AMTA Paper Archive
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Validation of a Compact Antenna Range for Broad Beam Antennas
A compact range was recently constructed at the Applied Physics Laboratory to measure broad-beam, fan-beam, and pencil-beam antennas (max aperture: 1 meters). Chebyshev absorber treatments, lightweight composite reflector, foam column mount for light-weight antennas, automated measurement software, and a novel feed spillover rejection algorithm are the technology elements implemented in this compact range measurement facility. This paper will describe a trade study that APL performed before the compact range antenna facility was built. Solutions to some of problems that were encountered during the construction will be discussed as well as the overall performance of the facility. The measurement of a broad-beam antenna will be compared to calculated pattern. This measurement will highlight the advantages of using a software range gate that was recently developed.
Correcting Dual Port Probe's Port-to-Port Calibration Using Near-Field Measurements
When a dual port probe is used for near-field measurements, the amplitude and phase difference between the two ports must be measured and applied to the probe correction files so that the measurements and calculations will have the same reference. For dual port linear probes, the measurement of this “Port-to-Port” ratio is usually accomplished during the gain or pattern measurements by using a rotating linear source antenna.1 When a dual port linear probe is used to measure a circularly polarized antenna, the uncertainty in this Port-to-Port ratio can have a significant effect on the determination of the cross polarized pattern. Uncertainties of tenths of a dB in amplitude or 1-3 degrees phase can cause changes in the cross polarized pattern of 5-10 dB.2 3 The paper will present a method for measuring the Port-to-Port ratio on the near-field range using a circularly polarized antenna as the AUT (Antenna Under Test). The AUT does not need to be perfectly polarized nor do we need to know its correct polarization. The measurements consist of two separate near-field scans. In the first measurement the probe is in its normal position and in the second it is rotated about the Z-axis by 90 degrees. A script then calculates the Port-to-Port ratio by comparing the crosspolarization results from the two measurements. Uncertainties in the Port-to-Port ratio can be reduced to hundredths of a dB in amplitude and tenths of a degree in phase. Measurements were taken at TRW’s Large Horizontal Near-field Antenna Test Range.
Measured Frequency Performance of a X-Band 4-Port Butler Matrix on a Planar Near-Field Range
Single-beam and composite-beam performance of a 4-port X-band waveguide Butler matrix was measured on the Georgia Tech Research Institute planar near-field range for wideband frequency performance. The techniques necessary to perform accurate measurements on a broad-beamed antenna in a near-field range will be discussed, and measured far-field pattern data collected at the design frequency of 9.3 GHz are presented and compared with predicted results of the Butler matrix. In cases where the measured data and the expected results do not compare well, aperture amplitude and phase data, transformed from the near-field data, are shown as a diagnostic tool for corrections. After correction, new data at 9.3 GHz are presented for comparison with predicted results, and selected farfield pattern data collected at 8.6 GHz and 11.0 GHz are presented.
Methods to Estimate and Reduce Leakage Bias Errors in Planar Near-Field Antenna Measurements
This paper describes two methods that can be used to measure the leakage signals in quadrature detectors, predict the effect on the far-field pattern, and correct the measured data for leakage bias errors without additional near-field measurements. One method is an extension and addition to the work previously reported by Rousseau1. An alternative method will be discussed to determine the leakage signal by summing the near-field data at the edges of the scan rather than summing below a threshold level. Examples for both broad-beam horns and narrowbeam antennas will be used to illustrate the techniques.
Measurement of Element Patterns in a Conformal Wing Array
In this paper, techniques are presented for the measurement of element radiation patterns of a belt-like C-band conformal array of microstrip patch elements, which wraps completely around the cross-section of an aircraft wing. The element patterns were measured, in situ, then analyzed in terms of phase and amplitude ripple versus element location around the wing. These results indicated trends in interference due to the experimental environment and the geometry of the wing itself. Experiments were conducted which minimized interference effects due to the environment, resulting in the true element patterns in the presence of wing platform interference. In an effort to identify platform-induced interference, anechoic absorber was used to minimize pattern ripple attributed to the edges of the wing, enabling validation of the measured element patterns against simulated data, which did not include platform interference. Thus, determining whether to include the platform effects in the measured data is dependent on the intended use of the results.
Detection of Antenna Pattern Measurement Errors by the Suppression of Permitted Spatial Frequency Modes
An error detection technique was developed for culling large masses of measured antenna pattern data by first removing information that is likely to be associated with the antenna. Since the maximum spatial frequency of radiation from the antenna can be determined by its electrical size, any energy outside that spatial band is not considered to be valid and may be used to flag suspicious data. This analysis can be accomplished rapidly and can be used to cull patterns containing such anomalies as spikes, notches, non-closures and multipath effects. This paper describes the method with examples from simulated and measured patterns.
