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All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.
Traditionally Taper chambers are constructed using a square based pyramidal shaped taper. The taper is then shaped into an octagon and finally transformed into a cylindrical launch section. This approach is related to the manufacturability of different absorber cuts. This presentation introduces a chamber where the conical shape of the launch in continued through the entire length of the taper chamber. The results are of the free space VSWR test over a 1.5m diameter quiet zone are presented at different frequencies. The conical taper appears to have a better illumination wave front and better QZ levels even at frequencies above 2GHz than the standard traditional approach. The implementation of this design was done in Singapore and the actual chamber was designed to have a secondary Near Field range for Planar and spherical scans.
This paper details a recent advance that, for the first time, enables the Mathematical Absorber Reflection Suppression (MARS) technique to be successfully deployed to correct measurements taken using plane-polar near-field antenna test systems with reduced AUT-to-probe separation. This paper provides an overview of the measurement, transformation, and post-processing. Preliminary results of range measurements are presented and discussed that illustrate the success of the new planepolar MARS technique by utilising redundancy within the near-field measured data that enables comparisons to be obtained and verified by using two existing, alternative, scattering suppression methodologies.
In this paper, a broadband EM wave absorptive structure using a periodic structure is proposed and its reflectivity characteristic examined. Based on the proposed structure, the design technique for improving absorption bandwidth of original structure is additionally proposed. Finally, the method for measuring the reflectivity of the absorber is presented, and the measured results of the absorbers, which show a broadband reflectivity response with a fractional bandwidth of approximately 76% below -10 dB and an increase of 17% of the bandwidth because of the proposed design technique, are presented.
We present a test procedure and results that compare RF performance of traditional pyramidal absorber to that with latex coating. Measurements were performed from 5 to 15 GHz, but the same measurement methodology can be applied to other frequency bands. Absorber with protective coating is being used in place of traditional absorber for outdoor antenna measurement facilities to reduce degradation of the absorber performance in harsh environments. Knowledge of the RF performance characteristics of coated absorber is especially necessary when it is used to replace uncoated absorber in an operational antenna measurement facility. Measurements are performed with a simple measurement setup based on a vector network analyzer and broadband horn antennas. Results from bi-static and mono-static measurements are presented.
Normally, field power density is inversely proportional to distance from a radiating antenna. In a compact range, however, the reflector focuses the radiated field onto the feed. This dramatically increases the power density – similar to the sun through a magnifying glass. Naturally, if the power density gets high enough, it could set the feed area absorber on fire. In order to determine the focusing effects on the feed horn and surrounding absorber, a series of transmit tests were conducted to measure feed absorber heating with an IR camera. This paper describes the test set up, the test results, and provides an analysis of the test results with suggestions for increasing power handling at the feed.
When specifying a Near Field scanner, intended to measure radiating systems under operational conditions, one of the requirements is the power flux density that the Near Field system and the absorbers on it have to with-stand. Today's trend is to use an EM-solver to calculate field intensities of (aperture) antennas. The advantage of these solvers is that they can handle any geometry but the disadvantages are that they can only handle limited dimensions and use approximations. Analytical solutions are not only more elegant and accurate but they also provide insight in the field behavior. For symmetrical cases, it is clear that the maximum field intensities will appear on the symmetry axis. The only (nearly identical) expressions in the literature are from Rudduck and Chen [1] and Yaghjian [2]. These analytical expressions describe the on-axis electrical field intensity of a circular aperture with uniform illumination. Rudduck and Chen have derived their equation via a Plane Wave Spectrum approach. Unfortunately, Yaghjian provided this version without reference or background about the derivation. It turns out that the expressions of [1,2] need a (minor) correction. Besides that, uniform illumination is not a very realistic case. This paper will also present an analytical expression for a tapered illumination. Graphs will be provided of the equation of [1,2], the corrected formula for the uniform illumination case and the new expression for the tapered illumination case.
