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A ground station antenna for Galileosat application operating in right hand circular polarization at P-band has been designed, manufactured, and tested. Other than stringent environmental requirements for typical ground station antennas the specification call for an antenna with very stringent requirements on pattern shape and symmetry and a very severe control on side and back lobes. In order to ease the requirement on the antenna positioner the antenna should have very compact size and low weight. The final antenna consists of an array of 7 medium gain, dual linear polarized yagi elements as shown in Figure 1. This paper describes the antenna design trade-off activity including the selection of the most suited antenna technology and manufacturing details. It also reports on the testing in the SATIMO SG-64 multiprobe spherical near field test range with considerations on the associated measurement uncertainty. The final acceptance of the antenna was based on measurements performed in CNES and SATIMO.
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
This paper shall discuss a method for measuring the current distribution – in both magnitude and phase - along the length of a floating antenna operating on the surface of the ocean. The method makes use of a novel toroidal current sensing device and balun arrangement, with a vector network analyzer serving as the measurement instrument. The current data obtained using this method can then be used to compute the far-field pattern of the antenna, both at the horizon and overhead, in a manner similar to near-field scanning of aperture antennas. This new method has significant advantages over the conventional far-field method of measurement in terms of accuracy, time, and cost, and can also be used to determine the realized gain of the antenna. Measured and theoretical data shall be presented on example antennas to illustrate the process of measuring the current distribution as well as computation of the far-field pattern.
The in-flight pattern measurements of a sub-millimetre space telescope may be improved by de-termining the actual reflector anomalies and then in-clude the knowledge to these in the final pattern de-termination. The pattern measurements with a celes-tial object as source often have an insufficient signal-to-noise ratio for a pattern prediction outside the main lobe. Repeated measurements may improve this but even better is the possibility to extract data from different detectors operating at different frequencies. With the Planck Space Telescope as example, simulations of a displaced and distorted reflector have been carried out for noise contaminated amplitude measurements of Jupiter by 5, respectively 10, differ-ent detectors. First, the main beams of the antenna patterns are retrieved in a regular grid. Here the ac-curacy is limited by the noise level. Then, by a Physi-cal Optics optimization the actual distortions of the telescope's reflector are determined so that the calcu-lated radiation patterns of the antenna are correlated to the measured main beams. The patterns for the optimized and retrieved reflector geometry are shown to be precise at levels far below the noise floor in the direct measurements1. 1 The work presented in the paper has been carried out under ESTEC Contract No. 18395/NL/NB
Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.
A widely used time-gating technique can be effectively implemented in near-field (NF) antenna measurements to significantly improve the measurement accuracy. In particular, it can be implemented to reduce or remove the effects of the following measurement errors [1]: -multiple environmental reflections and leakage in outdoor or indoor NF ranges -edge diffraction effects on measurement accuracy of low gain antennas on a ground plane [3] In addition, reflectivity in the range can be precisely localized, separated and quantified by using the time – gating procedure with only one addition (a subtraction operation) added to the standard near-field to far-field (NF – FF) transformation algorithms. In this paper a step by step procedure is described which includes acquisition of near-field data, transformation of the raw near-field data from the frequency to the time domain, definition of the correct time gate, transformation of the gated time domain data back to the frequency domain, and the transformation of the time gated near-field data to the far-field. The time gated results, as already shown in [2], provides for more accurate far-field patterns. In this paper it is shown how the 3D reflectivity/multiple reflections in the measurement chamber or outdoor range can be determined by subtracting the time gated results from the un-gated data. This technique is illustrated through use of several measurement examples. It is demonstrated that the time gated method has a clear physical explanation, and, in contrast with other techniques [4,5] is less consuming (does not require mechanical AUT precise offset installation, additional measurement and processing time) and allows for a better localization and quantization of the sources of unwanted radiation. Therefore, this technique is a straightforward one and is much easier to implement. The main disadvantage cited by critics regarding use of the time gating technique is the narrow frequency bandwidth used in many NF measurements. However, it is shown, and illustrated by the examples, that the technique can be effectively implemented in NF systems with a standard probe bandwidth of 1.5:1 and an AUT having a bandwidth as low as 5% to 10%.
