmit power, and other signal processing techniques. The radome design for an eight UWB MCoRDS antenna array with a cross-section that maintains the curvature of the supercritical airfoil is proven to be aerodynamically effective and electromagnetically feasible, and is expected to result in improved structural performance due to significantly reduced drag loading.