AMTA Paper Archive
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3-D Antenna Measurement System - Low Gain Antenna Measurements and CTIA OTA Testing
ABSTRACT We are in the era of wireless communications and devices. The antennas that enable these technologies are electrically small and can be challenging to test and analyze. Yet, the industry is becoming more standardized, and so too are the tests and certifications being adopted to validate these antennas. These antennas must undergo “antenna measurements” to characterize such information as far-field patterns and gain. Additionally, hand-held devices, such as cell phones, must satisfy requirements of the Over-the-Air (OTA) performance tests as specified by the Cellular Telecommunication and Internet Association (CTIA). These tests require a measurement system that can accurately collect data on a spherical surface enclosing the AUT. This system also has to provide the appropriate data analysis capabilities and has to be constructed from dielectric materials to minimize reflections.
The Development of a Mini-UWB Antenna
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.
Planar Passive Microwave Devices on Magneto-Dielectric Substrates
Traditional antenna loading materials are dielectrics with a large dielectric constant. This results in a physical reduction, proportional to the square root of the ratio of the dielectric constant used for the miniaturized antenna and the dielectric constant used for the full-size antenna. Unfortunately, loading the antenna in this manner results in a rather sever reduction in efficiency as seen by a reduction in bandwidth. This paper suggests the use of a magneto-dielectric material as the substrate in order to increase the bandwidth. A magneto-dielectric material is has a non-trivial permittivity and permeability. It is often a composite mixture of a magnetic material and a dielectric material. The magnetic material is often an oxide, such as nickel zinc ferrite, which is most readily available as a sintered ceramic. Unfortunately, such a material is rather brittle, and so there is an interest in forming a composite of such materials with a flexible binder, such as urethane. This paper also begins the experimental evaluation for the use of these materials in patch antennas and discusses the next step in investigating further results.
Automated Method for characterizing Temperature Dependent Dielectric Materials
Abstract Among the various methods to accelerate curing thermoset polymers, such as an epoxy, involves the use of radio frequency (RF) fields. In this, the polymer precursor materials are placed in a microwave reactor, high power RF fields are introduced, and the electrical (or for that matter, magnetic) loss mechanisms convert the RF power to heat and therefore inducing curing. One of the most challenging aspects of such curing methods lies in the fact that the materials being cured undergo chemical change during the process. This results in a time-dependent change in the electrical properties of the materials. It is therefore important to have accurate data on the material’s electrical properties as a function of both temperature and extent of cure. This paper describes an apparatus designed to facilitate such measurements.
Numerical Analysis of the Focused-Beam Measurement System
The focused-beam system can measure electromagnetic constitutive parameters of materials much more accurately than the classic free space arch system due to reduced scattering from the edge of the finite sample and its support structure. However, for a number of reasons, the system tends to perform poorly at frequencies below 4 GHz. In order to improve the system’s performance, we need to determine the causes of this degraded performance. One way to do this is by studying the field behavior of the system at the sample region. But instead of using Gaussian beam approximation for the exciting plane-wave and locating the antenna at focal lengths determined by geometric optics, we take advantage of recent advances in computational electromagnetic tools and high performance computing technology and find the field behavior near the sample by numerically solving the full Maxwell’s equations for the whole system. In this paper, we will present our approaches and our findings which lead to better understanding of the system performance.
Electromagnetic Material Characterization Using Partially Filled Rectangular Waveguide
A waveguide material measurement technique is developed for highly reflective or lossy materials. In order to extract the complex constitutive parameters from a material, experimental reflection and transmission scattering parameters are needed. In a traditional rectangular waveguide material measurement, the sample fills the entire waveguide cross-section, making it difficult to obtain a significant transmission scattering parameter with highly reflective or lossy materials. This paper demonstrates, through the use of a modal-analysis technique, how using a partially filled rectangular waveguide cross-section allows for better transmission responses to extract the complex constitutive parameters. Experimental results for acrylic and radar absorbing material are compared to stripline measurements to verify the modal-analysis technique.
The AFRL RF Materials Measurement Laboratory
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.
An Automated Slotted Line Dielectric Measurement System
The microwave slotted line is an accurate single frequency instrument for determining the dielectric properties of materials. The broadband application of materials requires accurate dielectric characterization across wide bands, which is a labor intensive procedure with a manually operated slotted line. The subject effort improves upon the manual slotted line dielectric measurement procedure by incorporating a steppermotor drive mechanism and automated measurement software with the standard slotted line carriage. A comparison of results obtained with the automated slotted line is made with those derived using a network analyzer system.
