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.)
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.
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.
There is a great interest in the automotive and military sectors for small and broadband antennas that meet modern communication needs. These needs require ultra-wide bandwidth (>10:1) UWB antennas, such as the spiral antenna. However, the physical size at the low-frequency end typically becomes too large for practical applications. To reduce the size of the antenna, miniaturization techniques must be employed such as the use of high-contrast dielectric materials. Size reduction using high-contrast materials has been demonstrated for narrowband antennas, such as patch antennas, but not for broadband antennas to our knowledge. Therefore, the concept of miniaturizing a broadband spiral antenna using dielectric materials will be investigated experimentally and numerically. Issues that arise from dielectric loading such as impedance reduction will also be addressed. It will be shown using the results from these studies that there are practical limitations to the amount of miniaturization which can be achieved.
A novel antenna miniaturization approach utilizing artificial transmission-line (ATL) structures whose impedance and phase velocity are mainly controlled by distributed reactive elements is explored. First, the slow- wave phenomena and impedance control in ATL will be demonstrated. Then, miniaturization of a resonating structure will be presented. Finally the application of ATL on antenna structure will be demonstrated. The proposed miniaturization approach is inherently suitable for broadband miniature antenna designs, such as spiral antennas, and provides additional design degree of freedom.
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.
The Air Force Research Laboratory (AFRL) Materials Measurement Laboratory (MML) is a state of the art facility for the characterization of the electromagnetic properties of materials at radio frequencies. The two-fold mission of the MML is to provide material characterization services to AFRL and to conduct R&D to develop or improve RF material characterization technology. The goal of the MML is to perform—or develop the ability to perform—material property measurements to the highest degree of accuracy possible with state of the art test equipment. Characterization measurements range from determination of RF reflection or transmission loss to the extraction of the dielectric permittivity and magnetic permeability of material samples. The MML has the ability to characterize material samples from below 100 MHz to above 18 GHz over material test sample temperatures ranging from – 150oC to greater than 1000oC. While maintaining capabilities using ‘standard’ material measurement techniques (circular coax and rectangular waveguide), the MML’s most highly utilized system is based on the GTRI focused arch apparatus. The MML also employs resonant cavity fixtures, open-ended coax probes and impedance meters to provide a capability to evaluate material samples of a wide variety of shapes and sizes.
Microwave measurement of intrinsic material properties can be performed with transmission-line fixtures such as waveguides or free-space focused beams. However, analyses of measured data usually assume idealized sample geometries. In this paper, Finite Difference Time Domain (FDTD) calculations are used to study the systematic error from non-ideal geometries, in free-space and waveguide measurements of impedance sheets. Analytical models of these errors are developed. FDTD analysis can be used to numerically invert intrinsic material properties from measured freespace transmission coefficients. The focused beam is simulated in FDTD with a sum of weighted plane waves with a Gaussian spectral distribution. The transmission coefficient is predicted by propagating the focused beam through a material slab or sheet; and the dielectric or impedance properties are derived from the transmission coefficient. The focused beam diameter is preferably several wavelengths, which requires large sample size (>1 square meter) at low frequencies. A modified focused beam technique is described that incorporates a finite aperture in a metal groundplane to measure samples with reduced dimensions, even at low frequencies. Calculations are compared to laboratory measurements. FDTD calculations are also applied to study the effect of gaps in waveguide fixtures, since gap and edge effects in both waveguide or free-space aperture fixtures contribute to measurement error.
MW HF phased antenna array, capable of very wideband frequency and scan coverage, is constructed as part of the High Frequency Active Auroral Research Program (HAARP). Operation of the HAARP Developmental Prototype (DP), consisting of 96 dipoles, demonstrated importance controlling even mode excitation originated by electrical asymmetry of the phased active array. For this purpose antenna matching units (AMU) are using hybrids with resistive termination on its even mode ports. Accurate predictions of the array ERP and safe operation of the extended to full 360 dipoles array require a trusted model. This paper presents an application of the measurement and analytic array characterization used for the active array performance prediction and optimization. Mixed mode sparameters of sampled dipoles were measured before multiport mixed-mode S-matrices were constructed using learned array symmetry properties. Created mixed-mode Smatrix model allows for estimation of the ERP, prediction of dipoles power distributions, the AMU and even mode termination impedance optimization for better efficiency.
