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MEASUREMENTS OF POWER HANDLING OF RF ABSORBER MATERIALS: CREATION OF A MEDIUM POWER ABSORBER BY MECHANICAL MEANS
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.
Accurate Radiation Pattern Measurements in a Time-Reversal Electromagnetic Chamber
In a recent paper , we have introduced the concept of the time-reversal electromagnetic chamber (TREC), a new facility for creating coherent wave-fronts within a reverberation chamber. This facility, based on the use of time-reversal techniques in a reverberating environment, is here shown to be also a useful tool for the characterization of the field radiated by an antenna under test (AUT). The TREC is proven to be capable of providing real-time measurements, with an accuracy comparable to that of spherical near-field facilities, while using a very limited number of static probe antennas. This performance is made possible by taking advantage of the reflections over the chamber’s walls, in order to gain access to the field radiated along all the directions, with no need to mechanically displace the probes, or to have a full range of electronically switched ones. A 2D numerical validation supports this approach, proving that the proposed procedure allows the retrieval of the free-space radiation pattern of the AUT, with an accuracy below 1 dB over its main lobes.
Single Antenna Method for Determining the Gain of Near-Field Waveguide Probes
Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.
Fault Monitoring of Patch Antenna Arrays Using Neural Networks
A technique to diagnose faulty elements present in patch antenna array from either measured far field radiation pattern or return loss characteristic is suggested. A linear array consisting of eight square patch elements with uniform excitation and ./2 spacing between them is considered. A method is developed using Artificial Neural Networks to detect one or two faulty elements present in the array. A neural network is trained with one third of the possible faulty radiation patterns and tested with two thirds of faulty patterns. ANN is implemented with Radial Basis Function neural network (RBF) and Probabilistic neural network and their performance is compared.
HIGH PERFORMANCE BROADBAND FEEDS FOR ECONOMICAL RF TESTING IN COMPACT RANGES
Compact test ranges are worldwide used for real-time measurements of antenna and payload systems. The Compensated Compact Range CCR 75/60 and 120/100 of Astrium represent a standard for measurement of satellite antenna pattern and gain as well as payload parameter due to its extremely outstanding cross-polar behavior and excellent plane wave field quality in the test zone. The plane wave performance in the test zone of a compact test range is mainly dependent on the facilities reflector system and applied edge treatment as well as on the RF performance of the range feed. To provide efficient and economic testing and maintaining the needed measurement accuracy the existing standard set of high performance single linear feeds covering the frequency range from 1 - 40 GHz had been extended to operate simultaneously in dual linear polarization. In addition several customer specific range feeds had been developed and manufactured and validated. More detailed information and achieved test results for the new high performance range feeds will be presented.
The Software Defined Antennaâ„¢ Programming and testing the prototype
The Ohio State University ElectroScience Laboratory in partnership with Syntonics Corporation has been developing a new type of programmable antenna over the past 4 years. The antenna is made of an array of small pistons where the top of the piston is conductive, followed by a dielectric layer and then a conductive remainder. When all of the pistons are in the down position, they form a ground plane. When a row or a region of pistons are in the up position, microstrip transmission lines and patch or rampart antenna elements can be created. We have several years of theoretical and experimental data showing the effectiveness of the transmission lines and antennas thus created. Small hand-emplaced elements have been used in the past, but at this time, a prototype programmable antenna system has been built and tested. We will show examples of array beam formation and beam steering. Now that the prototype is available, we are testing an automatic array generation software system that creates the radiating elements and the transmission line system so as to form the array at the desired frequency and pointing in the desired direction. This is particularly interesting because of the need to design the beam former feed based on only discrete increments but with the flexibility to shift the location of the radiating elements. This paper will show examples of the operation of the system and the resulting beam patterns. .
An accurate and efficient error predictor tool for CATR measurements
An accurate and efficient numerical model is developed to simulate the far field of an antenna under test (AUT) measured in a Compact Antenna Test Range (CATR), on the basis of the known quiet zone field and the theoretical aperture field distribution of the AUT. The comparison with the theoretical far-field pattern of the AUT shows the expected measurement accuracy. The numerical model takes into account the relative movement of the AUT within the quiet zone and is valid for any CATR and AUT of which the quiet zone and aperture field, respectively, are known. The antenna under test is the Validation Standard Antenna (VAST12), especially designed in the past for antenna test ranges validations. Simulated results as well as real measurements data are provided.
Numerical Calibration of Standard Gain Horns
The gain-transfer technique is the most commonly used antenna gain measurement method and involves the comparison of the AUT gain to that of another antenna with known gain. At microwave frequencies and above, special pyramidal horn antennas known as standard-gain horns are universally accepted as the gain standard of choice. A design method and gain curves for these horns were developed by the US Naval Research Laboratory in 1954. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach. Similar computational electromagnetic modeling is applied to predict the gain and pattern of open-ended waveguide probes which are used in near-field antenna measurements. This approach provides data for probes that are not available in the literature.
