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This paper describes a novel system that overcomes the inaccuracies in antenna radiation pattern measurements caused by multipath propagation. The system operates by specifically compensating for the effects of unwanted signals rather than by attempting to remove, or minimize, their effects through the use of screens or baffles or an anechoic chamber. Compensation is achieved through the use of an equalization technique, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration. The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy. Most importantly, however, the basic idea leads to the design of a completely new type of real-time 3-D range in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.
When using an array antenna for measurements, small phase imbalances between the sum and difference channels may occur due to a variety of factors. This imbalance may arise due to unequal line lengths, twists or bends in the cabling, leakage, improper connections, or a variety of other factors. In the ideal phase array antenna one lobe of the difference pattern should be inphase with the sum pattern, and the other lobe should be 180° out-of-phase. When a phase imbalance occurs between the sum and difference channels, errors occur in determining the antenna beam deflection due to the presence of a radome. By taking the ratio of the difference to sum channel data a phase correction factor may be determined. Application of this phase factor to the difference channel data will phase align the sum and difference channels so the correct deflection may be determined. This correction factor will be frequency dependent.
With the continued growth of mobile communications and the emergence of wireless LAN and personal area networks (PAN), there is an increased need to accurately measure the antenna properties for omnidirectional antennas and antenna systems. Furthermore, it is very desirable that antenna measurement systems be flexible to support a variety of antenna configurations and form factors. In this paper, we assess the performance of two measurement configurations utilizing dielectric positioners. These configurations comprise a traditional roll-over-azimuth antenna positioner and an arm-overturntable system such as that used in ORBIT/FR’s Advanced Spherical Cellular Near-Field (ASCENT) product. The results show that both configurations offer demonstrable improvements over conventional metallic positioners, and the arm-based system provides the highest accuracy for omnidirectional antennas.
This paper presents a 'mid-range' calibration technique, now being developed for a 60 ft. diameter reflector site. With this technique, near-field amplitude and phase is collected at a calibration tower as the reflector scans across it. The mid-range 'near-field' data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to standard measurement techniques. A particular advantage is that the system, once set-up, can be used on a regular basis without impacting site operations.
WINDSAT is a satellite system designed to be a demonstration of passive microwave polarimetry to measure ocean surface wind speed and direction. The polarimeter works off the crosspol components of the antenna, necessitating high performance requirements both in the building and testing of the antenna. The calibration of the reflector antenna system will be discussed in this paper, along with various analysis done for the project and verified by range measurement.
When making large scale RCS measurements at outdoor ground bounce ranges, vector background subtraction is often not performed. To get a clean measurement, range engineers must control the backscatter from the target supports that mainly dictate the background level. Presently, nearly all ranges use single high gain antennas to concentrate incident field energy onto the target, but single antennas have physical limitations for controlling the incident field energy in the target support region. To improve the incident field distribution, an array of transmitting elements can be used instead of a single radiator. With an array, engineers can control the illumination in both vertical and cross range dimensions, making it possible to concentrate the incident field energy on the target while reducing the field level over the target supports. This paper describes ground bounce range and incident field modeling, shows beamforming applied to foam column scattering, and demonstrates that a 2-dimensional array can improve the cross range phase taper. It also discusses design sensitivity issues.