The installation and operation of large horizontal UHF phased array antennas in arctic regions is challenged by severe environmental conditions. It can be shown that large and moderate snowfall can impact the operation of exposed dipole array elements and reduce aperture efficiency. Reflection coefficients measured at the antenna terminal of an embedded array element can vary drastically as a function of snow depth. In this paper we will describe several measurements that show embedded array element impedance variations versus snow height. Measured results will be presented for an array operating from 400 MHz to 470 MHz.
Recently, remarkable efforts have been spent to develop GPR (Ground Penetrating Radar) systems able to detect shallow anti-personnel mines. In order to achieve high resolutions, large bandwidths are necessary; furthermore antennas must operate detached from ground. The paper describes how an existing surface based antenna, developed for high resolution inspection of man-made structures, has been optimized following a combined measurementssimulation approach. The novel antenna is the basic element of a polarimetric array, composed of 35 elements, that will be part of a multi-sensors demining system under development in the frame of a European Union funded project (DEMAND). Measurements have been carried out in the frequency domain, by the means of an S-parameters modal decomposition. Results concerning bandwidth, leakage, impulse response of array channels and input impedance of the basic element are reported in the paper. Comparison between measurements results and simulations are presented.
Generally, microstrip patch antennas excited to radiate circular polarized waves have serious weakness for narrow bandwidth of axial ratio and impedance in comparison with others (lens, horns, and etc)[1-3]. For this reason, it has been difficult to use microstrip patch antenna for satellite communications in spite of several advantages which are low profile, light weight, ease to fabricate, low cost, and so on [4-5]. In this paper, novel microstrip patch antenna is presented for satellite communications at Ka band. The proposed antenna provides wide axial ratio and impedance bandwidth compared with conventional circular polarized (CP) microstrip patch antenna. These operating characteristics are analyzed.
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.
Conventional planar microwave absorbing materials may be divided into two main types: those that employ one or more thin resistive sheets separated by dielectric spacers, such as the Salisbury screen, and those comprised of one or more lossy layers such as the Dallenbach absorber. Both types operate by absorbing incident electromagnetic energy and converting it into heat. However, an alternative approach based on the concept of phase modulation has recently been proposed [1-3], in which electromagnetic energy scattered from an object is phase modulated to produce a reflected field with a low time-averaged energy spectral density. This new type of ‘absorber’, called the phase-switched screen (PSS), consists of one or more active layers whose impedance properties are controlled electronically. Previously published work in the area has concentrated on the scattering properties of PSS at normal incidence, and has shown that single layer screens exhibit similar characteristics to those of a Salisbury screen. More interestingly however, multi-layer PSS can be configured to provide an active scatterer with dynamic reflectivity null tuning properties [4]. In this contribution we extend the analysis to consider the characteristics of PSS at oblique incidence and present results to compare the performance of active PSS to those of conventional passive designs.
A series of experimental and theoretical tests designed to develop techniques for reliable computational modeling of automobile antenna performance is presented. The results from the experimental measurements are compared with the results of computational techniques to verify their accuracy and reliability. The Electromagnetic Surface Patch (ESP5) code, a theoretical Method of Moment (MoM) general-purpose code developed at the Ohio State University, is used for computational modeling. We progress from the simple geometry of a single square plate and a monopole, to the more complex structure of a small copper-coated plastic model of an automobile. The computational simulation and measurements are configured with both a monopole antenna mounted at the center of the automobile roof and a backlite heater grid FM antenna. The input impedance, pattern, and polarization are all measured. Comparisons between the results of the computational simulations are presented, as well as the procedures used to measure the antenna characteristics and compare the experimental data with the measured data.
