Carl F. Stubenrauch

A brief history of near-field measurements of antennas at the National Bureau of Standards

The US National Bureau of Standards (NBS) played a pioneering role in the development of practical planar near-field antenna measurement techniques. A brief history is presented of that role, which began with theoretical studies to determine corrections for diffraction in a microwave measurement of the speed of light. NBS contributions to the development of nonplanar near-field measurement theory and practice are also described. >

The effect of random errors in planar near-field measurements

Expressions that relate the signal-to-noise ratio in the near field to the signal-to-noise ratio in the far field are developed. The expressions are then used to predict errors in far-field patterns obtained from near-field data. A technique for measuring the noise in the calculated far-field pattern by calculating the spectrum in the evanescent region from a single-dimensional oversampled scan is also described. >

Phase Measurements of Electromagnetic Fields Using Infrared Imaging Techniques and Microwave Holography

Complex (magnitude and phase) measurements of the near field of a radiating antenna over a known surface (usually a plane, cylinder, or sphere) can be used to determine its far-field radiation pattern using near-field to far-field Fourier transformations. Standard gain horn antennas are often used to probe the near field. Experimental errors are introduced into the near-field measurements by mechanical probe position inaccuracies and electrical probe interactions with the antenna under test and probe correction errors. A minimally perturbing infrared (IR) imaging technique can be used to map the near fields of the antenna. This measurement technique is much simpler and easier to use than the probe method and eliminates probe position errors and probe correction errors. Current IR imaging techniques, which have been successfully used to rapidly map the relative magnitude of a radiating field at many locations (mXn camera pixels per image captured) over a surface, however, suffer from an inability to determine phase information. Absolute magnitude and relative phase data can be obtained by empirical or theoretical calibration of the IR detector screens (used to absorb the radiated energy over the measurement plane) and by using techniques from microwave holography. For example, magnitude only measurements of the radiating field of an antenna at two different locations (over two different surfaces) in the near field of the antenna can be used to determine its complex (magnitude and phase) far-field radiation pattern using plane-to- plane (PTP) iterative transformations. This paper discusses the progress made to data in determining both magnitude and phase information from IR imaging data (IR thermograms); thus, enabling near-field and far-field measurements of antenna patterns using IR thermal imaging techniques.

Calibration of microwave antenna gain standards

Techniques for precision calibration of microwave antenna gain standards are described with discussions of applicability and associated uncertainties. Included are the three-antenna, extrapolation, swept-frequency, and near-field techniques.

Quantitative images of antenna patterns using infrared thermography and microwave holography

In this paper, we apply microwave holography to infrared (IR) thermal images of electromagnetic (EM) fields to measure near‐field and far‐field radiation patterns of antennas. The phase of the field is retrieved from IR thermograms (magnitude‐only thermal images of the field) measured in the near field of the antenna using microwave holography. One method to extract the phase is to use an iterative plane‐to‐plane (PTP) two‐dimensional (2D) holographic phase retrieval method. After the phase is recovered, the near‐field thermographic/holographic data can be processed to determine the complex intensity (magnitude and phase) of the field at any distance in front of the antenna under test (AUT). Of particular interest is the far‐field radiation pattern of the antenna or, especially for phased array antennas, the aperture source‐plane distribution. Numerical simulations were performed to determine the feasibility, accuracy, and sensitivity of these IR thermographic/holographic phase‐retrieval techniques. The advantages and disadvantages of the technique are also discussed. To demonstrate the feasibility of the technique, actual IR thermograms were obtained at the Air Force Research Laboratory/Rome Research Site (AFRL/RRS) in Rome, (New York) and holograms were derived from a simple planar 6 × 6 phased array patch antenna using the PTP technique. The phase‐retrieval results are presented and compared with the known results for this antenna, as measured on the near‐field range at the National Institute of Technology and Standards (NIST) using standard hard‐wired probes. The agreement between the results for this antenna is very good. Published 2001 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 11, 210–218, 2000

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.

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.

Far-field antenna patterns determined from infrared holograms

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 measured and calculated antenna sidelobe coupling loss in the near field using approximate far-field data

Computer programs presently exist to calculate the coupling loss between two antennas provided that the amplitude and phase of the far field are available. It is shown that when this far-field information is not available it is possible to specify approximate far fields from a knowledge of the sidelobe of each antenna along the axis of separation and the electrical size of each antenna. The ENVLP computer program developed by M.H. Francis and A.D. Yaghjian (ibid. vol.AP-34, p.952-5, July 1986) was modified for this purpose. Measurements of near-field coupling loss between two moderately sized microwave antennas were made to determine the effectiveness of using approximate sidelobe level data instead of the detailed far fields. Comparison of the measured and computed coupling indicates that the use of approximate far fields gives an estimate of the coupling loss with an uncertainty of about +or-5 dB. >

International Intercomparison of Electric-Field Strength at 100 MHz

This paper discusses an international intercomparison of electric field strength at 100 MHz. Laboratories in four countries participated in the intercomparison. Measurements from each of the laboratories fell within a range of +0.75 to -0.5 dB with respect to the overall average. The transfer standard used in the measurement is described and the details of the results are presented.