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This paper validates the sub-reflectarray technique for main reflector antenna distortion compensation through simulations and measurements. First, axial defocusing of the feed creates spherical aberration distortion and it is corrected using sub-reflectarray by conjugate field matching method. Then, a ring-type distortion is created on the main reflector and it is also compensated using a similar approach. A hybrid HFSS/PO simulation approach was used for the design and analysis. Bipolar planar near-field measurements are performed to validate the compensation technique and back projection holography is used to locate the position of distortions and to study the effects of the distortion on the antenna performance.
The use of computational electromagnetics (CEM) prediction codes in the analysis and interpretation of RCS measurements has become increasingly prevalent. This is in large part due to rapid advances in computing capability over the last several years, particularly for rigorous techniques such as the method of moments (MoM). In many instances, however, these codes are still limited to plane waves and/or elementary dipoles as the sources of target illumination. Modeling of the illumination from an arbitrary antenna therefore requires meshing and solution of the combined antenna-target geometry for each frequency and aspect angle, with an associated increase in the computational complexity of the problem, even if the interactions between the antenna and the target are negligible. In this paper, we describe a method by which measurements or predictions of the antenna pattern are used to develop an equivalent representation of the antenna in terms of an array of non-interacting elementary dipole current sources in a MoM code that uses RWG basis functions. The representation can then be used to efficiently derive the antenna’s illumination on the target as a function of frequency and aspect angle with only a minor increase in the computational burden relative to plane wave illumination. Results are presented using antenna pattern predictions for an ETS-Lindgren 3164-01quad-ridged VHF antenna which illustrate the accuracy and efficiency of the technique.
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.
Electromagnetic Interference (EMI) in enclosed areas such as the inside of an automobile is often hard to deterministically measure and to geometrically characterize. EMI can be created by motors, actuators, power systems, computer-based control systems, etc. The non-sinusoidal signals can propagate to the radio system or data system by RF radiation, by coupling to power or data lines, or by direct conduction through the metallic structure of the vehicle or system suffering from the EMI issues. The technique to be described here uses direct A/D conversion at rates in excess of 2 GHz to collect two channels of data with up to 15,000 data points. One channel is captured from the potential source of the EMI. The other channel is captured along wiring harness points or over a region of space inside the vehicle. Transforms such as cross correlation or cross spectral analysis are used to characterize and/or map the relationship between the reference channel and the data channel. The algorithms will be discussed, and results will be shown for specific examples of EMI in an automotive body.
A free space transmission line measurement method for dielectric constant and loss tangent determination in low-loss dielectric materials has been analyzed and implemented. This method utilizes dielectric materials with thicknesses greater than half the wavelength in the material to obtain greater sensitivity for determining intrinsic dielectric properties. An analysis of the process sensitivities and experimental measurements has been utilized to estimate the accuracy and lower limits of the dielectric property extractions from the reflection loss magnitude.
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.
This paper presents a hybrid analysis algorithm, which is used at Radiation Group (UPM) to carry out the design of a conformal serrated-edge reflector for the mm-Wave compact range UPM facility. Main features of this algorithm involve its capability of handling conformal serrated rim parabolic reflectors, accuracy and computational efficiency.
A means of determining Antenna Sky Noise temperature reduction due to the use of choke rings is provided. This critical information is needed to understand the signal-to-noise limitations of a receiver in each of its RF bands. The approach is novel and straight forward, combining measurement and analysis. The methodology will show the S/N contribution due to the antenna and sky noise; and will separate it from the other noise contributors such as the receiver.s RF front end noise figure. Also it will show the specific noise reduction due to the choke ring. The procedure is shown by example for a dual band GPS antenna (L1 & L2 frequencies). The process is also applicable for antennas in other RF bands. This is done thru a process of covering and uncovering the antenna with an electrostatic type sheet covering. The sheet material has high RF absorption/reflection characteristics across broad RF bands including GPS. The covered antenna condition forms a 290 degree Kelvin reference for comparison. Both absolute and relative RF measurements are made during this process, which is performed with and without choke rings. The measured data is then analyzed to derive the total sky noise temperature.
