Electromagnetic Design and Performance of a Conical Microwave Blackbody Target for Radiometer Calibration

Published

May 29, 2017

Author(s)

Derek A. Houtz, William Emery, Dazhen Gu, Karl Jacob, Axel Murk, Dave K. Walker, Richard Wylde

Abstract

A conical cavity has been designed and fabricated for use as a broadband passive microwave calibration source, or blackbody, at the National Institute of Standards and Technology. The blackbody is used as national a national primary standard for brightness temperature and used to pre-launch calibrate space-borne radiometers and calibrate a ground-based system to provide traceability amongst radiometric data. The conical geometry provides performance independent of polarization minimizing reflections and standing waves, and thus having a high microwave emissivity. The conical blackbody has advantages over typical pyramidal array geometries, including reduced temperature gradients and excellent broadband electromagnetic performance over more than a frequency decade. The blackbody is designed for use between 18 GHz and 230 GHz, between temperatures of 80 K and 350K, and is vacuum compatible. To approximate theoretical blackbody behavior, the design maximizes emissivity and thus minimizes reflectivity. A newly developed microwave absorber is demonstrated with cryogenically compatible thermally conductive two-part epoxy with magnetic carbonyl iron (CBI) loading. The complex permittivity and properties of different CBI loading percentages are measured, the conical absorber is then designed and optimized using geometric optics and finite element modeling, and finally the reflectivity of the resulting fabricated structure is measured. We demonstrate calibrated normal incidence reflectivity considerably below -40dB at all relevant remote sensing frequencies.

Citation

IEEE Transactions on Geoscience and Remote Sensing

Pub Type

Journals

Keywords

calibration source, calibration target, brightness temperature standard, microwave absorber, blackbody, radiometer calibration, hot load, cold load, permittivity, permeability

Citation

Houtz, D. , Emery, W. , Gu, D. , Jacob, K. , Murk, A. , Walker, D. and Wylde, R. (2017), Electromagnetic Design and Performance of a Conical Microwave Blackbody Target for Radiometer Calibration, IEEE Transactions on Geoscience and Remote Sensing (Accessed March 14, 2025)