Derek A. Houtz

Reflectivity Study of Microwave Blackbody Target

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique is developed for this study by the use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures, along with the uncertainty analysis, are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well-documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this paper shows well-matched properties with a power reflectivity below -40 dB over the entire measurement band (18 to 26 GHz).

A finite element thermal simulation of a microwave blackbody calibration target

We introduce a method to determine the gradient between measured physical temperature and true radiating surface temperature of a passive microwave calibration target (load or blackbody). An empirical cooling-curve fit is employed to determine heat-transfer coefficients that then allow commercial finite-element software to solve for the physical temperature at the surface of the target. Only gradients in the direction parallel to the targets pyramidal structures are determined. Two target insulation thicknesses are investigated and a mean surface radiating temperature is determined. This surface temperature differs from the internally measured physical temperature by a maximum of 0.3 K in an ambient environment. Use of a thicker insulation assembly decreases this temperature bias by 0.1 K.

Extraction of Illumination Efficiency by Solely Radiometric Measurements for Improved Brightness-Temperature Characterization of Microwave Blackbody Target

We report our recent progress toward the development of microwave brightness-temperature (BT) standards. As one of the crucial parameters, the target illumination efficiency (IE) was traditionally determined from the relative antenna pattern. We propose a measurement technique to extract the target IE solely by the use of passive radiometric measurements for characterizing the BT of the blackbody radiator. Such a technique allows us to skip the complexities that are often encountered during the measurement and calculation of the antenna pattern. Taking advantage of the variable heating capability available on most blackbody targets, we varied the temperature of a heated blackbody target and ran a series of radiometric measurements when the target was separated at different distances away from the the antenna with the radiometer operating at a few frequencies. Our experimental results show excellent measurement accuracy on the IE, with uncertainty of about 1% at close separation distance between the antenna and the target. We further measured and computed the BT of the blackbody target at the locations where we had measured the extracted IE. The BT was slightly lower than the physical temperature of the target and exhibited 0.7 K to about 1 K uncertainty when the target was located no more than 1 m away from the antenna. A measurement uncertainty of 1 K already meets the accuracy requirements of some climate variables, and such results reflect a significant step toward the establishment of BT standards at microwave frequencies.

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

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 will be used as a national primary standard for brightness temperature and will allow for the prelaunch calibration of spaceborne radiometers and calibration of ground-based systems to provide traceability among radiometric data. The conical geometry provides performance independent of polarization, minimizing reflections, and standing waves, 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 and 230 GHz, at temperatures between 80 and 350 K, and is vacuum compatible. To approximate theoretical blackbody behavior, the design maximizes emissivity and thus minimizes reflectivity. A newly developed microwave absorber is demonstrated that uses cryogenically compatible, thermally conductive two-part epoxy with magnetic carbonyl iron (CBI) powder loading. We measured the complex permittivity and permeability properties for different CBI-loading percentages; the conical absorber is then designed and optimized with geometric optics and finite-element modeling, and finally, the reflectivity of the resulting fabricated structure is measured. We demonstrated normal incidence reflectivity considerably below −40 dB at all relevant remote sensing frequencies.

An Improved Two-Port Transmission Line Permittivity and Permeability Determination Method With Shorted Sample

A modified measurement technique and nonlinear least-squares solution method is introduced for determining complex permittivity and permeability in transmission lines. In addition to a two-port S-parameter measurement, a one-port measurement of the shorted sample is employed. For low-loss samples, material property determination using the traditional method can be plagued by resonances when the sample thickness is an integer multiple of the guided half-wavelength in the sample medium. The introduction of the shorted reflectivity measurement reduces the large uncertainties inherent in the resonance effect, and increases the likelihood of determining the correct zero in the fitting of the material properties. The short-circuited sample S11 measurement has an increased magnitude compared with the standard two-port measurement, which greatly reduces the uncertainty of this term. The resulting complex permeability and permittivity values obtained with this method are smoother improved solutions, and have lower uncertainty. Results are presented comparing the traditional Nicolson-Ross-Weir solution and National Institute of Standards and Technology iterative method with the proposed technique for a cast epoxy and a ferrite-loaded microwave absorber sample in WR42 waveguide. We also investigate finite element simulated S-parameters of a fictional material and compare the derived material parameters obtained with the proposed method to the “true” permittivity and permeability values.

Realization of a standard radiometer for microwave brightness-temperature measurements traceable to fundamental noise standards

We describe and demonstrate a standard radiometer for making microwave brightness-temperature measurements that are traceable to fundamental noise standards. The standard radiometer is based on a National Institute of Standards and Technology (NIST) waveguide radiometer for 18-26.5 GHz, fitted with an antenna to measure radiated power. The fraction of the antenna pattern subtended by the radiating target is determined by anechoic-chamber measurements in which we vary the temperature of the target and measure the received power. Sample measurement results with uncertainties are presented. The typical standard uncertainty for a brightness temperature of around 340 K is about 1 K. The approach should be extendable to other waveguide bands where NIST has radiometers and standards.

Comparison of microwave black-body target radiometric measurements

Accurate characterization of the brightness temperature (TB) of black-body targets used for calibrating microwave remote-sensing radiometers includes many inputs: antenna pattern and loss, target temperature, target emissivity, mechanical alignment, and radiometric TB measurements, all of which must be calibrated against physical standards. Here, we describe measurements made using several black-body targets and two different antennas within the WR-42 (18 to 26.5 GHz) waveguide band. Uncertainty estimates are also shown for the retrieved target TB measurements.

An investigation of antenna characterization techniques in microwave remote sensing calibration

We compare three methods of quantifying illumination efficiency (IE). The ratio IE describes the contribution of energy emitted from a blackbody target to the total energy measured at an antenna aperture in a free-space microwave calibration target radiometric measurement. Measurements are compared at three frequencies: 18 GHz, 22.5 GHz, and 26 GHz. An antenna pattern integration method is compared with a recently developed target-temperature fitting method. These two experimental approaches are also compared to a computational antenna pattern simulation. Results show that the simulation agrees with the experimental fitting method more closely at far-field distances, whereas the antenna pattern integration and experimental fitting method agree at closer distances.

Extraction of reflectivity from microwave blackbody target with free-space measurements

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote-sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique was developed for this study by use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this study shows well matched properties with a power reflectivity below −40 dB over the entire measurement band (18 GHz to 26 GHz).

Reflectivity studies of passive microwave calibration targets and absorptive materials

We report on the characterization of blackbody reflections as a part of the recent progress on the development of brightness standards for microwave remote sensing at National Institute of Standards and Technology (NIST). Three blackbody targets at variable temperatures used for airborne and/or satellite systems along with an aluminum plate were measured in terms of their reflection coefficients by horn antennas in connection with a vector network analyzer (VNA) in the WR-42 waveguide band. Precision measurements of reflection are needed for blackbody emissivity computation to check against the brightness temperature measurement of blackbody targets. All experiments were conducted in two distance ranges by free-space methods in an anechoic chamber. Preliminary results show negligible reflections from the calibration targets, indicating near ideal blackbody characteristics in the measured frequency range.