The Georgia Tech Research Institute (GTRI) analyzed a phased-array antenna for the purpose of testing phase-only defocusing methods. The array is defocused with the objective of broadening its beam at the cost of lower antenna gain. A design for the beam-steering computer is accomplished which adds the capability of focusing a beam, steering in azimuth and elevation, and performing beam defocusing using only element phase. Widening of the beam is accomplished using only 180° phase shifts in the elements, and it is compared with widening accomplished using gradual phase tapers. The antenna is measured in a near-field range to obtain amplitude and phase information as a function of each element in the array. Near-field testing of the antenna is also used to verify the capability of the beam-steering computer; two-dimensional antenna patterns and near-field hologram projections are compiled to prove this functionality. A software model is designed to mimic the behavior of the phased array antenna in its operational modes; it is also used to predict antenna gain and beamwidth prior to near-field testing. Measured and modeled antenna patterns are compared using focused and defocused modes. Metrics are performed on the near-field data to infer statistics of the individual phase shifters and on the computed far-field patterns to characterize the entire antenna. The defocusing methods under analysis are phase-only methods, due to the inability to control amplitude weighting of elements in this antenna. One method discussed uses only 180° shifting of elements in the antenna to achieve a desired beamwidth. This is compared with another method which gradually spoils the beam by applying a phase taper across the aperture. The results from near-field testing compare the defocusing methods and characterize the relationships between gain, beamwidth, and sidelobe levels for both defocusing methods.
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.
Recently, the smaller of the ESTEC CATR’s has been moved to a new location in the ESTEC Test Centre. In the frame of the relocation, the original reflectors of the range were positioned and aligned in a brand new anechoic chamber. The commissioning phase of the new range included a quiet zone field probing in order to verify the range performance in the new situation and to identify direction of arrival of major reflections. During this exercise, it was realized that the criteria for Quiet Zone dimensions are rather arbitrary. The paper addresses a new figure of merit for range comparison in terms of accuracy. Peak to peak values and RMS have been recorded depending on the size of a hypothetic AUT. This analysis resulted in accuracy nomograms that allow ESA staff to easily assess measurement accuracy depending on antenna size and operational frequency. Similar nomograms elaborated for different CATR’s could allow unbiased inter-range comparison. Moreover, a GRASP model of the facility has been developed based on the metrology measurement of the reflectors surfaces, relative position of range feed and AUT positioner.
Far field antenna measurements require specialized chambers, not very commonly available. The measurement process is inherently time consuming. If this time can be reduced it would increase the through put of the test chamber and would decrease the incurred expenses. This paper describes a novel adaptive far field measurement methodology for antennas, by varying the angular resolution and IF bandwidth in an adaptive manner. Different adaptive angular resolution techniques have been proposed and verified. Different type of antennas were measured with conventional antenna measurement methods and compared with proposed technique. It was observed that fine angular resolution can be achieved in main beam and first sidelobe levels with a little compromise on other side lobe and back lobe levels. The comparative results and their analysis are presented. On the average 20 to 40% measurement time is reduced with the proposed methodology. All measurements have been conducted in a CATR.
This paper discusses the designing of circular waveguide antenna, mode & field propagation analysis with in circular waveguides, cutoff frequency analysis & radius along with calculations for millimeter region Electromagnetic waves ranging between 1GHz-40GHz.This analysis will be based on Finite Element Method using Ansoft HFSS, therefore Finite element Method has also been briefly discussed. Circular waveguide’s cutoff frequency & radius can be directly calculated by using SAND’s constant; a method generated through the optimization of approximated cutoff frequency equation refined by using FEM numerical technique. Graphical analysis for cutoff frequencies ranging between 1GHz-40GHz against waveguide radii has also been discussed. SAND’s constant variation for entire frequency range of 1GHz-40GHz have also been discussed.
M rejection curve was described. The steps to generate this rejection curve consist simply of (1) translating the coordinate origin of the measured pattern to a new location (2) performing a spherical modal analysis of the pattern, and (3) taking the total power in the lowest order mode as a measure of the strength of the radiation source at that location. Stepwise repetition of this process then generates the IsoFilterTM rejection curve. The basis for the process of generation was an empirical recipe for which no theoretical basis was presented. In this paper we relate the rejection curve to conventional electromagnetic theory. We begin with the general free space Green's function assuming a general distribution of current sources, and show how one may plausibly describe the IsoFilterTM rejection curve, and how it operates to reveal an arbitrary source distribution.
