Achieving Impressive Global Positioning and Stability in a High Fidelity Antenna Measurement System

Author: Jacob Kunz, Eric Kim

Highly accurate antenna measurements can require precise alignment and positioning of the probe antenna to the antenna under test. The positioning of the antenna during acquisition can involve the movement of several simultaneous axes of motion. This places a global positioning accuracy requirement on the positioning system. To achieve precision in global positioning and alignment, an understanding of dominant error factors such as load induced deflection/resonance, thermal deflection, positioning error sources and mechanical alignment tolerances is essential. This paper focuses on how global accuracy and stability were achieved, addressing these factors, on a recently delivered large far field antenna measurement system. The system involved eight axes of positioning with the ability to position 950 lbs. antenna under test 5.94 meters above the chamber floor achieving 0.84 mm and 0.027 degrees positioning accuracy relative to the global range coordinate system. Stability of the probe antenna after motion was within 0.076 mm.

Non-Ideal Quiet Zone Effects on Compact Range Measurements

Authors: David Wayne, Jeffrey A. Fordham, John McKenna

Performance requirements for compact ranges are typically specified as metrics describing the quiet zone's electromagnetic-field quality. The typical metrics are amplitude taper and ripple, phase variation, and cross polarization. Acceptance testing of compact ranges involves field probing of the quiet zone to confirm that these metrics are within their specified limits. It is expected that if the metrics are met, then measurements of an antenna placed within that quiet zone will have acceptably low uncertainty.

Various methods for determining the uncertainty in antenna measurements have been previously developed and presented for far-field and near-field antenna measurements. An uncertainty analysis for a compact range would include, as one of its terms, the quality of the field illuminating on the antenna of interest. In a compact range, the illumination is non-ideal in amplitude, phase and polarization. Error sources such as reflector surface inaccuracies, chamber-induced stray signals, reflector and edge treatment geometry, and instrumentation RF leakage, perturb the illumination from ideal.

This paper will review, in a summary fashion, the equations that estimate the effect of a non-ideal incident electromagnetic field on an antenna. It will calculate the resulting antenna pattern for a candidate antenna and compare it to the ideal antenna pattern thus showing the induced errors. Parametric studies will be presented studying the error effects of varying illumination metrics on the antenna measurement. In addition, measured field probe data from a compact range will also be used with the candidate antennas to investigate induced errors.

The intent is to provide the reader with insight as to how the typical compact range metrics affect the accuracy of an antenna measurement.

A Calibration Method Using Interpolation to Reduce Measurement Errors in Electromagnetic Compatibility Measurements

Authors: Vince Rodriguez, Dennis Lewis

MIL STD 461 is the Department of Defense standard that states the requirements for the control of electromagnetic interference (EMI) in subsystems and equipment used by the armed forces. The standard requires users to measure the unintentional radiated emissions from equipment by placing a measuring antenna at one meter distance from the equipment under test (EUT). The performance of the antenna at 1m distance must be known for the antenna to measure objects located at this close proximity. MIL STD 461 requires the antennas to be calibrated at 1 m distance using the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 958. This SAE ARP 958 document describes a standard calibration method where two identical antennas are used at 1m distance to obtain the gain at 1m for each antenna. In this paper the authors show using simulations that the SAE ARP 958 approach introduces errors as high at 2 dB to the measured gain and AF. To eliminate this problem the authors introduce a new method for calibrating EMC antennas for MIL STD 461. The Method is based on the well-known extrapolation range technique. The process is to obtain the polynomial curve that is used to get the far field gain in the extrapolation gain procedure, and to perform an interpolation to get the gain at 1 m. The results show that some data in the far field must be collected during the extrapolation scan. When the polynomial is calculated the antenna performance values at shorter distances will be free of near field coupling. Measured results for a typical antenna required for emissions testing per the MIL STD 461 match well with the numerical results for the computed gain at 1 m distance. Future work is required to study the use of this technique for other short test distances used in other electromagnetic compatibility standards, such as the 3 m test distance used by the CISPR 16 standard.

