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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.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
A fundamental part of the work of the BAE SYSTEMS Advanced Technology Centres Materials Group at Towcester (UK) is the microwave characterisation of the electromagnetic parameters of lossy materials. This paper describes a Quasi-optical microwave system for the free space measurement of material parameters in the frequency range 5 GHz to 18 GHz. The system employs two spherical reflectors which are illuminated from the side by gausian beam forming antennas. This produces a well defined parallel beam between the reflectors. The 5 GHz ro 18 GHz frequency range is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that the beamwaist diameter (or illumination area) is essentially the same for each of the three frequency bands The measurements 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, permeabilty or lumped admittance (if the sample is very thin <ë/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.
CAMELIA is a large RCS measurement facility (45m.12m. 13m in dimensions) whose compact range is optimized in the SHF band (1-18 GHz). Exploiting it at lower frequencies requires the modification of the absorbers and the use of huge broad band horns as RF sources (since the compact range is now not well adapted). To help understanding the radioelectric behavior of the large scale facility, we have developed a 1:10 small scale model as well as 1:10 scale horns, that are operated in the SHF band. It enables the experimental simulation of RCS measurements in the V/UHF band. Thus, all dimensions and frequencies are homothetic, only electromagnetic properties of materials are not. RCS measurements of several canonical targets have been performed in both facilities and compared. Due to non directive transmitting/receiving antenna, coupling between the targets and the wans has been exhibited. A simple ray tracing model, taking into account the measured reflection coefficient of the walls and the bistactic RCS of the target, shows good agreement with the measurements.
Radar Cross Section measurements require the target to be in the far field of the illuminating and receiving antennas. Such requirements are met in a compact range in the SHF band, but problems arise when trying to measure at lower frequencies. Typically, below 500 MHz, compact ranges are no more efficient, and one should only rely upon direct illumination. In this case, the wavefront is spherical and the field in the quiet zone is not homogeneous. Furthermore, unwanted reflections from the walls are strong due to the poor efficiency of absorbing materials at these frequencies, so the measurement that can be made have no longer something to see with RCS, especially with large targets. We first propose a specific array antenna to minimize errors caused by wall reflections in the V-UHF band for small and medium size targets. Then an original method based upon the same array technology is proposed that allows to precisely measure the RCS of large targets. The basic idea is to generate an electromagnetic field such that the response of the target illuminated with this field is the actual RCS of the target. This is achieved by combining data collected when selecting successively each element of the array as a transmitter, and successively each other element of the array as a receiver. Simulations with a MoM code and measurements proving the validity of the method are presented.
The purpose for this advanced development program was to design, fabricate and test a physically small, broadband, dual-polarized, phased array antenna aperture using Flare Notch elements. The array was designed to operate in the 4 to 18 GHz frequency spectrum, having a VSWR of less than 2:1 and capable of handling 10 watts per element. The array was configured with polarization diversity, essentially, dual cross elements are used which are excited in phase or out of phase depending on the application. One of the significant accomplishments of this research effort was the elimination of grating lobes and the reduction of the size of the elements. Another significant accomplishment is the feeding of dual flare notch elements with a broadband microstrip match network. The antenna elements were implemented using Rogers 4003 materials. Fabrication of the elements and assembly of the elements is being done in a configuration of two rows by twelve elements of which only eight elements are normally excited. The remaining elements are used as parasitics to support the desired radiation pattern. The research work is being done in support of the next generation of solid state broadband radiation systems presently under development for ECM applications.
This paper examines candidate configurations for a smart radar absorber or reflector which is capable of self-tuning and perform while scan operation. The discussion is supported by both modelled and measurement data.
The purpose for this advanced development program was to advance the flatness level on an RF/IR Beam combiner. The developed beam combiner minimized transmission losses for RF signals between 1 GHz and 40 GHz and maximized total transmission for RF signals between 8 GHz and 18 GHz. The combiner maximized IR reflectance for IR radiation (2µm to 13 tm). Two 12 inch units were delivered to NAWC-WPNS for evaluation. The combiners produced an average transmission losses in the range of 0.4 dB between 1 and 33 GHz and 0.8 dB between 33 GHz and 40 GHz. Reflectivity in the Infrared was measured at 87% with the use of a 3.39 µm laser source. The combiners were manufactured on PolyOxyMethyle (POM); they are highly crystalline structures, very flat (mold driven), with unique acetal resins structures. POMs are a variant of thermo plastics, are made by free radical polymerization initiated by a peroxide or azo catalyst, or by redox polymerization. Four basic polymerization processes may be used to produce good RF transmission acrylic resins. Using POM as the host material, a Frequency Selective Surface (FSS) using a low pass configuration, was Gold sputtered on the host material surface. The results produced a mirror like surface, highly visible and IR reflective, and very transmissive in the RF domain. These combiners are to be used for the anechoic chamber testing of dual mode missile seeker systems. The missile systems required in an anechoic chamber measurements, far field illumination from both IR signals and RF signals. The dual mode beam combiner allows spatially coherent signals to illuminate the missile seeker under test. Results of these development, seems to indicate that larger combiners can be fabricated on optically flat materials (e.g. fused silica) with flatness of 12 µm. This will allow the next generation seeker heads, operating with large focal plane arrays, to be stimulated in an anechoic chamber environment.