Computational Electromagnetics (CEM) Techniques have found wide use in scattering analysis of structures due to the fact that they require less cost and time than doing physical measurements. Numerical methods both in the time and frequency domain such as the Finite Integration Technique (FIT) [1], Method of Moments (MoM) [2], Multilevel Fast Multipole Method (MLFMM) [3], Transmission Line Method (TLM) [4] and Finite Element Method (FEM), have been known to provide accurate results for Bi-static as well as Mono-static Radar Cross Section (RCS) analysis in general but their practical applicability to specific types of structures is frequently misunderstood thus leading to mistrust in the results obtained. A result comparison between the different techniques is typically the best way of gaining trust in the results obtained, however this involves the general principle of result convergence which must be achieved for each individual solution technique. Using one of the standard benchmark radar targets which is the Cone-sphere [5], a comprehensive description of how to achieve result convergence for each technique will be presented and the final results will be shown to agree with published measured results [7, 8]. This target will be used in different configurations (with and without a slot) as well as coated with Radar Absorbent Material (RAM).
We report on the initial phase of our study to assess the electromagnetic interference and electromagnetic compatibility measurement and calibration procedures at the National Institute of Standards and Technology. We are developing a measurement-based uncertainty analysis of calibrations and measurements in the anechoic chamber. We intend to characterize all sources of uncertainty, which include power and probe-response measurements, noise, nonlinearity, polarization e.ects, multiple re.ections in the chamber, drift, and probe-position and probe-orientation errors. We present simple and repeatable measurement procedures that can be used to determine each individual source of uncertainty, which then are combined by means of root-sum-squares to state the overall measurement or calibration uncertainty in the anechoic chamber. We report on work in progress and future plans to characterize other EMI/EMC facilities at NIST.
Many periodic structures are often built up of multiple layers to improve the electromagnetic performance such as the frequency bandwidth. Two approaches can be employed to analyze multi-layer structures: one is to formulate and analyze a multi-layer structure in its entirety; the other is to compute the generalized scattering matrix (GSM) for each layer and then obtain the total GSM of the structure by simple matrix calculations. The second approach is more flexible and efficient to practical problems where several layers may be cascaded in arbitrary sequences. This paper describes an efficient procedure to analyze multi-layer periodic structure using the finite difference time domain (FDTD) method. Based on the constant horizontal wavenumber approach, the procedure first computes the GSM of each periodic layer. The scattering parameters of the entire multi-layer structure are then calculated using the cascading formulas. The validity of this algorithm is verified through several numerical examples including frequency selective surfaces (FSS) with different periodicities and under different incident angles. The numerical results of the developed approach show good agreement with the results obtained from the direct FDTD simulation of the entire structure, while the new procedure saves the computational time and storage memory.
RF power harvesting enables controllable and simultaneous wireless power delivery to many RF devices. Devices built with this unique technology can be sealed, embedded within structures, or made mobile, thus eliminating additional service for battery. A key component of this technology is the “rectenna”, which is composed of antennas and rectifying circuitry to convert RF energy into DC power. Typically, multiple antenna elements are used to produce sufficient power for reliable device operation. This paper compares two different rectenna architectures (see Fig.1) for maximum RF-to-DC power conversion efficiency due to non-linear characteristics of the rectifying circuitry. A simple rectenna design example containing a 2x2 planar antenna array will be presented to demonstrate such RF power harvesting technology. The quantity, Rectenna Topology Indicator (RTI), is introduced for performance comparison.
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.
Instruments for Earth observation working from W-Band up to mm-wave frequencies mainly use quasi-optical feed feeds (QOF) to illuminate the corresponding reflector antennas. The final design of the QOF for the Cloud Profiling Radar System (CPR) for the EarthCARE satellite has been finalized. The QOF is designed to perform polarization and frequency tuning, as well as the separation of transmit and receive channels. The final design verification was performed theoretically by Astrium with QUAST, a new add-on to the GRASP software, especially developed by TICRA for a fast and accurate set-up and analysis of quasi-optical networks. Within the paper, at first a short description of the QOF working at 94.05 GHz will be given. Secondly, the modeling of the QOF will be explained. At last the RF test setup will be described and comparisons between resulting calculated and measured antenna pattern will be shown.