Microwave Characterisation of Materials in Free Space Over the Frequency Range from 1.7 GHz to 5.8 GHz.
The microwave characterisation of the electromagnetic parameters of lossy materials is an essential part of the work of the BAE SYSTEMS Advanced Technology Centre, Stealth Materials Group at Towcester (UK). The electromagnetic parameters of lossy materials change rapidly with frequency below 5GHz, therefore for stealth applications it is vitally important to be able to characterise materials at these frequencies. This paper describes a unique quasi-optical free space focused beam system for the measurement of microwave electromagnetic material parameters. The system employs two spherical reflectors which are illuminated from the side by gaussian beam forming antennas. The frequency range of 1.7GHz to 5.8GHz is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that a parallel beam is formed between the reflectors whose beam waist diameter (or illumination area) is essentially the same across each frequency band. The measurements from the system are taken using a vector network analyser under computer control. The parallel beam enables a “Through, Reflect, Line” calibration technique to be used. After calibration the sample under test is placed in the beam (mid way between the reflectors) and the four microwave ‘S’ parameters are recorded automatically in complex form. The permittivity, permeability or lumped admittance ( if the sample is very thin /50) for the material are then determined from the ‘S’ parameters. The operation and performance of the system is discussed and some material parameter measurement results are given.
A Broadband Materials Measurements Technique Building Upon the Implementation of Coaxial Probes
A Technique is presented that allows for broadband nondestructive material electrical parameter measurements. Electrical parameters of a large number of materials are not readily available over extremely broad bandwidths (multiple octaves as an example). This information is required for accurate modeling of microwave circuits and antenna(s). These parameters consist of complex permittivity and complex permeability that result in loss due to the types and thickness of materials to be used. A Method is required that allows for fast, accurate and low cost measurements of the materials under test. The technique of using dual coaxial probes provides a solution that can be applied to numerous materials including thin films. It takes advantage of the full frequency extent of the network analyzer. This measurement uses dual coaxial probes, as compared to the implementation of cavity resonators, coaxial lines, waveguides and free space measurements, and performs the measurement in a 2-port calibration procedure. The resultant analytical solution is a transcendental equation with complex arguments. The Coaxial probes are described and can be easily made with available components where the only limitation is the valid component frequency bandwidth. Several material examples show the expected accuracy versus frequency range of this measurement technique.
Stepped-Waveguide Material Characterization Technique
Electromagnetic material characterization is the process of determining the complex permittivity and permeability of a material. Rectangular waveguide measurements involving frequencies greater than several gigahertz require only a relatively small test sample. In an X-Band (8-12 GHz) waveguide, for example, sample dimensions in the crosssectional plane are only 0.9 by 0.4 inches. However, for lower-frequency applications waveguide dimensions become progressively larger. Consequently, larger quantities of materials are required leading to possible sample fabrication difficulties. Under these circumstances, a waveguide sample holder having a reduced aperture may be utilized to reduce the time and cost spent producing large precision samples. This type of holder, however, will cause a disruption in the waveguide-wall surface currents, resulting in the excitation of higher-order modes. This paper will demonstrate how these higher-order modes can be accommodated using a modal analysis technique, thus resulting in the ability to measure smaller samples mounted in large waveguides and still determine the constitutive parameters of the materials at the desired frequencies.
An Approach to the Evaluation of Uncertainties for Complex RCS Measurement Data
The Radar Cross Section (RCS) measurement facility operated by the Stealth Materials Department of BAE SYSTEMS Advanced Technology Centre in the UK is an invaluable tool for the development of low observable (LO) materials and designs. Specifically, it permits the effect of signature control measures, when applied to a design, to be demonstrated empirically in terms of the impact on the RCS. The facility is operated within a 3m by 3m by 12m anechoic chamber where pseudo-monostatic, co-polar, stepped frequency data for a target can be collected in a single measurement run over a frequency range of 2- 18GHz, and for a range of azimuth and elevation angles using a Vector Network Analyser (VNA). The data recorded consists of the complex voltage reflection coefficients (VRC) for the chosen range of aspect angles. This includes data for the target, mount, calibration object, and the associated calibration object mounting where significant. All data processing is conducted offline using a bespoke post processing software routine which implements software time domain gating of the raw data transformed into the time domain prior to calibration. The significant sources of type A (random) and B (systematic) uncertainties for the range are identified, grouped, and an approach to the determination of an uncertainty budget for the complex S21 data is presented. The method is based upon the UKAS M3003 guidelines for the treatment of uncertainties that may be expressed by the use of real, rather than complex numbers. However, a method of assessment of the uncertainties in both real and imaginary parts of the complex data is presented. Finally, the uncertainties estimated for the raw VRC data collected are propagated through the calibration and the uncertainty associated with the complex RCS of a simple target is presented.