Ultra-Wideband Antenna Pattern Characterization in a Non-Ideal EM Facility
We present and analyze a procedure for performing relative, ultra-wideband antenna pattern measurements in a non-ideal EM facility. Ultra-wideband, shortimpulse, TEM horn transmission measurements were performed and compared with computer-modeled radiation pattern results. These measurements allowed us to analyze radiation lobes and nulls in both boresight and off-axis antenna positions. The results show that the measurements performed in this testing environment agreed well with computer models.
New Antenna Design and Measurement for Automobile Applications Using Genetic Algorithm
New automobile antennas must be developed to satisfy the growing requirements of the automobile industry. The uses of GPS band antennas for vehicle applications are growing very rapidly in the modern telecommunication area. In automobile antenna design, there exists geometrical constraints and several requirements for antenna specifications, for example, a Right-Hand Circular Polaization (RHCP) for a GPS antenna. In this paper, a new antenna for the automobile applications is designed using a Genetic Algorithm. It is well known that the GA can be used efficiently in the designing of various antennas. The GA searches the solution space of the possible antenna geometries satisfying the design goals. The design goals are RHCP with low cross polarization, a low SWR, and an omni-directional gain pattern in the upper-half plane. These design goals will be included in the cost function. The GA produces a set of new optimal antenna geometries. A series of experimental tests of the new antennas is presented, and the results are compared with the theoretical prediction. The ESP 5, a theoretical Method of Moment (MoM) general-purpose code developed at the Ohio State University, is used for an analysis tool.
Spherical Pattern Measurement Techniques for Low Directivity Antennas
Requirements for pattern measurement of antennas with low directivity continue to increase. The wireless communications industry is a significant driving force behind this change, but other fields such as electromagnetic compatibility (EMC) have an emerging need of low directivity antennas that work well to microwave frequency ranges. Traditional microwave techniques used for highly directional antennas are not suitable for testing more broad-beamed or omnidirectional antennas. Spherical pattern measurement systems using dielectric support materials with low permittivity are required to obtain acceptable results. This paper will review several different spherical pattern measurement techniques proffered by the Cellular Telecommunications & Internet Association (CTIA) for testing cellular handsets. It will present a benefit analysis of each method and provide useful information for both the novice and experienced antenna user. It can be shown that with appropriate care, several different techniques can generate the same resulting data, but each method has its own unique benefits and drawbacks. Spherical surface plots of measured data will be provided to illustrate some of the pitfalls related to this type of pattern measurement, and results from a certified test site will be presented.
Development, Measurement, and Analysis of a Wideband Optically Multiplexed Beamformer Architecture (WOMBAT)
A Wideband Optically Multiplexed Beamformer Architecture (WOMBAt) was developed and characterized at the Crane Naval Surface Warfare Center Active Array Measurement Test Bed (AAMTB) facility. The project included development and integration of the WOMBAt photonic beamformer with the Active Array Measurement Test Vehicle (AAMTV). The AAMTV is a 64-channel transmit-receive (TR) module based phased array beamformer that is integrated with the AAMTB facility 12’x9’ planar near-field scanner. The AAMTV provided phase trimming and a small amount of electrical delay while the WOMBAt provided longer optical delays using commercial-off-the-shelf (COTS) components typically manufactured for the telecommunication industry. By integrating the WOMBAt with the AAMTV, a highly flexible test environment was achieved that included system calibration, multi-frequency scanning, and antenna pattern analysis. This paper presents antenna pattern results showing less than 0.7 dB of amplitude variation over the frequency range from 9 to 10 GHz at each of the measured nominal steering angles. The beamformer was steered to greater than ±69 degrees with an observed beam squint from 9 to 10 GHz of less than 1 degree.
Measurement of Wideband Arrays
Determination of Scan Element Pattern (SEP) and of Scan Impedance (SI) of wideband arrays is desirable, in addition to patterns and gain. Scan Element Pattern gives array gain versus scan angles and frequency, while Scan Impedance is the impedance versus scan angle and frequency that must be matched. Some organizations have been measuring SEP in transmit mode, with all elements terminated and the center element driven. This procedure gives erroneous results, as the mutual couplings are all passive. The way of properly measuring SEP is to place the array in a gain measurement setup as a receive antenna, so that all elements are terminated and properly excited. The nominal center element is connected to the receiver; the Scan Impedance mismatch is included in SEP. Knowledge of Scan Impedance is important, as it controls the impedance matching possibilities. It is however difficult to measure. Network analyzers (HP8510) measure impedance both (S11 and S22) by transmitting a signal and measuring the reflected signal, thus do not allow operation in a mode with all elements excited. A full feed network can be employed, with the network modified to allow measurement of the current and voltage at the center element. This method is seldom used. Because of the importance of SI, use is often made of waveguide simulators, and simulation codes. The infinite array Floquet unit cell codes must be used with caution as these codes omit edge effects; these may be very important in some types of coupled arrays. A planar array code is used to simulate both transmit (single element excited) SEP, and receive SEP. Data on SEP and SI are presented.
