AMTA Paper Archive
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Antenna Measurement Techniques for SDARS Antennas
The purpose of this paper is to present antenna measurement techniques of antenna modules for Satellite Digital Audio Radio System (SDARS). SDARS employs dual-transmitter broadcasting formats which include simultaneous transmission of signals from both satellites and terrestrial transmitters. An SDARS antenna efficiently receives both satellite and terrestrial signals: it has relatively good circularly polarized gain at high elevation angles and acceptable linearly-polarized gain at the horizon. Popular SDARS antennas are small ground- depended patch antennas etched on ceramics and ground- independent mast antennas such as quadrifilars. Ceramic patch antennas have a relatively narrow bandwidth of operation. Thus, tuning such antennas to the right frequency is critical. The measurement techniques presented help engineers and technicians evaluate SDARS antennas and determine whether they are correctly designed. We shall describe hardware platforms for evaluating impedance, radiation characteristics, and real-world performance. Parameters such as VSWR, antenna gain, axial ratio, as well as receiver satellite C/N and terrestrial BER will be discussed.
Pattern Measurement Demonstration of an Untouchable Antenna
A method is presented to measure the antenna pattern of an AUT where the antenna port is inaccessible. That means that it is not possible to connect a test cable, nor can the termination be changed physically. In some cases there is no test port at all. The only variation possible is to change the input impedance of the first receiver or LNA by switching it on and off. An RCS-technique can be used to retrieve the radiation pattern. By experimental comparison between the conventional pattern measurement technique and the RCS-technique it is shown that pattern determination via RCS-measurements is feasible. In addition, the measurement method offers the advantage of directly reducing the influence of systematic measurement errors. On the other hand, the penalty is put on power efficiency and a subsequent limited dynamic range.
Measurement of Backscattering from RFID Tags
This paper presents a method for measuring signal backscattering from an RFID tag and calculating tag radar cross-section (RCS), which depends on the chip input impedance. We present a derivation of a theoretical formula for RFID tag radar cross-section and an experimental RCS measurement method using a network analyzer connected to an antenna in an anechoic chamber where the tag is also located. The return loss of the antenna measured with and without the tag present in the chamber allows one to calculate the power backscattered from the tag and find tag RCS. Measurements were performed in anechoic chamber using RFID tag operating the base station (called “RFID reader”). RFID tag antenna is loaded with the chip whose impedance switches between two impedance states, usually high and low. At each impedance state, RFID tag presents a certain radar cross section (RCS). The tag sends the information back by varying its input impedance and thus modulating the back-scattered signal.
An Apparent Discrepancy Between Impedance Mismatch Factors for Near-Field and Far-Field Measurements
In making accurate measurements of antenna gain one must correct for the impedance mismatches between (1) the signal generator and transmitting antenna, (2) between the receiving power sensor and the receiving antenna and (3) between the signal generator and receiving power sensor. This is true for both far-field gain measurements and near-field gain measurements. It has recently come to our attention that there is a lack of clarity as to the form the mismatch factor should take when correcting near-field measured data. We show that a different form of impedance mismatch factor is to be used with the voltage domain equations of near-field than has been used with the power domain Friis transmission equation.
Effects of Array Panel Joint Discontinuities on RF Calibration
In this paper is presented an experimental investigation of conventional array calibration in the presence of various kinds of joint discontinuities between array panels. Two rigid array panels were positioned such that the element lattice was continuous across a narrow joint. Three kinds of discontinuities were applied to the joint: (1) an angle, (2) a gap (including an edge), and (3) a step between panels. Each type was investigated for joints oriented in the E-plane and the H-plane. Each discontinuity was also varied in magnitude so as to observe parametric effects. Planar near-field-range (NFR) measurements were made in a conventional array calibration mode and a near-field pattern mode. Processing included separating the pattern component due to element transmission (impedance) change from that due to pattern shape change. Results show that conventional calibration methods quickly become inadequate to calibrate these discontinuities because they change element pattern shapes.
Three-Antenna Method for Group Delay Calibration
Antenna systems are increasing in complexity at a rapid pace as advances are made in electronics, signal processing, communication, and navigation technologies. In the past, antenna design requirements have focused on parameters such as gain, efficiency, input impedance, and radiation pattern (e.g., beamwidth and sidelobe level). For some new systems, the group delay characteristics of the antenna are important, where the group delay is proportional to the derivative of the insertion phase as a function of frequency. The group delay is required to stay within certain bounds as a function of frequency and pattern angle. Unfortunately, there are not well established methods or standards for calibrating antenna group delay like the standard methods used for gain and input impedance. This paper presents a method for calibrating the group delay of three antennas based on an extension of the widely used three-antenna gain and polarization calibration methods. No prior knowledge of the gain or group delay of the three antennas is required. The method is demonstrated by a measurement example where it is shown that multipath errors and time gating can be critical for calibrating the group delay.