Compact Frequency-Independent Inverted-Hat Antenna for UWB operations
Small size ultra-wideband (UWB) antennas are attractive for aircraft and ground vehicle communication systems. In this paper, we presented a novel compact low-profile Inverted-Hat Antenna (IHA) to work from low VHF frequencies up to 2GHz. Such a large bandwidth is achieved via excitation of traveling waves between the ground plane and the top portion of the antenna. As one UWB radiator, the IHA is designed to have frequency-independent behavior by introducing a moderate number of elliptical segments. In particular, an 11-ellipse IHA is fabricated and tested to validate the concept. Fairly good impedance matching and radiation properties are achieved. In addition, quad-blade IHA is investigated to show the flexibility of both impedance and pattern control. The proposed antenna is simple and rugged for various UWB applications.
ACCURATE INFINITE GROUNDPLANE ANTENNA MEASUREMENTS
The accurate measurement of the infinite ground plane antenna patterns are needed in different applications as discussed in [1–12]. The comprehensive performance of a general antenna in a complex environment including interaction can be evaluated fast and accurately using ray tracing techniques [1,2]. This approach requires a reliable representation of the local source behaviour either through measurements or simulation. A good source approximation for this method is the infinite ground plane pattern assuming a perfectly conducting plane. The infinite ground plane condition can be achieved easily in simulation using full-wave computational tools but is very difficult to measure on a general antenna due to the finite dimensions of the measurement systems. Different measurements and post processing approaches have been investigated in the past to determine the infinite ground plane pattern of a general antenna. Spherical mode truncation/filtering have been used as means to eliminate edge diffraction from finite ground plane measurements. This method suffers from the dependence on the selection of filtering parameters as discussed in . Time-gating can give some information about the isolated antenna pattern in most directions as discussed in [4-6] but is not completely general and require special equipment and setup for the measurement. Other approaches to eliminate the edge diffraction by special design of the ground plane shape have also been pursued as discussed in [7-10]. This paper introduces a simple formulation to accurately determine the infinite ground plane pattern of any antenna from measurements on a small finite ground plane. The theory of the method is presented and its accuracy and suitability demonstrated with measured examples.
The Effect of the Absorber Collar on Open Ended Waveguide Probes
This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for near-field measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the far-field patterns of the probe and this in turn could affect the probe correction process when the probe was used in near-field measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.
Radiation Pattern Measurements of NASA Mars Science Laboratory UHF Entry Antenna using a Spherical Near-field Range
Measurements of a conical micro-strip Wraparound™ antenna array mounted on a portion of the entry vehicle for NASA’s Mars Science Laboratory mission were completed at Nearfield Systems, Inc.’s new spherical near-field range facility. The Wraparound™ antenna, designed and manufactured by Haigh-Farr, Inc., provides nearly full spherical coverage and operates in the UHF frequency band for telecommunications to orbiting assets at Mars. A summary of the measurements techniques and results are presented, along with a comparison of the measured and calculated patterns.
3-D Antenna Radiation Pattern Measurement System Using Ultra Light Phantom
A 3-D radiation pattern measurement system using an ultra light phantom is proposed for the evaluation of handset antennas in mobile communication systems. The problem of phantom imitating human electrical characteristics is its heavy weight to mount on the 3-D pattern measurement system for the conical or great circle cut method. This difficulty is removed by a light-weight phantom. This paper presents a novel phantom consisting of a plastic shell and wave absorbers whose electrical parameters are optimized to obtain electrically equivalent performance with the human head and body. Single and double layered absorbers are used for the phantom in the frequency range of 800 to 2000 MHz. The weight is less than 7 kg for an upper body phantom with an arm and a hand holding handset under test. This light weight phantom is easily installed on the rotating table of great circle cut measurement facility. This paper presents a design method of ultra light phantom, the characteristics of the phantom and 3-D antenna pattern measurement system in detail.
DUAL POLARISED CORRUGATED HORNS FOR ADVANCED MEASUREMENT APPLICATIONS
Recent advances in the design of orthomode junction (OMJ) have created new devices capable of achieving as much as 1:4 bandwidth while maintaining the high performance of traditional OMJ designs [1–6]. The new OMJ technology is based on inverted ridge structure with four symmetrical feeding points for external balanced feeding stabilizing the frequency dependence of the OMJ. Probes with one or more corrugations on the aperture can greatly enhance the radiated performance in terms of return loss, pattern symmetry, stability with frequency and minimizing the cross polar levels within the main beam of the probe. In the standard corrugated horn literature  the upper limit on achievable bandwidth for corrugated horns is often stated to be somewhere between the 1:1.5 and 1:1.8 ratio depending on the performance requirements. New strategies in corrugated horn design and optimization are therefore required in order to take advantage of the increased bandwidth of the wideband OMJ technology. This paper discusses the achievable performances and limitations of wide band probes with multiple corrugated apertures and show measured and predicted design examples of apertures covering up to 1:2 bandwidth in the L to Ka band range.