This paper shall discuss how to correct for the presence of groundwave propagation on a low frequency (3 MHz
As part of an ESA/ESOC development project [1] a set of three planar Ka band Frequency Selective Surface (FSS) mirrors were designed. For design verification purposes a set of FSS samples of sufficient size (about 220mm by 250mm) to demonstrate their design was manufactured. The RF performance of these samples was measured on a planar near field scanner at ERA Technology and on a compact antenna test range (CATR) at ESA/ESTEC for test comparison purposes. The FSS were designed to operate over a narrow range of RF incidence angles (30º + 1º). Hence, it was important for test purposes that the FSS samples be illuminated by a near plane wave. This was achieved with a 220mm offset reflector. As the FSS samples were relatively small it was also desirable that any edge effects should not significantly influence the measurement. Hence, the test reflector antenna was designed to operate with a tapered RF distribution of –20 dB at its edges. A precision framework was constructed to hold the FSS samples at the required incidence angle. The two methods of measurement show remarkable agreement and indicate that the differential pass-band phase and amplitude performance of the FSS samples can be measured to within approximately 1 degree of phase and about 0.03 dB of amplitude
The fundamental concepts of operation of a Planewave Generator1 (PWG) were described in an earlier paper. In the present paper the measured performance of a proof of performance experiment are reported. First, we will briefly describe the concepts and architecture of the experimental configuration. Next, measurements of the electromagnetic field created by the PWG prototype in a specified test zone will be presented. A planewave figure of merit (FOM) has been defined earlier, and the measured FOM of the PWG will be compared with the FOMs of the field of a single antenna, and of a uniformly illuminated transmit array. We first discuss the experimental strategy, and our use of superposition to minimize the hardware required for the demonstration. We then present measured data that shows that a minimally configured PWG can produce field distributions that are planewave-like over a limited spatial extent, and that its field demonstrates planewave qualities that significantly exceed those achievable by a typical transmit antenna, or uniformly-illuminated array. We also present data that relates the size and quality of the planewave-like field in a test zone in terms of the number of elements in the PWG transmitting array. Finally, we present a scaling relation for the PWG, and show that the radiated field produced by the experimental hardware agrees well with the field predicted by simulation.
In real life, most radar targets are located outdoors. Here we present results from outdoor broadband RCS measurements at the X-, Ka- and W-band of “Holger”, a metallized model-scale aircraft with cavities. RCS vs. angle data in the wing plane (0° elevation) were recorded at discrete frequencies (9, 35 and 94 GHz) in both horizontal (HH) and vertical (VV) polarizations. ISAR data at 7-13, 32-38 and 92-97 GHz were acquired. Results from a 104.1 m ground range and a 162.7 m free space range will be compared.
This paper presents a pragmatic approach in the determination and measurement of RF Power on RF systems in the presence of Voltage Standing Waves Ratio (VSWR). Information is provided to determine the RF Power measurement ambiguities produced by the VSWR of cables, pads and loads which ultimately affect and perturb the readings associated with both CW and Peak Power meters. A comparison of presently available commercial RF Power Meters is also provided which include their maximum metering ambiguities, their performance under dynamic range and their absolute metering accuracies. The paper also provides a simple equation for determining your system ambiguities (in dBs) based on the aggregate of the individual components within the RF path and their individual VSWR effects. Experimental data and calculated data are also presented with excellent correlation. Recommendations are also provided in the process of enhancing RF power measuring procedures.
The S parameter measurement of large sheet materials has been limited to microwave frequencies due to a lack of test apparatus that could properly illuminate the materials with a uniform electromagnetic wave in a confined space at lower frequencies. Boeing Mesa has developed a unique test system that overcomes this limitation. The current design will perform S11 and S12 measurements from 0.125 to 40 GHz in five bands. Sample sizes up to 4 ft. wide, 8+ ft. long, and 12 inches thick, can be tested in sections by sliding the sample into the test fixture. Apertures up to 46 inches square can be provided in the aperture plate. The angle of incidence can be adjusted from 0 degrees to 45 degrees.
Measurement of material properties, at radio frequencies (RF) including microwave and millimeter-wave frequencies, is a critical issue for RF polymer research and development. RF polymers are polymer composites specifically designed for specified electrical and magnetic properties within a required frequency band of operation. In this paper, we investigate the methods required to determine the RF permittivity and permeability in the RF band for RF polymers.