Impedance Measurement in Millimeter-Wave Near-Field Antenna Measurements
Measurements of the insertion loss and impedance in antenna characterization are very important and should be traced back to national attenuation and impedance standards. Vector network analyzers commonly used to measure the impedance are not suitable for millimeter-wave antenna measurements because movement of DUT (Device Under Test) during measurement is required and long cable of high loss for connection between the network analyzer and the DUT mounted high above the floor increases measurement uncertainty. In this paper, a conventional microwave subsystem based on external mixer configuration is modified to measure the impedance of DUT without using the vector network analyzer in millimeter-wave frequency range.
FSS-Loaded Pyramidal Absorber
This paper describes a new approach to improving the low frequency reflectivity performance of geometric transition radar absorbent materials through the use of impedance loading in the form of one or more included FSS layers. The discussion includes theoretical predictions and measured data on modified commercially available RAM which confirm the validity of the concept.
Simulation and Experimental Verification of Wideband Planar Power Dividers
Planar power dividers with a good match over a wideband of frequencies are designed using Klopfenstein impedance taper. To validate the proposed design procedure a 2-way stripline and a 2-way microstrip power divider are designed based on simulation, fabrication, and measurement. The measured return loss reveals better than –24 dB (from 4.3 GHz to 19.5 GHz) for a stripline configuration and –27 dB (from 2.2 GHz to 12 GHz) for a microstrip line configuration. Guidelines for accurate simulation and experimental verification are also presented.
Sub-Wavelength Measurement of Electromagnetic Inhomogeneities in Materials
Microwave microscopes that measure surface impedance or roughness have been demonstrated with fine spatial resolutions of less than a micron. These microwave probes are practical only for samples less than a few inches in size. However, composite materials in applications such as multi-layer radomes, embedded frequency selective surfaces, or integrated EMI shielding, have larger length-scale features embedded within a multilayer laminate. Diagnosing larger-scale, subsurface features such as joints/seams, periodic elements, imperfections, or damage is driving a need for methods to characterize embedded electromagnetic properties at mm or cm length-scales. In this research, finite difference time domain (FDTD) simulations and experimental measurements were used to investigate a probe technique for measuring sub-wavelength sized features embedded within a dielectric composite. For these applications, the probe interacted with the sample material via both evanescent and radiating fields. A dielectrically loaded, reduced size, X-band waveguide probe was designed in a resonant configuration for improved sensitivity. Experimental measurements demonstrated that the probe could characterize small gaps in ground planes embedded within a dielectric laminate. Simulations also demonstrated the possibility of detecting more subtle imperfections such as air voids.
A Probe Station Based Setup for On-Wafer Antenna Measurements
A probe station based antenna measurement setup is presented. The setup allows for measurement of complex impedance and radiation patterns of an on-wafer planar antenna, henceforth referred to as the device under test (DUT), radiating at broadside and fed by a coplanar waveguide (CPW). The setup eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the DUT with a coplanar RF probe. In addition, the DUT is probed exactly where it will be connected to a transceiver IC later on, such that no de-embedding of the measured data is required. The primary sources of measurement errors are related to calibration, insufficient dynamic range (DR), misalignment, scattering from nearby objects and vibrations. The performance of the setup will be demonstrated through measurement of an on-wafer electrically short slot antenna (.0/35 × .0/35, 5 mm2) radiating at 2.45 GHz.
Investigation into the Physical Mechanisms Underlying High-Frequency Pattern Degradation in Broadband, Double-Ridged Horns
Double-ridged waveguide horns can provide better than 10:1 relative frequency bandwidth over which they exhibit excellent impedance match and power transfer characteristics. However, the radiation pattern of such an antenna generally becomes more complex at the high end of its operating frequency range. That is, the pattern degenerates from being predominantly single-lobed at lower frequencies to a more complicated pattern exhibiting four gain maxima around the principal axis, all of which are greater than the gain on the principal axis. Here, we present some numerical simulations that appear to indicate that this behavior might not be directly related to higher order modes in the feed region and is not due to manufacturing imperfections, but rather is simply due to the overall taper of the horn itself.
Surface Impedance Modeling of an Outdoor Groundwave Antenna Range with a Non-Uniform Ground Plane
The groundwave correction method of measuring the gain of a vertical antenna over a lossy ground plane is an accepted means of performing a gain measurement without the need for a standard reference antenna. However, on antenna ranges where the ground plane is not uniform, this approach may not yield accurate results over certain portions of the test band due to discontinuities in the ground. This paper shall present a method for using surface impedance methods to predict the performance of an outdoor antenna test range that has a non-uniform ground. Comparison with measured data shall also be presented over the commercial HF and VHF bands.
The device of the embedded control of parameters of the microwave feeder of airborne radar
A device and algorithm of measuring of microwave airborne radar antenna impedance and input power level are presented. A compact five-port microwave reflectometer, p-i-n diodes switch, single microwave detector are used. The output detector signal is processed. All of that results in decreasing of the cost of equipment, elimination of instrument components non-ideality and reaching of high equipment accuracy.