Recently ORBIT/FR Inc. has introduced a far – field antenna measurement anechoic chamber design method called “ Two Level GTD “ , which combines shaped chamber walls with a specific absorber layout intended to achieve a better level of reflectivity in the test zone [1-3]. The sidewalls may have the shape of an “inverted open book “, while the back wall may be a pyramidal shape with a small subtended angle at the base. A wedge type foam absorber with a variable orientation of the wedge tips can treat the sidewalls in a “fishbone” layout, while the back wall may be treated by using conventional foam based pyramids. The’ fishbone’ like layout is intended to adverting the reflected waves by the sidewalls out of the test zone, while the back wall pyramid layout is applied to utilize both: the optimum pyramid reflectivity at almost normal incidence; the back wall shape diverting the reflected incident plane or quasi - plane wave out of the test zone. It well known that GO and GTD principles are widely applicable to electrically large structures, delivering a high quality simulation accuracy and good correlation with measurement results. Therefore, the application of the “Two –Level GTD “ is expected to deliver well predicted improvement in the reflectivity of anechoic chambers operating at relatively high frequencies , where the chamber sidewall characteristic dimensions may reach 30. where . is the wavelength at lowest operating frequency. The key question to be answered is - Can the method be successfully applied to cases where the chamber sidewall characteristic dimensions are only – 2-3.? This represents a typical situation in anechoic chambers designed for operation at VHF/UHF frequency bands. In order to answer the question, a full wave 3D simulation has been performed on two anechoic chambers having similar dimensions: 20’ x 20’ x 33’ (L). The two cases are a conventional anechoic chamber and a shaped wall chamber designed based on the “Two – Level GTD” principle. The simulation results were compared, and the “Two - Level GTD” has shown superior performance. Based on these encouraging results, the anechoic chamber was constructed and measurements were performed in the tests zone at a number of frequencies down to 100 MHz. The chamber construction, simulation and measurement results are discussed in the paper below.
In the paper [1] the principles of operation of high performance absorbing materials were described and the criterion for absorber performance optimization at UHF/VHF frequency bands was proposed and confirmed experimentally on a number of absorber components optimized for operation at low frequencies such as the VHF/UHF bands. C:\Publishing 2011\AMTA 2011\Papers\Absorbing Material Performance\freq dom 18 24 36 60.jpg The experimentally verified optimization criterion is intended to determine the optimum carbon loading of the absorber components, thus delivering optimal reflectivity of the full absorbing assembly (absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity of the criteria was confirmed by measurements of the reflectivity of pyramidal absorbing components of various heights, (3’, 5’, 6’ and 8’ [3]) in a 40’ long coaxial line terminated in a metallic back wall [2,4]. In this paper, more details are highlighted explaining how the criterion is delivering the best absorber reflectivity at low frequencies. This is accomplished by implementing time gating post-processing to isolate two primary concurrent peaks corresponding to the reflections from the front surface and metallic backing substrate. It is shown that the improved reflectivity is achieved by a self-cancellation of the two signals delivering the “null” in the frequency domain, which, in turn determines the lowest operating frequency attributed to an absorber of a given height. It is shown that the “null” property of the reflectivity pattern, as well as the properties of the peaks in between “nulls”, can be scaled and, therefore, predicted based on the height of the absorber almost everywhere in the UHF band. Thus, it is possible to optimally choose the grade of the absorber necessary to meet or exceed given reflectivity specifications, or to manufacture the appropriate absorber grade which can deliver the optimum reflectivity at the specified frequency.
Mathematical Absorber Reflection Suppression (MARS) has been used successfully to identify and extract range multi-path effects in a great many spherical [1, 2], cylindrical [3, 4], and planar [5, 6] near-field antenna measurement systems. This paper details a recent advance that enables the MARS measurement and post-processing technique to be used to correct antenna pattern data from far-field or compact antenna test ranges (CATRs) where only a single great circle pattern cut is taken. This paper provides an overview of the measurement and novel data transformation and post-processing chain that is utilised to efficiently correct far-field, frequency domain antenna pattern data. Preliminary results of range measurements that illustrate the success of the technique are presented and discussed.
When making antenna measurements, great care must be taken in order to obtain high quality data. This is especially true for near-field antenna measurements as a significant amount of mathematical post-processing is required in order that useful far-field data can be determined. However, it is often found that the integrity of these measurements can be compromised in a large part through range reflections, i.e. multipath [1]. For some time a technique named Mathematical Absorber Reflection Suppression (MARS) has been used to reduce range multi-path effects within spherical [2, 3], cylindrical [4, 5] and most recently planar [6, 7] near-field antenna measurement systems. This paper presents the results of a recent test campaign which yields further verification of the effectiveness of the technique together with a reformulation of the post-processing algorithm which, for the first time, utilises a rigorous spherical wave expansion based orthogonalisation and filtering technique.