UHF Antenna Phased Array Measurements
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
Performance Characterization of the Full Performance and Ultra-Light Cylindrical Communications Arrays
Two cylindrical phased array antennas were characterized at the NAVSEA Crane.s Active Array Measurement Test Bed (AAMTB) facility. The antennas include the Full Performance Antenna (FPA) and the Ultra Light Antenna (ULA) that are intended for land mobile test sites for the United States Department of Defense. These air breathing, low-cost antennas are candidates for a new communication system. Crane.s role as the program Technical Advisor (TA) includes integration and performance testing at the component level, antenna level, and system level. This paper discusses issues related to the antenna-testing phase including pattern measurements, G-F, and high power safety concerns. The final goal of the integration and testing phase was to verify that the antenna RF performance specifications were met. To this end, conventional cylindrical near-field pattern testing was adequate for many items such as beam width, pointing angle, and side lobe levels. However, two issues required additional effort: G-F measurement and high-power transmit safety concerns. Since the majority of required measurements could be made using the near-field chamber and the antenna required special controllers and prime power sources, it was desirable to make all measurements in the same location. Hence, a new measurement process was required for G-F using a near-field range and the high-power safety concerns needed to be addressed.
Pattern Measurement of X-Band Standard Gain Horn Antenna Using Photonic Sensor and Planar Near Field Scanning Technique
We have measured the amplitude and the phase of the electric field on a planar area very near (about 0.3 wavelengths) to the aperture of a X-band standard gain horn antenna using a photonic sensor and transformed the aperture field distribution to the far field pattern. The measured aperture field distributions and antenna patterns agreed well with those calculated by the method of moments. Comparing the far field patterns by the photonic sensor and the conventional open-ended rectangular waveguide probe reveals that the antenna measurement using the photonic sensor has advantages over the conventional probe.
Wide-Band Dual Polarized Probes for High Precision Near Field Measurements
High precision near field measurement systems in dual polarization have stringent requirements on the probe performance in terms of radiation pattern shape, on-axis and off-axis polarization purity and port-to-port isolation. In general, these specific requirements can be fulfilled using single polarized probes for narrow frequency bands (about 10% relative bandwidth) and using mechanical rotation for polarization diversity. Consequently, several sealed probes and complicated procedures are necessary to cover the operational frequency band of most common antenna applications leading to inefficient and time-consuming measurement procedures. SATIMO has developed high precision wide-band, dual polarized near field probes covering the frequency range from L to Ka-band to overcome this problem. Two different probe technologies have been applied, each particularly well suited for the appropriate low (L to X-band) and high (X to Ka-band) frequency range. The low frequency probe design is based on a compact corrugated horn with capacitive orthogonal excitations. The high frequency probe design consists of an axially symmetric corrugated horn, a square to circular wave-guide transition, and a wide-band, high isolation ortho-mode junction (OMJ) exciting the orthogonal polarizations. Probes in C-band and Ku-band have been delivered to and tested by ALCATEL SPACE INDUSTRIES in their planar near field antenna test range in Toulouse. The C-band probes have operational bandwidths of 25% covering the entire commonly used transmit and receive frequency bands for C-band communication satellites. The Ku-band probes have operational bandwidths of 40% covering the entire commonly used transmit and receive frequency bands for Ku-band communication satellites.
The Effects of Target Motion on ISAR Imagery
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
RF Antenna Beam Blockage of a New CWI Illuminator Antenna
The Antenna Branch of NAVSEA Crane was tasked to design and formulate a plan to pattern test a CWI Illuminator antenna in line of sight with a SATCOM antenna. The Navy has problems finding new places aboard ship to mount antennas without having interaction between them. The separation would be about 19 feet, within the near field zone of the Illuminator. The testing was performed on a 2000 foot outdoor range. A special test fixture was designed by Crane Engineering to mount both the Illuminator and the SATCOM to their proposed mounting locations. The unique characteristic of this measurement approach is the mounting of both the FCS antenna and the SATCOM antenna and radome on a test fixture, which allows complete pattern measurements with different antenna orientations relative to each other. A limited raster scan was used for collecting the data in a 3-D result. Baseline data files were collected without the SATCOM present for comparisons. A Matlab program written to evaluate the results. The proposed mounting location produced unacceptable results in the radiating pattern of the Illuminator antenna. Crane Engineering calculated a new mounting location from the results of the data taken in the Raster scan. Subsequent testing was done and proved to be a valid location for the Illuminators test requirements.
Improved Antenna Radiation Pattern Measurements Using an Equalization Technique
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.
Adaptive Phase Correction Method to Account for Electrical Phase Imbalances Between Sum and Dfference Channels of an Array Antenna, An
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.
Near Field Calibration for Large Reflectors
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.
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