A Comparison of Laser-Correction Approaches for Planar Near-Field Scanners

Author: Scott T. McBride, Ping Yang, Robert L. Luna

MI has recently developed and installed two separate real-time laser-correction mechanisms for large planar scan-ners. One mechanism employs a spinning laser, while the other uses a tracking laser with multiple SMR constellations. The spinning-laser system is limited to planarity correction, and is appropriate for any planar scanner up to a diagonal of about 15 meters. The tracking-laser system compensates X, Y, and Z, and is intended for a horizontal planar scanner of larger size or when X and Y positions also require dynamic correction. This paper will provide an overview of the two correction mechanisms, contrast the two approaches, and include measured performance data on scanners employing each mechanism.

A Reduced Uncertainty Method for Gain over Temperature Measurements in an Anechoic Chamber

Author: Vince Rodriguez and Charles Osborne

P Gain over Temperature (G/T) is an antenna parameter of importance in both satellite communications and radio-astronomy. Methods to measure G/T are discussed in the literature [1-3]. These methodologies usually call for measurements outdoors where the antenna under test (AUT) is pointed to the “empty” sky to get a “cold” noise temperature measurement; as required by the Y-factor measurement approach [4]. In reference [5], Kolesnikoff et al. present a method for measuring G/T in an anechoic chamber. In this approach the chamber has to be maintained at 290 kelvin to achieve the “cold” reference temperature. In this paper, a new method is presented intended for the characterization of lower gain antennas, such as active elements of arrays. The new method does not require a cold temperature reference, thus alleviating the need for testing outside or maintaining a cold reference temperature in a chamber. The new method uses two separate “hot” sources. The two hot sources are created by using two separate noise diode sources of known excess noise ratios (ENR) or by one source and a known attenuation. The key is that the sources differ by a known amount. In a conventional Y-factor measurement [4], when the noise source is turned off, the noise power is simply the output attenuator acting as a 50 ohm termination for the rest of the receive system. But by using two known noise sources, the lower noise temperature source takes the place of T-cold in the Y-factor equations. The added noise becomes the difference in ENR values. An advantage of this approach is that it allows all the ambient absorber thermal noise temperature change effects to be small factors, thus reducing one of the sources of uncertainty in the measurement. This paper provides simulation data to get an approximation of the signal loss from the probe to the antenna under test (AUT). Another critical part of the method is to correctly define the reference plane for the measurement. Preliminary measurements are presented to validate the approach for a known amplifier attached to a standard gain horn SGH) which is used as the AUT.

An Articulated Swing Arm System for Spherical Near-Field Antenna Measurements at Millimeter Wave Frequencies

Authors: Pieter N. Betjes, Daniël J. Janse van Rensburg, Stuart F. Gregson

NSI has developed a high precision articulated swing arm system for millimeter wave spherical near-field measurements. This paper presents this system along with structural analysis and characterization, and explains what error corrections are performed in order to produce high accuracy results. Both simulated and measurements are shown to demonstrate the effectiveness of these correction measures.

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Bridging the Gap: Bringing Measurements and Computational Results Together

Author: Vince Rodriguez

Is there a gap between Computational and Measurement Electromagnetics? The author believes that there is. That those involved in engineering electromagnetics using numerical methods and those performing measurements of electromagnetic devices have drifted apart in the recent years. The improvement of numerical tools available commercially seems to have some part in the widening of this gap. Additionally the improvement of measurement tools and instrumentation seem to have given the community of EM metrologist, the belief of better measurements and results. As the author studied this apparent gap he believes that the ease of use of these tools has reduced the amount of training necessary. Also the author believes that the confidence on the tools has eliminated the use of a priori knowledge of the problem’s solution as well as a dose of skepticism. While computational electromagneticists seem to have a blind faith on their tools, the metrology group seem to believe that the computational results are models with little place in the real world where they perform the measurements. As a way to try to bridge this gap the author looks at a series of case studies where numerical results benefit form measurements and measurements benefit from numerical results. In conclusion, the author believes that the most important ingredient in closing the apparent gap between measurements and computed results is to question the results of the simulation or measurement and to understand if they are physical results or errors of some kind. The lack of that skepticism may be tied to the easy-to-use tools available that minimize the need for training on the underlining theory of the phenomena being measured or computed.


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