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
Obtaining a quantitative accuracy qualification is one of the primary concerns for any measurement technique [1, 2]. This is especially true for the case of near-field antenna measurements as these techniques consist of a significant degree of mathematical analysis. When undertaking this sort of examination, room scattering is typically found to be one of the most significant contributors to the overall error budget [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in quantifying and subsequently suppressing range multi-path effects in first spherical [3, 4] and then, cylindrical near-field antenna measurement systems [5, 6]. This paper details a recent advance that, for the first time, enables the MARS technique to be successfully deployed to correct data taken using planar near-field antenna measurement systems. This paper provides an overview of the measurement and novel data transformation and post-processing chain. Preliminary results of computational electromagnetic simulation and actual range measurements are presented and discussed that illustrate the success of the technique.
Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3.. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a “Two Level GTD” approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also [6]).
This paper describes the principles of operation of high performance absorbing materials and the criterion for its performance optimization at UHF/VHF frequency bands. The optimization criterion is intended to determine the optimum carbon loading of the foam based absorber components, thus delivering optimal reflectivity of the full absorbing assembly (foam based absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity is confirmed by measurements of the reflectivity of pyramidal absorbing components of varying heights, (3’, 5’, 6’ and 8’) in a 40’ long coaxial line terminated in a metallic back wall. In addition, it is shown that the “aging” process of the absorbing components can be characterized by the change of the effective reflectivity in the time-domain of the components as a function of aging time. It is possible to determine whether the absorber performance is stabilized and the “aging“ process is complete, and whether the loading of the absorber carbon mix is optimum, or is otherwise under-loaded or over-loaded. In particular, it is possible to determine prior to the time when the “aging” process is stabilized whether the loading is excessive.
In this paper we explore how placing the same amount of RF absorber in different locations within a reverberation chamber can have different loading effects. This difference can have a significant impact on measurement reproducibility, both for measurements in the same chamber and measurements between chambers (i.e., round robin style testing). We begin by discussing some of the theories behind this and show some experimental results from different absorber placements in a reverberation chamber. Our experimental results will be presented in a fundamental format and in a practical sense (RMS-delay spread). We conclude with some suggestions on how to ensure that absorber is placed consistently.
This paper analyzes the applicability of near field scanners into existing compact test ranges. The analysis is motivated by creating multi-purpose test chambers having the advantages of both, near field systems and compact test ranges. This contribution comprises the discussion of near field scanners at several positions inside a typical compact test range. A ray tracing analysis is presented taking these positions into account in the assessment of near field errors due to multi-path reflections. It is presented how reflections from the absorbers and reflectors are differently impacting near field measurements of low, medium and high gain antennas. The impact is quantified in terms of error levels used in common near field error budgets. It is shown that the combined approach is realizable for specific configurations only.
A well designed absorber configuration is a key factor for precise antenna measurements. Unfortunately, even a scanner covered with pyramidal absorbers can cause reflections that could degrade the measurement accuracy. A novel scanner absorber configuration using bent absorbers is presented in this paper. Another problem is that in most cases it is necessary to remove the absorbing material at the scanner to change the antenna under test. The absorbers covering the scanner suffer abrasion caused by the frequent manual movement. For this reason it was also the intention to find a faster and easier solution which also preserves the absorbing material. The new and the old absorber layout were benchmarked using a number of spherical nearfield measurements as well as time domain reflection measurements with a broadband probe antenna. A comparison of the results is also shown in this paper.
The rise in the number of active antennas used in radar applications calls for changes to the common absorber treatment used in chambers. The electronic circuits that are imbedded into these scanning arrays are non-linear in nature so they must be tested at the correct power outputs to get the correct pattern behavior. The combination of higher power and narrow beams means that areas of the anechoic treatment in a chamber can be subjected to high power densities. High power absorber has been used in the industry for many years. The substrate used in these absorbers makes the material very expensive. While in the past it was common to use this material only in regions where the main beam was going to be illuminating the absorber treatment the new electronically scanned arrays will have main beams that can illuminate several areas of the chamber. In cases where Near Field systems are used the absorber material will be in a radiation region where the main beam has not been formed, but the Absorber is closer to an array that is radiating high power so a large area of higher power absorber is needed to treat the chamber. In the present paper the authors present a medium power absorber 3kW per square meter (versus 775w per square meter) using the same material used in common RF absorber. Mechanical changes to the absorber are performed to increase the thermal dissipation of the EM energy absorbed. A series of measurement of the absorber is performed with a without additional air flow for cooling. The result is an absorber that can handle higher power densities without the need for exotic substrates.
Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements [2, 3, 4, 5]. Whilst the technique presented herein is also a general purpose measurement and post-processing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the post-processing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.