This paper presents the advantages of the next generation of RF Transistors and Amplifier units based on Silicon Carbide (SiC) and Gallium Nitride (GaN) materials. The use of these devices, having higher output and input impedances, allow easier matching to antenna impedances without compromises in power levels. These devices are basically wide bandgap semiconductors having superior properties to other competing technologies such as Silicon (Si) or Gallium Arsenide (GaAs). Implementation of SiC RF transistors will provide higher temperature operation than Si, higher breakdown voltages and extremely good ft operation. A typical SiC unit with a 0.7 ìm configuration will have an ft of 10 GHz.; similarly, a 0.4 ìm configuration will have an ft of greater than 20 GHz. Typical power density is up to 4.5 watts per mm. of transistor structure. In general, SiC Metal Semiconductor Field Effect Transistor (MESFET) will have up to 10 times higher impedances than a Silicon LDMOSFET (input and output). The devices are also very low noise, which allows the use of SiC as high dynamic range Low Noise Amplifiers (LNAs). The paper presents measured data on both SiC Power Amplifiers units and LNAs operating in the frequency domain between 30 to 2800 MHz.
The automobile antenna industry is facing two rapidly growing trends: (1) the incorporation of effective, low cost, AM/FM conformal antenna designs and (2) the antenna capability to handle diversity FM radio receivers. The development of techniques for testing automotive conformal diversity antennas' performance becomes necessary to evaluate and compare them. Testing techniques to obtain the antenna Input Impedance (Zin), Standing Wave Ratio (SWR) and Mismatch Loss (MML) as well as the azimuth gain patterns and the combined diversity signal (maximum of the diversity signals) are described. Experimental results for the Annular Slot Windshield Diversity Antenna using polarization diversity are shown. It is demonstrated that the Annular Slot Windshield Diversity Antenna can be used effectively to reduce multipath fading.
Two types of cellular handset testing are presented. The first studies models of a cellular handset near the human head. A comparative analysis is done between simulation and measurement of an inexpensive head mockup compared to a more expensive head mockup. Peak gain values have good agreement within about 1 dB. The second type of cellular handset testing is for a PCS band PIFA antenna integrated to a cellular handset. This paper describes the design and experimental study of the radiation patterns of a PCS band (1850-1990MHz) cellular handset with an internal PIFA. The PIFA described in this paper has good gain, impedance matching, and reduced sensitivity to human body interaction. This PIFA is a good cellular internal antenna.
Errors arising in the measurement of reflection coefficient are identified and analyzed. The presence of multiple reflections due to poor connectors, transmission line discontinuities, and terminal loads is described, modeled and applied. Various measurement scenarios are analyzed, and measured results are presented as a guide for laboratory troubleshooting and as a validation of the measurement models. Improvements to Vector Network Analyzer calibration methods are proposed, including computer corrected calibration for one-port radiating elements and elementary improvements to two-port TRL calibration. An extensive error evaluation of the somewhat forgotten slotted line measurement is finally presented as a robust alternative, and computer automation, acquisition, and calibration of this measurement is outlined.
This paper is concerned with the measurement and analysis of a circularly polarized, flat plate patch array receiving antenna at 12.5 GHz. Input impedance and far field pattern measurements of the antenna over the frequency band from 10 to 15 GHz were performed. The small Compact Range (CR) facility of the Ohio State University Electro Science Laboratory OSU/ESL was used to measure the gain pattern. Gain pattern measurement of the antenna was done by using the gain comparison method. A broadband (2-18 GHz), constant phase pyramidal horn antenna was used as a reference. The data were analyzed to determine the radiation efficiency of the antenna.
Bowtie dipole antennas have been widely used for surface-based ground penetrating radar ( GPR) applications. This type of GPR antennas share common problems such as low directivity, antenna ringing, unstable characteristic impedance, RFI and large size. Special treatments have been used to improve their performance. Resistive terminations have been used to reduce the antenna ringing at t he price of efficiency. Some use reflectors to increase directivity at the price of bandwidth and the risk of cavity ringing excitation. Absorbing material is also used to shield RFI with increased size and weight. Some people use horn antennas because of bet ter gain. However, they are limited to high frequency applications where their size are still reasonable to handle. This means they can only do shallow target measurements. Horn antenna approach also faces the strong reflection arising at the air-ground interface. A new type of GPR antenna design presented in this paper has been developed to overcome the above difficulties.