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AMTA Paper Archive

Frequency and Impedance Agile Real-Time Tuning Network for 200-400 MHz Antennas
N. Smith,C. Chen, J. Volakis, November 2011

Mobile communication devices have many different requirements; namely they often have stringent size constraints, and must efficiently radiate over several frequencies in a myriad of different environments. Furthermore, the antenna is often electrically small or unintentionally loaded by environmental effects which cause unpredictable changes in antenna impedance. Therefore an agile matching network that is self-tuned to increase matching efficiency is desired. Most existing tuning approaches minimize reflections looking into the matching network. It will be demonstrated that this approach does not guarantee optimal performance due to circuit losses. A better approach is to also maximize power transmitted through the matching network. This paper presents a real-time frequency and impedance-agile tuning design that automatically matches a very wide load impedance range (0.5. < Re{ZL} < 1K. and -1K. < Im{ZL} < 1K.) from 200 to 400 MHz by the use of varactor diodes with impedance tuning stubs.

Advances In Planar Mathematical Absorber Reflection Suppression
S. Gregson,A. Newell, G. Hindman, November 2011

When making antenna measurements, great care must be taken in order to obtain high quality data. This is especially true for near-field antenna measurements as a significant amount of mathematical post-processing is required in order that useful far-field data can be determined. However, it is often found that the integrity of these measurements can be compromised in a large part through range reflections, i.e. multipath [1]. For some time a technique named Mathematical Absorber Reflection Suppression (MARS) has been used to reduce range multi-path effects within spherical [2, 3], cylindrical [4, 5] and most recently planar [6, 7] near-field antenna measurement systems. This paper presents the results of a recent test campaign which yields further verification of the effectiveness of the technique together with a reformulation of the post-processing algorithm which, for the first time, utilises a rigorous spherical wave expansion based orthogonalisation and filtering technique.

Achieving the Desired Performance from a Radar Cross Section Pylon Rotator
M. Hudgens,T. Schwartz, J. Ward, November 2011

The desire to acquire Radar Cross Section (RCS) data on full scale models poses a number of challenges to the users of pylon / rotator systems. Typically, these full scale models have significant mass but have a relatively small foot print on which it is acceptable to mount the model to the rotational flange. The challenges to be addressed in this paper include designing a rotator that will have sufficient strength to support the weight of the model and the stress generated by the overturning moment. This rotator must have a sufficiently low profile and small volume so that it will conveniently fit within the model volume but still achieve a sufficient elevation travel to meet test objectives. This rotator must still properly close out the pylon at all elevation angles to prevent unwanted reflections. Additional design considerations include the test conditions and the test environment. A rigorous test requirement can demand special engineering features to mitigate the demands of relatively high scan speeds and extended run times. Environmental concerns including wind loads, temperature, humidity, and contaminants, must be factored into the design of modern RCS rotators. This paper presents the system design approach to address the requirements of a full scale model rotator. The paper examines consequences of selected potential design solutions and demonstrates the importance of performing trade studies.

Reduced Cost, Automated, RTCA-DO-213 Compliant – Commercial Radome Test System
J. Wilber, November 2011

The commercial aviation industry faces several issues in regard to servicing and maintaining the radomes that abound in the aircraft fleet flying today. The first issue is the historically high cost of radome test systems. As a result of this, there are limited numbers of test systems in operation today and some geographic regions have insufficient radome test capacity. Advances in weather radar and increased reliance on them for turbulence avoidance and more efficient route planning around storm systems will increase the importance of ensuring that weather radar systems are performing well and consequently that weather radar radomes are in good condition and have been adequately tested. Because of the potential consequence of flying with a bad radome and the demands of new radar systems, its more important than ever to ensure test systems in use adhere to requirements and to spread awareness of these challenges within the aviation community. Recently, a design effort was conducted specifically geared towards developing a system concept for radome testing that would both provide a robust test capability that fully meets the RTCA-DO-213 after repair test requirements and one that is much lower in cost than traditional systems that are fielded today. This paper describes the issues cited above and provides a description of the low cost -compliant solution