A method to reduce the noise power in far-field pattern without modifying the desired signal is proposed. Therefore, an important signal-to-noise ratio improvement may be achieved. The method is used when the antenna measurement is performed in planar near-field, where the recorded data are assumed to be corrupted with white Gaussian and space-stationary noise, because of the receiver additive noise. Back-propagating the measured field from the scan plane to the antenna under test (AUT) plane, the noise remains white Gaussian and space-stationary, whereas the desired field is theoretically concentrated in the aperture antenna. Thanks to this fact, a spatial filtering may be applied, cancelling the field which is located out of the AUT dimensions and which is only composed by noise. Next, a planar field to far-field transformation is carried out, achieving a great improvement compared to the pattern obtained directly from the measurement. To verify the effectiveness of the method, two examples will be presented using both simulated and measured near-field data.
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.
The cloud profiling radar (CPR) for the Earth, clouds, aerosols and radiation explorer (EarthCARE) mission has been jointly developed by JAXA and NICT in Japan. The development of CPR has required several technical challenges from the aspects of hardware designing, manufacturing and testing, because very large antenna reflector of 2.5m diameter with high surface accuracy, high pointing accuracy and high thermal stability had been required to realize the first space-borne W-band Doppler radar. In order to verify the RF design, we have just begun to perform antenna pattern measurement by using a CPR Engineering Model (EM). For this RF testing, we introduced a Near-Field Measurement (NFM) system with necessary capabilities for high accuracy measurement. This paper will present the summary of preliminary test results of the CPR EM antenna and the other technical efforts being taken for the antenna pattern measurement.
Spherical near-field to far-field transformation techniques allow for the reconstruction of the complete radiation pattern of the antenna under test (AUT) from the knowledge of the tangential electric field over a spherical surface [1-2]. However, in practical spherical near field measurements there are zones on the measurement sphere where data are either not available or less reliable. When the spherical wave coefficients (SWC) are calculated from incomplete near-field data by setting to zero the unknown samples, the abrupt discontinuity in the field values at the edge of the scan area may lead to erroneously large values of the higher-order spherical harmonic coefficients. Different solutions have been proposed to circumvent this problem [3-4] and have been demonstrated effective for small truncation areas [3]. In this paper a novel approach is proposed for the reduction of the truncation error in spherical near-field measurements. The method is based on a proper filtering of the SWC in accordance with the extent of the minimum sphere enclosing the AUT. More specifically, it consists in iteratively imposing the matching of the near-field with the measured samples and performing a spectral filtering in the spherical harmonics domain, based on the knowledge of the physical extent of the AUT [5-8]. The procedure has been tested on synthetic as well as measured near-field data and has proved to be effective and stable against measurement errors. The approach has shown to be effective even for increasing truncation areas.
The design of a specialized reflector antenna set that supports dual polarization, dual beam widths, and an integrated wideband monopulse tracking capability in the X-band range is described in this paper. The reflector antenna code available at The Ohio State University has been used as the design tool. The design of such an antenna has posed several challenges in the feed and reflector assemblies. The requirement for an integrated wideband monopulse has resulted in a feed array that contains 5 rectangular feed elements with a center-to-center spacing of 1" and a diamond configuration. The 5 feed design has been selected to enable a shared feed array and reflector surface for both transmit and receive beams that eliminates the need for a high-power wideband receiver protector in the radar system. The center feed element is used for transmit waveform and the 4 outer elements are used as receive elements only. Each feed element operates with horizontally and vertically polarized waveforms, requiring a total of 8 waveguide input ports. In this paper, the challenges of the dual beam widths, dual polarized, integrated RCS and tracking antenna are delineated and the tradeoffs among several design configurations are shown. The final design is selected based on the performance predictions using The Ohio State University Reflector Antenna Code. The performance of the antenna has been validated at the OSU compact range for pattern and gain. Both the design and measurement data are presented in this paper.
This paper presents the results of radiation pattern measurement of small reconfigurable slotted ultra wideband (UWB) antennas. The measurements were conducted by using RF measurement and instrumentation facilities, software tools available at WCC of Universiti Teknologi Malaysia. The proposed slotted UWB antennas are having band notched frequency at Fixed Wireless Access (FWA), HIPERLAN and WLAN bands. Band-notched operation is achieved by incorporating some small gaps instead of PIN diodes into the slot antenna. It is found that by adjusting the total length of slot antenna to be about a half-wavelength or less at desired notched frequency [1-3], a destructive interference can take a place, thus causing the antenna to be non-responsive at that frequency. It was also observed that the measured radiation patterns, H-planes, are omni-directional with slightly gain decreased at boresight direction for measured frequencies. There are also more ripples occurred in the measured pattern compared with the simulated one.
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.