This paper presents the design and performance characteristics of a novel active stability control capability that Aeroflex Incorporated has developed and implemented in the élan-2000 pulsed-IF instrumentation radar. The real-time technique incorporates an internal power reference loop that continuously monitors and compensates for phase and amplitude drifts within the radar RF analog circuitry through high-speed processing of the streaming data collections. Vector corrections are applied to each recorded data point, using internal loop samples of the transmit pulse from a common RF channel and digitizer, without degrading other overall system performance capabilities. Demonstrated stability levels exceed –50 dB over the full operational RF bandwidth, for periods of several hours, with environmental temperature variations of several degrees. This measurement mode provides ~30 dB of improvement over conventional instrumentation radar systems under similar test conditions, which consequently enables significant improvements in measurement applications incorporating background subtraction or where extremely stable system parameters are required.
Sensor Concepts Inc. has prototyped a fast, lightweight, dechirp-on-receive radar called the SCI-Lr to provide the capability of a range instrumentation radar in a highly portable package. The small weight, size and power requirements of the SCI-Lr allow a variety of new deployment options for the user including in a small general aviation aircraft or on a mountaintop that is accessible only by four wheel drive. Pulse rates up to 20 KHz enables investigation of high Doppler bandwidth phenomenon such as ground vehicle microdoppler features. The dual integration from dechirp-on-receive matched filtering in fast time and Doppler processing in slow time provides high sensitivity with low output power. Planned enhancements of waveform bandwidth up to 2 GHz , frequency operation between .2 and 18 GHz and pulseto- pulse polarization switching will provide high information content for target discrimination. The flexibility provided by the hardware is augmented by software tools to examine data in near real time to monitor data quality and sufficiency. A variety of applications are being investigated including RCS measurement, SAR and ISAR imaging, Ground Moving Target Indication, and signature collection for ATC.
Traditional data collection strategy for antenna measurement is to perform a step and scan operation. This method moves a particular axis while holding all other source and AUT axes in a fixed location. Modern radome measurements require the coordinated motion of two or more axes due to the desired measurements, the radome testing geometries or a combination of both. An example would be transmission efficiency testing of a radome associated with a tracking antenna. In this measurement scenario, the antenna azimuth and elevation axes must maintain an orientation along the range axis while the radome is moved in front of the antenna. The axis coordination could be linear or non-linear in nature. This paper describes the concept of coordinated motion and the needs for coordinated motion in radome measurements that have been identified. Additional potential applications for coordinated motion in radome measurements are described. Two methods of coordinated motion that have been implemented in instrumentation are described. They are geared motion, which is a linear master/slave relationship between two axes and generalized coordinated motion where the relationship of axes motion is described via linear or non-linear equations.
The true RF spectral response represents In- phase and Quadrature (I and Q) data in the frequency domain, and is identical to that mea- sured in many anechoic chambers including the one at Mission Research Corporation. Given a Linear FM (LFM) response, a method is derived that extracts the true RF spectral response. In the process some basic features of LFM systems are explained. The derivation depends on the assumption that the received signal is zero outside a de¯ned interval. Validation consists of applying the extraction process to both sim- ulated and measured LFM data from the ERIM DCS radar system.
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.
In a conventional manner, a majority of compact ranges are currently used between 2 GHz and 200 GHz. Mechanical stiffness limits compact ranges at high frequency and diffraction effects are dominant at low frequency. However, CNES has installed a single reflector with dedicated serrations to perform accurate measurements between 800 MHz and 2 GHz. These serrations are 2 meters long and minimize the ripple in both amplitude and phase within the quiet zone. In order to further improve its measurement capabilities at lower frequencies, CNES has installed, in co-operation with SATIMO, a spherical near field measurement system directly inside of its compact range building. The goal is to measure antennas within the frequency range 80 MHz – 400 MHz with a relatively good accuracy. The spherical near field measurement facility has been tested and validated with four antennas that had been previously measured in the compact range of CNES and other external ranges. This paper focuses in this smart approach, which allows to extend the lower frequency domain of compact ranges. This paper describes in details the measurement facility, the test and the validation of the system.
We at MI Technologies have employed the Hansen error analysis [1] developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design. I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges [2]. The result is an appropriate extension of the TUD uncertainty analysis. Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field [5]. In this paper, I offer such an extended tabulation for spherical measurements.