Effect of Geometry on the Reflectivity Spectrum of Radiometer Calibration Targets

The reflectivity spectrum of radiometer calibration targets with alternate geometries is studied in this letter. We have presented normal incidence plane-wave reflectivity of conical array and truncated square pyramid array in the frequency range [6,200] GHz. Irrespective of whether the pyramid cross-section is circular or square, a base-to-height ratio of 1:4 or better is required to achieve reflectivity of -50 dB or lower for frequencies higher than the first Floquet harmonic cutoff frequency. It was found that the variations in the reflectivity at high frequencies are less pronounced for conical arrays compared to square pyramids. This could make conical structures more useful for broadband calibration purposes. In the case of truncated square pyramids, a reduction in tip sharpness affects the reflectivity at high frequencies substantially. It was found that the use of MF117 or MF124 as coating material instead of MF112 can reduce the reflectivity by ~ 15 dB. The study concludes with the use of the unscented transformation to overcome time consuming Monte Carlo simulations in order to quantify the effect of material property uncertainties in computational electromagnetic simulations.