Katherine MacReynolds

Standard radiometers and targets for microwave remote sensing

We describe the NIST effort to develop brightness-temperature standards for microwave and millimeter-wave frequencies. Results of preliminary measurements at 26 GHz are presented.

Intercomparison of Standard Gain Horn Antennas at

An intercomparison of two W-band (75-110 GHz) standard gain horn antennas of nominal gains 24 and 27 dB has been performed at the Korea Research Institute of Standards and Science (KRISS), National Physical Laboratory (NPL), and National Institute of Standards and Technology (NIST). The measurement parameters for this comparison are the power gain and complex reflection coefficient of the traveling standards measured at 75, 95, and 110 GHz, and, as an option, the swept-frequency power gain measured over the frequency range of 75-110 GHz with a finite frequency step. All participants performed the fixed-frequency antenna measurements, whereas the swept-frequency antenna measurements were carried out by the NPL and the NIST. This paper describes the comparison and its measurement results with uncertainties. Generally, the agreement between the results in all the measurements is within the uncertainty of each participant, except for fixed-frequency gain results of the high-gain antenna at 110 GHz, reflection coefficient results at some fixed frequencies, and swept-frequency gain results of the high-gain antenna in the low- and high-frequency regions of the W-band.

W

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.

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We describe a technique based on optical holography which allows determination of the amplitude and phase of an unknown antenna on a near-field plane from amplitude-only measurements. We measure the interference pattern between the fields radiated by the antenna under test (AUT) and a known reference field. The reference field is produced by a standard gain horn radiating at an angle to the AUT and positioned so that the peak of the radiation occurs approximately at the same place as the peak from the AUT. The fields are detected by a resistive screen which absorbs some of the incident energy and heat as a function of the electric field intensity distribution. We use an infrared camera to record the temperature distribution caused by the interference of microwave energy radiated by the reference and the AUT. Data are processed using an enhanced algorithm based on conventional holography for recovery of the complex near field. The new algorithm allows elimination of the spurious images commonly present in optical hologram readouts using illumination of the hologram with the reference wave.

Comparison of microwave black-body target radiometric measurements

An intercomparison of power gain for W-band (75 GHz to 110 GHz) standard horn antennas has been performed among KRISS, NPL and NIST. This paper describes the comparison and its measurement results.

Far-field antenna patterns determined from infrared holograms

A bi-lateral comparison of power gain for a V-band (50 GHz - 75 GHz) Cassegrain antenna and a standard horn antenna has been performed between KRISS and NIST. This paper describes the comparison and its progress so far. Measurement results will be presented at the conference.

Intercomparison of W-band standard gain horn antennas

A comparison of the planar, spherical and cylindrical near-field techniques was completed at the National Institute of Standards and Technology (NIST) for a Ku-band cassegrain reflector antenna. This paper discusses the measurement results for the near-to-far field measurements.