Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
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.
Wireless sensors network desires a vertically-polarized, omnidirectional antenna. In the case where the sensor network was deployed close to ground, the communication range of the network degrades. To quantify the communication range near the ground, several field measurements have been reported with monopole and microstrip patch antennas in the literature. In this study, a low-profile antenna design suitable for wireless sensor network is introduced. Its performance near a metallic ground plane is measured and simulated.
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%.
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. .
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.
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 [3]. 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 IsoFilterTM technique was originally demonstrated to operate by rejecting secondary signals that derive from reflections off of a nearby metallic object – namely, the ground plane surface supporting a small pyramidal horn.[1,2] The aperture of the horn was located several wavelengths above the ground plane and the sidelobes and backlobes of the horn illuminated the ground plane itself. The success of this demonstration has been sufficient to encourage us to pursue further the question of how well the IsoFilterTM technique will work to suppress other types of secondary signals– such as signals coming from other elements of an array antenna or another individual first-order primary radiator nearby. Here we report on some of the results of that investigation. We have calculated the far-field patterns of a sparsely populated array and applied the IsoFilterTM technique. The goodness of the suppression is judged by how well the “IsoFiltered” result agrees with the calculated pattern of the individual radiator.
Currently there is a lack of facilities capable of measuring the full upper hemisphere radiation patterns of antennas mounted on an infinite ground plane. Measurements performed with a finite ground plane suffer diffraction interference from the truncated edges. To circumvent this problem, a new measurement setup was developed at the Ohio State University ElectroScience Laboratory (ESL) for fully characterizing upper hemisphere radiation gain patterns and polarization for antennas up to 4” in diameter from 1-18 GHz. A probe antenna is positioned 46” away from the antenna under test (AUT). The ground plane end diffractions are removed using time-domain gating. The key design consideration is to position the probe antenna in the far-field region and yet shorter than the radius of the ground plane. This paper will present the calibration procedure necessary for the measurement system and it’s limitations due to ground plane probe antenna coupling at low elevation angles. In addition, the complete radiation pattern of a 4” monopole measured from 1-5.5GHz to demonstrate the systems capability for the lower third of the systems operating frequency range.
This paper presents a novel method to evaluate very small stray signals in compact range. The ripples of signals probed by an omni-directional antenna along the orthogonal direction of the bore sight could be treated as signals in time domain. Transforming the probed data with fast Fourier transform (FFT), the direction and amplitude (relative to the test signal) of each stray signal could be obtained. To improve the accuracy, time domain software gating should also be used in calibrating the measurement error of amplitude and phase. The presented method has the ability to measure very small stray signals with good angle resolution. The method has been tested by both simulation using MATLAB and experiment in the compensated compact range CCR120/100 in CAST using a monopole antenna centered on a circular ground plane as a probe. Good results were obtained.
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.
In low profile applications, a perfect electrical conductor (PEC) ground plane closely placed behind the radiating element significantly degrades antenna performance when its electrical height is less than ./15. Existing approaches such as lossless electromagnetic band gap (EBG) structures and artificial magnetic conductors (AMC) tend to operate over a small bandwidth. Herewith, we introduce two alternative ultra wide bandwidth (UWB) approaches by coating the ground plane with a magneto–dielectric (or ferrite) layers. The first approach employs a layer of high ratio to generate in-phase reflection similar to that caused by a perfect magnetic conductor (PMC). The second approach adapts a lossy layer having similar and values. Radiation enhancements achieved via these ground planes treatments will be demonstrated by simulated and measured results. Design guidelines based on our parametric study will be also given.
The goal of this research is to develop an unconstrained reconfigurable programmable array antenna. The concept is to build patch arrays using individual controllable pixels. The aperture of the system is made up of a large array of small (1/10 .min) pixels. Each pixel is a small piston made up of a metal top, a dielectric shaft, and a metal base. The pistons can be moved up and down under computer control. When all pistons are in the down position, a ground plane is created. When a line of pixels is raised into the up position, a microstrip transmission line (a metal line over a dielectric substrate) is created. A patch antenna is created when multiple pixels are raised into the up position to form a larger rectangle or other shape. In the final design, a set of feed lines and antennas can be created in any pattern within 1 millisecond. Under computer control, it is possible to change the beam direction, the beamwidth, the polarization, and the frequency of operation of the array. Design details, theoretical models, and the behavior of test fixtures and configurations will be discussed during this presentation.