Over-the-air (OTA) performance testing of wireless devices has become a major component of product qualification for today's wireless networks. The methodologies used currently expand on traditional passive antenna pattern measurement techniques and operate under the fundamental assumption that the radiation pattern of the DUT does not change for the duration of the test. Emerging wireless technologies such as MIMO that use multiple antennas and adaptive algorithms for communication violate that assumption. In addition, the enhanced feature set and higher complexity of these newer technologies means that there are other performance criteria to be evaluated beyond just radiated power and sensitivity. A new OTA test system has been developed that can simulate complex multipath systems in a fully anechoic environment. This technology allows evaluating the performance of the DUT in a host of simulated real-world environments. Existing spatial channel models used for conducted testing of these radios can be easily adapted to OTA testing. This paper describes the details of such a system, including requirements for calibration and validation, as well as showing typical test results and available metrics.
Anechoic chambers simulate open air test conditions and are advantageous for testing avionics systems in a secure, quiet electromagnetic environment. The 412th Electronic Warfare Group’s Benefield Anechoic Facility (BAF), located at Edwards AFB, California was designed for testing systems in the radio frequency (RF) range from 500 MHz to 18 GHz. For frequencies below 500 MHz, the installed radar absorbent material (RAM) does not effectively absorb incident RF energy, thereby allowing undesired RF scattering off the chamber’s floor, ceiling, and walls. This leads directly to measurement inaccuracies and uncertainty in test data, which must be quantified for error analysis purposes. In order to meet the desired measurement accuracy goals of antenna pattern and isolation measurement tests below 500 MHz, RF scattering must be mitigated. BAF personnel developed a test methodology based on hardware gating, range tuning and improved RAM setup, to improve chamber measurement performance down to 100 MHz. Characterizing the chamber using this methodology is essential to understanding test zone performance and thus increases confidence in the data. This paper describes the test methodology used and how the test zone was characterized with resulting data.
We introduce a new type of planar near-field measurement technique for testing small antennas which, heretofore, have been traditionally tested via spherical or cylindrical scanning methods. Field acquisition in both these procedures is compromised to a certain extent by the fact that probe movement induces change in relative geometry with respect to, and thus interaction with, the anechoic chamber enclosure. Moreover, obstructing equipment, such as antenna pedestals, may significantly impede, or even reduce the available angular scope of any given scan. Our proposed procedure, by contrast, minimizes both the residual interaction contaminant and the threat of obstruction. We have in mind here a variant, a hybrid version of planar scanning wherein, on the one hand, we limit severely the size of the acquisition rectangle (and thus minimize the contaminating influence of a variable probe/chamber interaction), while, on the other, we really do collect near-field data throughout a complete range of solid angle around any candidate AUT, front, back, above, below, and on both sides. Such completeness is achieved through the mere stratagem of undertaking six independent planar scans with the AUT suitably rotated so as to expose to measurement, one by one, each of the faces of an enclosing virtual box. In the meanwhile, the inevitable AUT pedestal per se remains immobile and removed from any occupancy conflict with the scanned probe. We have accordingly named our new planar near-field data acquisition scheme the “Boxed Near-Field Measurement Procedure.” With subsequent use of our Field Mapping Algorithm (FMA), elsewhere reported, we obtain the entire field exactly, everywhere, both interior and exterior to the surrounding (virtual) box. In particular, we achieve enhanced accuracy in the far-field patterns of primary interest by virtue of the completeness of data acquisition and its relative freedom from spurious contamination. The angular completeness of data acquisition conferred by our procedure extends in principle to antennas of arbitrary size, provided, of course, that due provision is made for the necessary scope of measurement rectangles. The benefits are seen to be especially valuable in the case of narrow-beam antennas, whose back lobe pattern details, usually deemed as inaccessible and hence automatically forfeited during conventional (i.e., utilizing a “onefaced box,” in our new way of thinking) planar near-field testing, are thrust now into full view. Our new, full-enclosure planar acquisition technique as now described has been verified by analytic examples, as well as by hardware measurements, with excellent results evident throughout, as we are about to demonstrate.
A novel approach using Artificial Neural network (ANN) is proposed to identify the faulty elements present in a nonuniform linear array. The input to the neural network is amplitude of radiation pattern and output of neural network is the location of faulty elements. In this work, ANN is implemented with three algorithms; feed forward back propagation neural network, Radial Basis Function neural network (RBF) and Probabilistic neural network and their performance is compared. The network is trained with some of the possible faulty radiation patterns and tested with various measurement errors. It is proved that the method gives a high success rate.
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.
In a recent paper [1], 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.
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.