Designing Small Size Low Frequency Antennas
This paper presents the results of an intensive investigation for trading off size vs. frequency for a large bandwidth antenna. Theoretical limits were established to determine minimum size as a function of gain and frequency. Bandwidth for the antenna developed was 50 MHz to 2000 MHz; stimulation was done with 50 ohm input impedance. The antenna broadband elements were located in a unique cavity, 6 inches in depth and 15 inches in diameter. The unique ground plane was composed of a combination of a Ferrite Region and Perfect Electric Conductor region which was implemented using Silver particles embedded in MEK. The cavity was fabricated using Carbon Composites to reduce weight. FireFly Opus antenna is in the testing phases.
Improvements on High Performance Solid State Frequency Extention Units for Full-Band Frequency Coverage at mm-wave Frequencies.
Solid state diode based multipliers and sub-harmonic mixers are enjoying increasing popularity as frequency extenders for the mm-and sub-mm wave frequency bands. Nowadays, models are available with 40% bandwidth, thus covering full frequency bands, with a reasonable amount of output power. When driven by a frequency synthesizer, they exhibit excellent frequency and phase stability, in contrast to tube devices like a Backward Wave Oscillator (BWO). Drawbacks of the multipliers and sub-harmonic mixers (SHM’s) include their low efficiency, requiring high power amplifiers (HPA’s) to drive them, and the difficulty of achieving broadband impedance matching, which makes it hard to get a constant performance level over the band. For the transmit module, a single HPA driving the multiplier directly turned out to be a satisfactory solution. On the receive side, a feedback circuit regulating the LO power amplifier was introduced. This circuit is based on pilot tone injection in the IF channel of the SHM. The modules have been breadboarded and tested.
Antenna miniaturization using impedance-matched ferrites
Antenna miniaturization has already been demonstrated using equal inductive and capacitive loading to improve antenna impedance at high frequencies, before and after loading. Inductive loading was introduced by coiling the antenna arms to form an inductor like coil, whereas the capacitive loading was achieved using dielectric material. However, this approach can only be applied to miniaturize wire antennas. Here, an alternative miniaturization technique is introduced, using low-loss ferrite composites. The inductive and capacitive loading is now provided by the permeability and permittivity of the ferrite composite, respectively. Of course, the ferrite should possess equal permeability and permittivity (i.e. e’r = µ’r) and must be of low loss for large bandwidths. The basic concept of this approach is to match the impedance of the material to that of free-space, and thus minimize reflections caused by impedance mismatches. In this paper a miniaturized spiral antenna is presented, using the above technique. The challenges of fabricating such a unique ferrite material will also be discussed. .
CHARACTERISTIC IMPEDANCE ANALYSIS OF A SLOT LINE WITH THE USE OF ADAPTIVE NEURO-FUZZY INFERENCE SYSTEM
The slot line, a transmission line suitable for application to microwave integrated circuits, may be used in place of or in association with microstrip. This paper presents an alternative method based on the adaptive neuro-fuzzy inference system (ANFIS) for computing the characteristic impedances of slot lines. The ANFIS is a class of adaptive networks which are functionally equivalent to fuzzy inference systems. The ANFIS has the advantages of the expert knowledge of the fuzzy inference system and the learning capability of neural networks. Different optimization algorithms, hybrid learning, genetic, simulated annealing, and least-squares, are used to determine optimally the design parameters of the ANFIS. The algorithm performances for the optimization of the ANFIS model parameters are compared with each other. The results of ANFIS are compared with the results of a commercial electromagnetic simulator IE3D and closed form expressions (CFE) obtained by curve fitting technique to the numerical results.
ANALYSIS, DESIGN, OPTIMIZATION AND IMPLEMENTATION OF A CIRCULARLY POLARIZED, X-BAND MICROSTRIP 2 X 2 SEQUENTIALLY ROTATED PHASED ANTENNA ARRAY
Paper discusses the design, optimization and implementation of a Circularly Polarized (CP) microstrip 2 x 2 sequentially rotated phased antenna array for an X-band onboard satellite transceiver. In the final design, CP radiation is constructed by using CP elements, having unique sequential rotation along with sequential phase shift feeding–giving wider 3dB Axial Ratio (AR) Bandwidth. CP in each patch element is achieved by a perturbation segment, in this case a pair of truncated corners and with a single point feed–reducing complexity, weight and RF loss of the array feed. First analysis based on cavity model approach for the single CP patch is carried out, which is used to determine the normalized perturbation parameter. The initial dimensions are calculated using perturbation analysis. Optimization initially for individual patch and then for the array is performed using full wave analysis tools based on Method of Moments (MoM), and verified using Finite Difference Time Domain (FDTD). Finally, the measured input impedance and radiation patterns are correlated with the calculated results. It is observed that the measured Gain and 3db Beamwidth agrees well with the simulated results of the array optimized using MoM, while the measured results of Axial Ratio, VSWR and reflection coefficients Sxx follows closely the results from the simulations based on FDTD.
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.
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