After a brief history of near-field antenna measurements with and without probe correction, the theory of near-field antenna measurements is outlined beginning with ideal probes scanning on arbitrary surfaces and ending with arbitrary probes scanning on planar, cylindrical, and spherical surfaces. Probe correction is introduced for all three measurement geometries as a slight modification to the ideal probe expressions. Sampling theorems are applied to determine the required data-point spacing, and efficient computational methods along with their computer run times are discussed. The major sources of experimental error defining the accuracy of typical planar near-field measurement facilities are reviewed, and present limitations of planar, cylindrical, and spherical near-field scanning are identified.

An overview of near-field antenna measurements

Reverberation chambers are becoming a popular alternative testing facility for a wide range of electromagnetic applications. Because of the statistical environment created inside a reverberation chamber, they offer a unique test facility. In particular, these chambers are ideally suited for performing radiated power measurements of either an antenna or device under test, and as such, it is possible to determine the efficiency of antennas. There have been several reverberation chamber techniques proposed over the years for measuring the antenna efficiency; however, these techniques require either the use of a reference antenna (i.e., an antenna with a known efficiency) and/or require the assumption that the two antennas used in the test have identical efficiencies. In this paper, we present three different approaches for determining both the radiation and total efficiencies of an unknown antenna that overcome these limitations and assumptions. We present a one-antenna approach, a two-antenna approach, and a three-antenna approach. We present measured data for three different antennas in order to compare these three approaches. We also discuss the uncertainties related to these types of measurements.

Reverberation Chamber Techniques for Determining the Radiation and Total Efficiency of Antennas

/pdf/non-planar-near-field-measurements-spherical-scanning-3x02tydyue.pdf

Non-planar near field measurements: Spherical scanning

In less than a decade since the mainstreaming of cellular wireless technology, spectrum has become saturated by data-intensive smartphones, driving the so-called spectrum crunch. As a solution, the wireless community is pursuing the use of alternatives to current wireless technologies, including multiple-input/multipleoutput (MIMO) antenna arrays that allow increased simultaneous transmission capacity; the millimeter-wave (mmW) spectrum (30-300 GHz) to alleviate the spectrum crunch in current frequency bands; and ultradense networks transmitting wide-band modulated signals to allow short-range, high-speed data transfer.

Measurement Challenges for 5G and Beyond: An Update from the National Institute of Standards and Technology

It is the objective of this chapter to review the progress in electromagnetic inverse scattering over the past three decades by placing major emphasis on the radar target imaging problem. The complete description of electromagnetic scattering processes implies polarization and to recover the descriptive parameters of a scatterer from the measured field given the incident field requires the knowledge of the target scattering matrices and their particular properties. In. radar target discrimination, identification and imaging use of the entire spatial frequency domain of the radar cross section must be made leading to various approximate approaches whose measurement inputs differ greatly from those used for the inverse problems in optics. We begin with an introductory section (Sect.7.1) providing general definitions of the electromagnetic inverse problems and their relation to other chapters in this volume. Next, we introduce in Sect.7.2 vector diffraction integrals and formulas both in frequency and time domains which are required to derive all components of the scattering matrices, show their relations, and briefly outline novel computer-assisted numerical methods. In Sect.7.3 properties of the radar cross section, its related scattering matrices and various radar target operators are derived placing major importance on optimal polarization descriptors, i.e., the cross polarization (maximum) and co-polarization (minimum) null pairs which in the radar case are of great use in the unique description of target and clutter. We then review pertinent inverse scattering theories in the various electromagnetic spatial frequency regimes (of the radar cross section) in Sect.7.4 and we will identify their limitations and polarization sensitivity. In Sect.7.5 we show how vector holography is intrinsically related to the electromagnetic inverse problem and how polarization plays a dominant role in electromagnetic imaging. Finally, in Sect.7.6 we summarize the state of the art, define still existing open problems, and we suggest new approaches by using methods developed in other physical fields.

Polarization Utilization in Electromagnetic Inverse Scattering

A new technique is described for determining power gain and polarization of antennas at reduced range distances. It is based on a generalized three-antenna approach which, for the first time, permits absolute gain and polarization measurements to be performed without quantitative a priori knowledge of the antennas. The required data are obtained by an extrapolation technique which includes provisions for rigorously evaluating and correcting for errors due to proximity and multipath interference effects. The theoretical basis provides a convenient and powerful approach for describing and solving antenna measurement problems, and the experimental method employed illustrates the utility of this approach. Examples of measurements are included which exhibit errors in gain as small as \pm0.11 dB ( 3\sigma ).

Accurate measurement of antenna gain and polarization at reduced distances by an extrapolation technique

An improved three-antenna measurement of polarization that greatly reduces the uncertainty due to phase measurement errors is described. This technique is used to calibrate polarization standards and probes used in near-field antenna measurements. >

/pdf/improved-polarization-measurements-using-a-modified-three-3qyisu4afy.pdf

Improved polarization measurements using a modified three antenna technique

Results of spherical near-field measurements on narrow-beam antennas

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Accurate measurement of antenna gain and polarization at reduced distances by an extrapolation technique

Improved polarization measurements using a modified three antenna technique

Results of spherical near-field measurements on narrow-beam antennas