Evaluating the Time Domain Performance of Spiral Antennas Using Near Field Measurements
M. Elmansouri,M. Radway, D. Filipovic, November 2011

Ultra wideband (UWB) systems use short pulses in order to achieve high data rate wireless communications and/or radar resolution. Thus, UWB antennas should be designed carefully, both in time and frequency domains, with the system performance in mind. Time domain characterization of an antenna can be performed first by measuring the frequency domain transfer function of a direct link consisting of two identical antennas. Then, the time domain response is obtained by post processing the frequency domain data using the Inverse Fast Fourier Transform (IFFT). This paper discusses frequency and time domain performance of four-arm equiangular and Archimedean spiral antennas operating in mode 2. The frequency domain transfer function is synthesized using complex far field information measured in a spherical near-field chamber from 2GHz to 12GHz. The synthesized approach is validated using simulation and direct link measurements. The quality of radiated pulses is evaluated in terms of fidelity factor over a full field of view, a task not trivial for the direct link measurements.

Controlling the Far-Field Resolution in Near-Field Antenna Characterization
A. Capozzoli,C. Curcioi, A. Liseno, November 2011
Wideband Performance for Planar Antenna-PMC Configuration
D. Voltmer,E. Wheeler, E. Wandel, November 2011

Planar low-profile antennas over high-impedance surfaces show improved performance compared to that over metal ground planes. Unfortunately, these high-impedance surfaces often operate over narrow bandwidths. This paper describes an approach to high-impedance surfaces which permits improved performance over a broader bandwidth. Current approaches to the design of high-impedance substrates typically employ identical unit cells with the same resonant frequency to produce high-impedance behavior over a relatively narrow frequency range. The wide bandwidth performance described in this paper derives from cells having a size and subsequent resonant frequencies that vary with position on the PMC substrate. This approach is explored through simulations using CST Microwave Studio which show the improved performance of these wideband structures.

Probe Performance Limitation due to Excitation Errors in External Beam Forming Network
L. Foged,A. Giacomini, R. Morbidini, November 2011

New developments in ortho-mode junctions (OMJ) and probe technologies has enabled near field probes with up to 1:4 bandwidth, while maintaining the high performance standards of traditional narrow band probes [1–3]. The new probe technologies are based on inverted ridge structures providing four symmetrical feeding points for external balanced feeding. The inverted ridge structure stabilizes the frequency dependence of the OMJ while the external balanced feeding is a crucial feature to achieve the desired high performances. This paper briefly review the theory of balanced feeding and derive performance guide lines on the external beam forming network for achieving high port-to-port isolation and matching on a wide bandwidth with the inverted ridge probe technology. The relationship between excitation errors in the balanced feeding scheme and the spherical mode index µ.1 content of the probe is also investigated and upper bounds on acceptable excitation errors are derived.

Non-Invasive Radiation Pattern Measurement of Electrically Small Antenna using Fiber Optic Link
S. Bories,G. Le Fur, L. Cannavo, C. Lach, C. Delaveaud, November 2011

The proposed method is based on fiber-optic link connected to the antenna under test in order to measure the radiation pattern of electrically small antennas without any metallic cable effects. The experimental radiation characterization of electrically small antennas reveals significant effects of the RF cable strongly disturbing the antenna gain and radiation patterns for both polarization components. With a very simple use and a low cost, the presented systems can characterize radiation of both narrow and UWB compact antennas in the 0.1-4 GHz frequency band. The proposed test-bench is firstly validated then its performances are compared to classical cable measurements and antenna simulation results. Both measured and simulated results are compared and the agreement is excellent (±1 dB) for co polarisation in the different cut planes. Limitations of the proposed method are also addressed. For very low gain levels (for the presented antennas at 950 MHz and 473 MHz, a cross polarization level below -30 dBi) the 42 x 20 x 23 mm3 autonomous optical receiver is too large and alters the antenna under test radiation. In this last case, and for a using at higher frequency, it is recommended to miniaturize the optical receiver whose size is mainly governed by battery performances (6 hours).

