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.
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.
Investigation of Several Miniature Antenna Design for Tri-Band GPS Applications
Abstract The undergoing development of GPS system requires the integration of all three GPS bands: L1 (1575MHz), L2 (1227MHz), and L5 (1176MHz) into one miniature antenna. The objective of work is to develop a small GPS antenna (less than 1/10 wavelength) to cover all the three GPS bands with a minimum bandwidth of 24MHz in each band and a minimum gain of 0 dBic (RHCP). Three novel miniature antenna designs: Quad-F antenna (QF), proximity-fed stacked patch (PFSP) antenna and channel fed ring (CFR) antenna were investigated. These antennas utilize high-index inhomogeneous dielectric materials to achieve antenna miniaturization, mode excitation and mode control. The full-wave simulation (HFSS) and measurement results are presented and discussed in this paper.
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.
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.
A Dual-Linear Polarization UWB Dielectric Rod Probe Design
A dual-polarization ultrawide bandwidth (UWB) dielectric rod antenna containing two concentric dielectric cylinders was developed for near field probing applications. This antenna features more than 4:1 bandwidth, dual-linear polarization, stable radiation center and symmetric patterns. The antenna begins with a tapered wave-launching section consisting of shaped conducting plates and resistive films. This launcher section is followed by a guided section where the excited HE11 modes are transported to the radiation section. The radiation section contains specially shaped dimensions and materials to generate similar E and H plane patterns with 3-dB beamwidths greater than 55° over 4:1 bandwidth (2 to 8 GHz).
Electromagnetic Material Characterization Using a Rectangular Waveguide to Excite a Homogeneous Media in a Parallel-Plate Waveguide Through a Slot
A material measurement technique is developed to simultaneously characterize electric and magnetic properties for homogeneous lossy material in a parallel-plate region. The material is excited by a rectangular waveguide which interacts with the parallel-plate region through a slot. In order to extract the complex constitutive parameters from the material two independent measurements are required. If the material is attached to a PEC surface and is unable to be removed, the most obvious manner to characterize the material is a parallel-plate region. This paper demonstrates through the use of a magnetic field integral equation (MFIE) how a rectangular waveguide interacting through a slot with a parallel-plate region can be used to obtain two independent measurements, which are necessary for characterizing the homogeneous lossy material. Experimental results for various lossy materials are compared to stripline and waveguide measurements to verify the MFIE
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.
Simultaneous Extraction of Permittivity and Permeability from a Conductor-backed Lossy Media
Nondestructive evaluation (NDE) of a conductor-backed lossy material plays an important role in electromagnetic shielding applications. Current NDE techniques limit such evaluation to non-magnetic materials and/or use of approximate solutions. In this paper a technique employing a flanged opened-ended rectangular waveguide and a rigorous integral equation model is presented allowing simultaneous extraction of both permittivity and permeability from a lossy magnetic shielding material. The technique reduces sample preparation time leading to rapid measurement. Calculation of both material parameters is accomplished using reflection measurements from two different thicknesses of a given material sample. A theoretical solution to the reflection coefficients is developed using a magnetic field integral equation (MFIE) formulation. The theoretical solution, along with experimental results of the reflection measurements, allows extraction of the material parameters using a two-dimensional Newton root search. Results using single-mode calculations are presented and compared to those obtained using traditional waveguide material characterization techniques. Future work will also be discussed.
Novel Method for Antenna Material Characterization
This paper describes the method and hardware implementation of a test bed that was designed and built to characterize the reflection characteristics of various types of reflector materials. The system described measures reflection amplitude and phase from flat test panels relative to a metal panel standard at normal incidence and for dual linear polarizations simultaneously. The measurement’s theoretical concept is based on a focused free space technique with time domain gating to remove the effect of multi-path coupling between the test panel and the feed assembly. The system as a whole demonstrates a novel method for measuring the reflection from reflector materials and characterizing their potential impact on polarization purity. The measurement system consists of: 1) A fixed reflector, 2) An alignment fixture accommodating feed assemblies, which include corrugated horns that operate over a 40% bandwidth that may be swapped out in order to cover a continuous frequency band from 18 to 75 GHz and Orthomode Transducers (OMT) in order to measure dual linear polarizations simultaneously, 3) An additional alignment fixture for mounting the flat panels under test, and 4) A Vector Network Analyzer (VNA) and computer for data collection and processing. The system is assembled on a bench top and aligned utilizing a Coordinate Measurement Machine (CMM). Sample results demonstrating the measurement of various types of reflector materials including composite reflector lay-ups with graphite face sheets and mesh samples for deployable reflectors are presented.
Low Frequency Optimization of 72 Inch Absorbers
The purpose of this paper is to detail the process used to optimize the low frequency performance of 72 inch absorber. The loading optimization was required to provide enhanced performance of a twisted 72 inch absorber which was to be used in the building of a large aircraft test facility. The chamber performance requirements are over a frequency range of 30 MHz to 18 GHz. The chamber dimensions are 30 meters x 30 meters x 20 meters high. This chamber will be used to measure a variety of fighter aircraft for many EW scenarios. The mission of this facility is to “perform radiated immunity testing of aerospace vehicles with high electromagnetic field intensity, radiated emissions measurements, EMC testing, electronic warfare testing, antenna pattern testing”. Due to the broad frequency range and the fact that the chamber is desired to test both in the low frequency EMC domain and high frequency antenna measurements, an extremely broad band absorber material had to be developed and optimized. The use of ferrite hybrids was considered. Due to the roll off at microwave frequencies and the expense of such a high volume of materials, they were eliminated for cost and due to the limited performance in the 1-2 GHz frequency range. The ideal candidate is a 72 inch twisted pyramidal geometry. The standard loading of these materials is ideal for frequencies above 150 MHz.. The performance level in the 30 MHz to 150 MHz range is less than ideal. A design for the chamber was established with specific target performances required of the 72 inch absorbers. This paper describes the effort taken to optimize the loss properties of the dielectric foam to meet the target absorber performance required for the implementation of the design. Key Words: Absorber Measurements, Absorber Performance, Computer Modeling of Absorbers, Dielectric Properties of Absorber
VHF/UHF High Performance Absorbing Material Measurements in a Coaxial Line Using Time-Gating Techniques: Validation & Error Analysis
This paper describes the Rectangular Coaxial 40’ long measurement system recently designed and installed at AEMI with the primary purpose of measuring the reflectivity of its high performance VHF/UHF absorbing materials in the frequency range 30 – 510 MHz. The basic principles of the system are described in detail in  and are based on S11 – measurements of absorbing material reflectivity by a Vector Network Analyzer (VNA). In order to improve the system productivity and measurement accuracy it was enhanced by the time-gating software option – the standard option of ORBIT/FR Spectrum 959 automated measurement software package .The measurement system performance was thoroughly evaluated and validated by a number of tests performed in the “empty” coaxial line, and in the line loaded by absorbing materials. The list of RF uncertainties – various measurement error sources - was generated, the main measurement error contributors were identified, the corresponding errors – estimated and the overall RSS measurement errors were calculated for the absorber reflectivity varying in the range of -30dB to – 40dB.