This paper will present techniques used to simulate semi-hemispherical spiral antennas with measured VSWR and antenna pattern data for performance verification. Previous work on semi-hemispherical spiral antennas has been done by Lobkova, Protsenko, and Molchanov [1]. GTRI researchers have built on this work by developing a MATLAB computer model to create a general semi-hemispherical spiral antenna pattern model. Parameters that can be adjusted include the radius of the sphere, the number of turns of the spiral, the creation of a 1-arm or 2-arm spiral, and the inclusion of dielectric material between the spiral and ground plane. In creating the MATLAB computer model, GTRI researchers found errors in the notation of the elliptical integral in [1] and added additional details for the calculation of the antenna pattern. The paper will then present the characterization of a specific example of a semi-hemispherical spiral antenna. First, the VSWR of a single antenna was measured using a standard HP8510 Network Analyzer setup. Next, antenna pattern data was measured for a single spiral antenna and a pair of spiral antennas on both the GTRI planar near-field range and the GTRI anechoic chamber. The paper will conclude with the presentation of the modeled and measured antenna pattern data for the single antenna case.
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 transmission line system has been developed for an electromagnetically transparent antenna array. The goal was to provide equal signal distribution to the array elements while maintaining the transmissivity of the antenna. The transmission lines consist of microstrip directional power couplers which are fed in series. This reduces the transmission line length needed. The transmission line was built, tested, and integrated with an array of circular polarized array elements mounted over a frequency selective surface (FSS) ground plane. Preliminary bench tests performed on the integrated array with a small test dipole indicated that the transmission lines provided uniform signal distribution. Outdoor far field measurements of the integrated antenna indicated that the antenna performance was satisfactory. The integrated antenna array was tested in the compact range located at the ElectroScience Laboratory at The Ohio State University. These tests were used to accurately characterize the antenna performance at S band and the transmissivity properties of the integrated array at L band. The measured antenna pattern and beamwidth were consistent with predictions. Transmissivity of the antenna as viewed by a second antenna was also consistent with predictions.
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.
This paper describes the design and testing of a feed network for a transparent flat plate array antenna. This antenna is the top of a stack of three antennas that must occupy the same volume while pointing in different directions. At many pointing angles, the antenna will create blockage for the antennas underneath. In order to minimize the blockage, the array and its transmission lines must be as transparent as possible to the antennas underneath. The flat plate array consists of active elements over a frequency selective surface (FSS) ground plane that is transparent at the frequencies of the antennas below. The feed lines must also be transparent to the antennas below. This is achieved by minimizing the total area occupied by the feed lines. Rather than the traditional corporate feed network, a series feed network was designed. Such a network requires that each individual feed point must be fed with a coupler where the coupling coefficient is adjusted to distribute the same power to each array element. We will show the details of the design of the network as well as a set of measurements that show the performance.
A strong interest exists in the commercial and military sectors for small and broadband antennas. For instance, in the automotive industry there is a need for a single antenna operating in the frequency range of 825-2500 MHz (AMPS, DAB, GPS, PCS, SDARS). For military applications, there is also a need to have a single aperture which permits operation in different communication bands and can be also used for imaging and guidance applications. These needs require wide band antennas, such as the miniaturized spiral antenna. In this paper we present the implementation of a spiral antenna situated on a ground plane that is fully functional at the size of 0.16 wavelength onward. Low profile (0.05 wavelength) and broadband operation design goals bring unique challenges, which must be confronted with multiple-front techniques. A combination of antenna geometry design and material loading results in the desired miniaturization effect. Further techniques, including the use of distributed resistors ensure good axial ratio and VSWR. Pattern uniformity and phase linearity of the antenna was also improved. In addition, we also examine the effectiveness of broadband spiral antenna miniaturization as a function of loading material’s dielectric constant.
A hemi-spherical near-field test system with to be considered. This type of test system offers a added far-field capability is described. The facility has practical solution to the test problem in that combined been constructed for the characterization of automotive motion of a probe antenna and the object under test, antennas. The test system consists of an 11m tall allows for spherical data acquisition covering one half of dielectric gantry, a 6.5m diameter in-ground turntable and the spherical surface. The configuration also allowsa 28m-diameter radome enclosure. Special software integration of a conducting ground plane as well as a required to compensate for the reflectivity in the facility radome enclosure for weather protection andand the hemi-spherical truncation was developed and confidentiality. forms an integral part of this test system. The characteristics of this facility are described in this paper The characteristics of this newly developed and measured data is presented. facility are described in the following section of this paper.
We summarize here a number of advances in Impulse Radiating Antennas. These devices are composed of a reflector and a broadband feed. We have demonstrated improved gain and reduced crosspol by using feed arms located at ±30° to the vertical, as opposed to the original design that placed the arms at ±45°. We have reduced the return loss (flattened the TDR) at the splitter, at the feed point (focus), and at the resistors in the feed arms. We have added a ground plane that enhances mechanical stability and reduces crosspol. We have also improved the mechanical stability of the feed point.