Minimizing G/T Measurement Errors Using the Y-Factor Method
L. Albers, November 2011

G/T is a standard figure of merit describing the sensitivity of receive, active, microwave antenna systems. It is therefore critical to have an accurate test technique for measuring the G/T of systems with varying levels of sensi­tivity. The Y­factor method is a commonly used and well­understood technique for cha­racterizing receive microwave systems. While the concepts behind the Y­factor measurement techniques are generally understood, little quantitative work has been done establishing limits and rules regarding selection of mea­surement hardware and design of the mea­surement system. This paper seeks to provide examples of test systems and quantify the er­ror induced into the measurement due to mis­matched standard hardware. The variables and techniques involved in a G/T test are ex­amined using simulated results of measure­ment systems performed in VSS (Visual System Simulator).

RF Target and Decoy Simulator
D. Wayne, November 2011

RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile defense developers, whose job it is to defend against guided missiles, require a similar test environment adding the ability to insert decoy RF targets that can spoof the seeker. Both seeker development and counter-measure development can benefit from an RF test facility that can provide RF targets and decoys controlled by a real-time simulation. This paper addresses an RF Target and Decoy Simulator developed by MI Technologies that provides this test capability. The direction of the target and decoy emitters is independently controlled such that the centerlines of their radiated main antenna lobes are always directed at the RF seeker. Each emitter can be independently and simultaneously commanded along a spherical surface. High rates of acceleration and velocity are achieved all the way out to the ends of the test area to simulate the high line of sight rate that occurs at missile closure. The simulator is capable of safely stopping a decoy racing to the ends of the target area with minimal over-travel. Collision avoidance provisions prevent target and decoy from damaging each other during the simulation. The paper presents a description of the simulator, pertinent tradeoffs considered in the design and accuracy data of the simulator’s performance.

A Conformal X-Band Cylindrical Patch Antenna Array System
U. Olgun,C. Chen, November 2011

Fig. 1: Cylindrically conformal antenna array system This paper discusses an example of a unique cylindrical conformal array design approach. This exemplary design is composed of six subarrays of series-fed microstrip patch antenna operating in X-band. The diameter of the conducting cylinder is 7.5 inches in diameter. Each subarray contains seven patch antennas connected in series configuration and is connected to a single coaxial probe located at the center element. Such feed arrangement greatly reduces the number of feeding cables, increases the feed-line isolations, and minimizes the feed-line radiation. The elevation pattern of each subarray is controlled by the number of patches and impedance matching tapering via varying the width of connecting microstrip lines. The azimuth pattern is controlled by the subarray height above cylinder surface, substrate width, cylinder radius, and surface treatments between subarrays. Measurement results exhibited good impedance matching and broad antenna coverage in X-band.

Simultaneous Beam Characterization and Active RDP of a Multi-Beam Antenna
B. Rizzuto,C. Jones, P. Kolesnikoff, November 2011

The Enhanced Antenna Subsystem (EAS) is a 12 beam, receive only antenna which uses a combination of switched elements and phase delay to accomplish independent beam steering. The upper portion of the dome­shaped antenna is populated with 45 circularly polarized antenna elements in an icosahedron pattern along with 15 additional circularly polarized elements along the cylindrical skirt extension. The antenna was tested in our 35’ by 35’ by 65’ compact range. Pattern testing was accomplished by mounting the antenna to a roll positioner atop a high load tower, which was then mounted to an azimuth turntable. The range has a 20’ by 20’ reflector providing an 8’ quiet zone. Using a switching network, we simultaneously characterized 11 statically pointed beams while tracking the range source antenna with the 12th beam. Post processing of the data was performed to separate the beam data and calibrate out losses through the switching network.