Noise Radar Correlation Patterns of Human and Non-Human Objects at Various Look Angles
In a search and rescue effort following a natural disaster, rubble and debris within the search environment can obscure human victims. A digital noise radar operating at UHF can penetrate the types of materials typically found in these situations with relatively little loss. This paper compares and contrasts measured correlation values of human and non-human objects taken from a digital noise radar. A noise radar works by cross correlating the received signal with a replica of the transmit signal. A high correlation indicates range to the target. Measured results with the noise radar show that patterns of peaks and valleys exist near the range of the targeted object. This paper looks at the correlation patterns generated by two different sized hollow metal tubes and a human at various look angles. The results show that the correlation patterns of the tubes are similar, but the correlation patterns of the human differ. Full characterization of human and non-human noise radar correlation patterns could confirm the presence of a human victim and information about his location within the rubble.
Improving and Extending the MARS Technique to Reduce Scattering Errors
The Mathematical Absorber Reflection Suppression (MARS) technique is a method to reduce scattering errors in near-field and far-field antenna measurement systems. Previous tests by the authors had indicated that NSI's MARS technique was not as effective for directive antennas. A recent development of a scattering reduction technique for cylindrical near-field measurements has demonstrated that it can also work well for directive antennas. These measurements showed that the AUT shouldbeoffsetfromtheorigin byadistanceatleastequal to the largest dimension of the AUT rather than only 1-3 wavelengthswhich hadbeenusedfor smallerantennasin the earlier MARS measurements. Spherical near-field measurementshaverecently beenconcludedwhich confirm that with the larger offsets, the MARS technique can be applied to directive antennaswith excellent results. The MARS processing has recently been modified to produce significantly improved results. This improvement isespeciallyusefulfor antennaswherethephasecenterof the horns is located inside the horn and varies with frequency like pyramidal Standard Gain Horns (SGH). Fewermodesarerequired for thetranslatedpatternandthe filtering is more effective at reducing the effect of scattering. The improvement is very apparent for pyramidal horns.
A NOVEL SPHERICAL SCANNER SYSTEM FOR WIRELESS TELEMATICS MEASUREMENTS
Modern vehicle telematics subsystems often employ wireless interfaces. The design and evaluation of these subsystems involves measurement of antenna characteristics or Over-The-Air (OTA) performance of the subsystem as installed in a vehicle. Several subsystems servicing multiple user applications may be installed in a single vehicle, with antenna structures located anywhere on or within the vehicle. In general, the radiation characteristics of each subsystem must be measured over a partial spherical surface surrounding the vehicle and of sufficient radius to be outside the reactive near-field of the Device Under Test (DUT). This paper describes a distributed axis spherical scanning system designed for vehicle applications. The elevation axis which supports the probe antenna has a measurement radius of 25 ft (7.62m). The elevation positioner is supported on a hydraulic vertical lift axis to permit the adjustment of the measurement coordinate origin to be in the same horizontal plane as the DUT phase center. The measurement instrumentation system supports VNA based antenna pattern measurements or active OTA testing of telematics subsystems. The system is suitable for outdoor or indoor measurement facilities. An outdoor installation is described.
Rapid Continuous Linear Spiral Planar Measurements for Millimeter-Wages
Bipolar planar antenna measurements have been used as an alternative to other planar scanning techniques such as plane-rectangular or plane-polar scanning. Bipolar scanning features important advantages such as the elimination of linear motion in measurement, increased stability, compact footprint, and a variety of data acquisition modes. The most rapid data acquisition mode for planar measurements overall, depending on range implementation, is the linear spiral sampling mode. This technique involves simultaneous incrementation of both the radial and azimuthal positioners to create a data grid in a spiral configuration. Data sampling and interpolation for linear spiral sampling has been obtained previously through rigorous development and modification of bipolar sampling requirements and interpolation techniques . Implementation of the continuous linear spiral technique is not a trivial task. Positional program requirements require non-uniform acceleration and velocity for each axis. Data acquisition requires precise synchronization of both positional and RF equipment. Finally, post-processing is complicated by the inherent nature of a linear spiral data grid. This paper will describe, in detail, the implementation of the linear spiral technique with our portable millimeter-wave bipolar planar measurement system with emphasis on the issues mention here. In addition, measurements of a 31GHz rectangular patch array using both the conventional bipolar and linear spiral techniques are compared for both measurement time requirements and pattern accuracy. The continuous linear spiral technique has shown a significant measurement time reduction and has shown excellent agreement with results obtained in comparison to previously implemented stepped spiral measurements.
Some Detailed Implementation Aspects of an Automated Error Assessment Scheme for Antenna Spherical Near-Field Measurements
This paper elaborates on certain aspects of a new measurement process that permits an assessment of spherical near-field (SNF) measurement errors based on a set of practical tests that can be done as part of any SNF measurement. It provides error bars for a measured radiation pattern in an automated fashion.
LABORATORY RESULTS ON THE COMPENSATION OF PROBE POSITIONING ERRORS IN THE NF â€“ FF TRANSFORMATION WITH HELICOIDAL SCAN
– far-field transformation with cylindrical scanning are efficiently determined by using an optimal sampling interpolation algorithm. The comparison of the far-field patterns reconstructed from the acquired irregularly distributed measurements with those obtained from the data directly measured on the classical cylindrical grid assesses the effectiveness of the approach.
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