Critical Technologies for Performing RCS Target Measurements Using a String Support System
Target support pylons and foam columns have been in use since the late 1970’s to provide target support for RCS measurements. Pylons currently limit our low frequency measurement capability due to the moderately high scattering from the pylon edges. Additionally both foam column and pylon support structures interact with the target scattering which can limit our ability to completely subtract the target support scattering from the target signature data. Target suspension using a string support system has the potential to eliminate these limitations. MRC has recently completed a string support technology demonstration program to identify the critical components for implementing an indoor string support system for RCS measurements. Critical components identified and demonstrated under this program included a survey of string materials for RCS measurements, development of low coefficient of friction swivel bearings, structural target to string interfaces, and three different techniques for providing target rotation. This presentation will highlight the results from the demonstration program showing viability of string support systems to provide an enhanced RCS measurement capability for indoor RCS measurement ranges
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.
Broadband Dual Polarized Material Test System
The S parameter measurement of large sheet materials has been limited to microwave frequencies due to a lack of test apparatus that could properly illuminate the materials with a uniform electromagnetic wave in a confined space at lower frequencies. Boeing Mesa has developed a unique test system that overcomes this limitation. The current design will perform S11 and S12 measurements from 0.125 to 40 GHz in five bands. Sample sizes up to 4 ft. wide, 8+ ft. long, and 12 inches thick, can be tested in sections by sliding the sample into the test fixture. Apertures up to 46 inches square can be provided in the aperture plate. The angle of incidence can be adjusted from 0 degrees to 45 degrees.
Compact RCS Imaging System
The implementation of low observable (LO) materials and the fielding of aircraft with controlled signatures creates a new degree of difficulty for maintaining, executing prompt accurate inspections and achieving meaningful evaluations. To address this problem, Sensor Concepts, Inc (SCI) has prototyped a new radar system, (the SCI-Xe) to provide a test bed for a lighter, smaller RCS measurement and imaging system. The hardware consists of a suitcase containing RF hardware, computer and display and a hand-held or rail-mounted unit containing two X/Ku band antennas. In the rail-mounted application, imaging is followed by registration and image differencing, which allows an operator reproduce a baseline measurement geometry and evaluate RCS changes. The hand-held application forms a synthetic aperture by moving the antennas by hand. This can be used to quickly investigate an object under test.
Characteristics of Phase-Switched Screens at Oblique Incidence
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 . 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.
The History of NBS/NIST in Boulder, Part 3 - Everything Else
Although the original reason for creating the Boulder Laboratories of NBS (later known as NIST) was to accommodate the radio program, as described by Dennis Friday and Allen Newell at this conference, the first major program in Boulder was in Cryogenics, which was created in response to a perceived emergency in national security and went on to provide basic cryogenic data to serve the national space program. It was also the origin of programs continuing to the present in the reliability of materials, the thermophysical properties of fluids, and cryoelectronics. The early work at NBS on radio propagation led to the development of new tools for meteorology and became an essential part of the newly formed National Oceanic and Atmospheric Administration. The Joint Institute for Laboratory Astrophysics was formed in collaboration with the University of Colorado and quickly exceeded the field of activity suggested by its name. The Time and Frequency program created a series of radical innovations in frequency standards and the dissemination of time, including a new definition of the meter and an experimental frequency standard based on a single mercury ion.
A Comparison of Material Measurements Using a Standard A.S.T.M. Measurement Cell and a Stripline Field Applicator
Current methods of measuring the electromagnetic properties of materials (i.e., RF shielding effectiveness) include an ASTM (American Society for Testing and Materials) standard test cell and a stripline field applicator, among others. This paper outlines the basic theory of operation of each measurement setup, and compares measured data of similar samples from each setup.
Advantages of Silicon Carbide (SiC) RF Transistors for Driving Antenna Impedances
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.
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