Ronald C. Wittmann

The receiving antenna as a linear differential operator: Application to spherical near-field scanning

The general receiving antenna is represented as a linear differential operator converting the incident field and its spatial derivatives at a single point in space to an output voltage. The differential operator is specified explicitly in terms of the multipole coefficients of the antennas complex receiving pattern. When the linear operator representation is applied to the special probes used in spherical near-field measurements, a probe-corrected spherical transmission formula is revealed that retains the form, applicability, and simplicity of the nonprobe-corrected equations. The new spherical transmission formula is shown to be consistent with the previous transmission formula derived from the rotational and translational addition theorems for spherical waves.

Spherical wave operators and the translation formulas

Translational formulas for both scalar and vector spherical wave solutions of the Helmholtz equation are developed in a straightforward manner using differential operator representations for the modal functions and well-known expressions for the scalar and dyadic free-space Greens functions. The expansion coefficients are given in compact integral or differential operator forms useful for analytic investigation. >

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.

Spherical near-field scanning: determining the incident field near a rotatable probe

A spherical near-field scanning algorithm is developed for determining incident fields inside a probes minimum sphere. This differs from the well-known spherical near-field scanning formulation which determines fields outside the sources minimum sphere. The scanner size depends on the extent of the region of interest and not on the extent of the (possibly much larger) source. The data can be collected using a standard roll-over-azimuth positioner. The practical implementation of this technique is discussed.<<ETX>>

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.

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.

Improved spherical and hemispherical scanning algorithms

A probe-corrected hemispherical-scanning algorithm has been developed which is applicable when the antenna under test radiates negligibly into its rear hemisphere. For a hundred-wavelengths diameter antenna, hemispherical scanning would be about three times more efficient computationally than prior full-sphere scanning algorithms. Improvements have also been made to full-sphere scanning, significantly increasing that algorithms computational efficiency.

An All-Metal, 3-D-Printed CubeSat Feed Horn: An assessment of performance conducted at 118.7503 GHz using a robotic antenna range.

Three-dimensional (3-D) printing is finding applications across many areas and may be a useful technology for antenna fabrication for cube satellites (CubeSats). However, the quality of an antenna produced using 3-D printing must be considered if this technology can be relied upon. We present gain and far-field pattern results for the feed horn of the radiometer payload of the CubeSat PolarCube. The corrugated feed horn is constructed from AlSi10Mg alloy and fabricated using powder bead fusion (PBF). Measurements were performed at the atmospheric oxygen line of 118.7503 GHz with the National Institute of Standards and Technology (NIST) Configurable Robotic Millimeter-Wave Antenna (CROMMA) facility in Boulder, Colorado. A comparison of these measurements to theoretical predictions provides an assessment of the performance of the feed horn.

Computation of causal characteristic impedances

We develop a numerical method of determining the magnitude of characteristic impedance required by causal power-normalized circuit theories from its phase using a Hilbert-transform relationship. We also estimate the uncertainty in the method.