Beam-Steering Computer Design for Space-Fed Phased-Array Antenna
P. Brady,D. Mauney, J. Skala, November 2011

In this paper, a beam-steering computer design is explored for a large space-fed phased-array antenna. GTRI previously developed a beam-steering computer for a smaller phased-array antenna which accomplished spherical propagation focusing and multiple phase-only beam-broadening modes. In a subsequent effort, the beam-steering computer design was scaled for a large phased-array antenna to accomplish similar tasks. To verify the design, a series of far-field measurements was initiated to characterize the performance of the antenna by comparing with past measured near-field data and modeled results. One of the primary responsibilities of the beam-steering computer was the focusing of the spherical propagation wave front. A measurement technique is discussed to accomplish this focusing for the large space-fed phased-array antenna by correcting measurement errors in the spherical propagation routine of the beam-steering computer. Additional patterns were taken using the updated feed horn focal point for spherical propagation correction. By correcting the phase errors caused by spherical propagation defocusing in the original beam-steering computer, significantly better antenna performance was obtained, including higher peak gain, reduced nearby sidelobe levels, and removal of beam-pointing errors. Another important responsibility of the beam-steering computer was the ability to realize multiple antenna modes, including a focused pencil beam as well as defocused broadened-beam modes. A stochastic gradient descent algorithm was utilized to obtain several phase tapers to accomplish beam-broadening for the antenna modes. These modes were implemented in the beam-steering computer and tested on a far-field range. The antenna patterns were compared with modeled results and with previous measured data to ensure validity of the implementation.

Adaptive Acquisition Techniques For Planar Near-Field Antenna Measurements
D. Janse van Rensburg,D. McNamara, G. Parsons, November 2011

The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements is described. A decision function is used to track the accuracy of a measurement as the data acquisition proceeds, and to halt such acquisition when this is considered sufficient for the measured quantity of importance. Possible decision functions are defined and compared. Several test cases are presented to show that significant test time reduction is possible when compared to traditional acquisition schemes.

Accuracy of Near Field Pattern Measurements Performed with Analytical Probe Models
F. Boldissar,A. Haile, November 2011

Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.

A New Planar Antenna Element For Ka-Band Applications
R. Glogowski,J. Zurcher, C. Peixeiro, J. Mosig, November 2011

One of the most promising bands for long-range radiocommunications is the Ka-band (25-40 GHz). This is due to the existence of a natural radio transmission window around 30 GHz. Both terrestrial and satellite transmission systems are planned on this frequency band. For satellite applications, circular polarization is needed and the antennas or antenna arrays must frequently exhibit specially tailored radiation patterns. This paper proposes an efficient planar element for Ka-band telecom and remote sensing applications. The element has reduced losses (and hence good efficiency), while providing circular polarization (AR better than 3 dB) and good matching (better than 10 dB) in the 25.5-27.0 GHz frequency band. The element is fed by an integrated low loss transmission line (suspended strip line, SSL). This modular design allows an easy grouping into high efficiency subarrays, which include beam forming networks (BFNs) built in the same SSL technology and an innovative transition to the final standard waveguide feeder.

Antenna Pattern Measurements of an S-Band Satellite Communications Phased Array Antenna Panel
A. Lyons,B. Thrall, M. Weiss, M.B. Davis, M. Huisjen, R. Haupt, November 2011

A spherical array designed for hemispherical coverage of satellite communications at S-band that is approximated by hexagonal and pentagonal planar panels. Ball Aerospace built a segment of a 10m diameter spherical array that has one center pentagonal panel and five surrounding hexagonal panels. This paper de­scribes our efforts at testing one large hex­agonal panel in a compact range.

Spherical Near-Field Measurements at UHF Frequencies with Complete Uncertainty Analysis
A. Newell,P. Pelland, B. Park, T. White, November 2011

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.

An Empirical Near-field to Far-Field Convergence Study for Antenna Measurements
P. Nelson,J. Henrie, November 2011

Pattern distortion due to finite range measurement of antennas in close proximity to electrically large metallic media is examined. The ubiquitous yet arbitrary 2D2/. distance requirement cannot be blindly applied to scenarios where antennas couple to nearby structures. A C-band standard gain horn antenna is analyzed near a circular metallic plate at 6 GHz using the commercial software FEKO. The near-fields are computed at various radii, which are set to multiples of D2/., where D is defined as the largest dimension of the complete structure. The radiating near-field patterns are normalized and compared to the far-field pattern. Results indicate that measurement at 2D2/. may not be necessary. Increasing fractions of D2/. results in a diminishing measurement error that may be tolerable, depending on the intended application.







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