Abstract-This paper presents an approach for calibrating backscattering measurements from 860-960 MHz Ultra-High Frequency Radio Frequency Identification (UHF RFID) tags. An Sparameter model is formulated to relate diode switch and antenna input circuit parameters with the scattering performance of the calibration device. Measurements of modulated backscattered power agree with the model to within ±0.1 dB. Tag backscatter measurements can then be calibrated by comparing them to the reference signal. In an example testbed, the expanded uncertainty of these measurements is estimated to be ±0.4 dB, compared with uncertainties worse than -0.9 dB, +1.2 dB for methods that calibrate against radar cross section (RCS) standards in the same testbed.

Accurate characterization of the brightness temperature (T B) 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 T B 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 T B measurements.

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Although the theory is straightforward, practical implementation of spherical near-field scanning for evaluating test chambers presents some significant challenges. Among these are the selection of an appropriate probe and the difficulty in minimizing support-structure blockage. We report on recent NIST efforts to address these difficulties and present our most recent results.