Michael H. Francis

Near-field antenna measurements using nonideal measurement locations

We introduce a near-field to far-field transformation algorithm that relaxes the usual restriction that data points be located on a plane rectangular grid. Computational complexity is O(Nlog N) where N is the number of data points. This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable to other measurement approaches such as plane-polar scanning, where data are collected intentionally on a nonrectangular grid.

Near-field, spherical-scanning antenna measurements with nonideal probe locations

We introduce a near-field, spherical-scanning algorithm for antenna measurements that relaxes the usual condition requiring data points to be on a regular spherical grid. Computational complexity is of the same order as for the standard (ideal-positioning) spherical-scanning technique. The new procedure has been tested extensively.

Accurate determination of planar near-field correction parameters for linearly polarized probes

A procedure used by the US National Bureau of Standards (NBS) for accurately determining the plane-wave receiving parameters of both single- and dual-port linearly polarized probes is described. Examples are presented, and the effect of these probe receiving characteristics in the calculation of the parameters for the antenna under test is demonstrated using the required planar near-field theory. The planar near-field theory necessary to accomplish probe correction and to formulate probe parameter errors is presented in a concise and meaningful way to help understand when probe correction is or is not needed. >

HF RFID electromagnetic emissions and performance

We examined the emissions of commercial HF (high-frequency) proximity RFID (radio frequency identification) systems and the performance of a typical RFID system in the presence of electromagnetic (EM) interference. Some initial investigations into security and reliability were also performed. These investigations highlight detectability and readability of an RFID transaction at a distance. We performed measurements to determine the power radiated by some commercial systems and monitored the RFID transaction in adverse EM environments.

Millimeter-Wave Near-Field Measurements Using Coordinated Robotics

The National Institute of Standards and Technology (NIST) recently developed a new robotic scanning system for performing near-field measurements at millimeter-wave (mm-wave) frequencies above 100 GHz, the configurable robotic millimeterwave antenna (CROMMA) facility. This cost-effective system is designed for high-frequency applications, is capable of scanning in multiple configurations, and is able to track measurement geometry at every point in a scan. The CROMMA combines realtime six-degree-of-freedom optical spatial metrology and robotic motion to achieve antenna positioning to within 25 μm rms. A unified coordinated metrology approach is used to track all positional aspects of scanning. A vector network analyzer is used to capture amplitude and phase. We present spherical near-field measurements of the forward hemisphere of a 24-dBi standard gain horn at 183 GHz. Using the configurable scanning ability, two different scanning radii were used. Near-field data were taken at a 100-mm radius. Direct far-field measurements were also taken at 1000-mm radius. The E- and H-plane patterns are determined from the measurements and compared to theoretical patterns. We describe the system components of the CROMMA and the coordinated metrology approach used. An analysis of the positional repeatability and accuracy achievable is also presented.

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.

Planar Near-Field Measurements of Low-Sidelobe Antennas

The planar near-field measurement technique is a proven technology for measuring ordinary antennas operating in the microwave region. The development of very low-sidelobe antennas raises the question whether this technique can be used to accurately measure these antennas. We show that data taken with an open-end waveguide probe and processed with the planar near-field methodology, including probe correction, can be used to accurately measure the sidelobes of very low-sidelobe antennas to levels of -55 dB to — 60 dB relative to the main beam peak. A special probe with a null in the direction of the main beam was also used for some of these measurements. This special probe reduced some of the measurement uncertainties but increased the uncertainties due to probe-antenna interactions. We discuss the major sources of uncertainty and show that the probe-antenna interaction is one of the limiting factors in making accurate measurements. The test antenna for this study was a slottedwaveguide array whose low sidelobes were known. The near-field measurements were conducted on the NIST planar near-field facility.

Comparison of measured and calculated mutual coupling in the near field between microwave antennas

Measurements of near-field mutual coupling between two moderate sized microwave antennas were performed and compared to coupling calculated using recently developed computer programs. Required input data for the programs are the complex far-field radiation patterns of the antennas and various geometrical factors describing the relative positions and orientations of the two antennas. Measured and calculated coupling as a function of both transverse and radial displacement showed good agreement.

Comparison of ultralow-sidelobe-antenna far-field patterns using the planar-near-field method and the far-field method

The development of very-low-sidelobe antennas raises the question of whether or not the planar-near-field method can be used to accurately measure these antennas. Previously, scientists at several organizations showed that data taken and processed with the planar-near-field methodology, including probe correction, can be used to accurately measure the sidelobes of very-low-sidelobe antennas. This can be done to levels of -55 dB to -60 dB, relative to the main-beam peak. The present paper highlights these results, including a comparison of the far field, from the planar-near-field method, with the far field, found on a far-field range. The test antenna for the study was a slotted-waveguide array, the low sidelobes for which were known. The near-field measurements were conducted on the NIST planar-near-field facility.

Using Truncated Data Sets in Spherical-Scanning Antenna Measurements

We discuss the mitigation of truncation errors in spherical-scanning measurements by use of a constrained least-squares estimation method. The main emphasis is the spherical harmonic representation of probe transmitting and receiving functions; however, our method is applicable to near-field measurement of electrically small antennas for which full-sphere data are either unreliable or unavailable.