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An effort to ascertain the accuracy of the rectangular waveguide measurement technique for permittivity and permeability characterization of materials, has led to the development and application of a waveguide notch filter as a scattering parameter (S-parameter) reference standard. The S-parameters of this reference can be determined accurately using simulations that implement a full wave model of the waveguide measurement technique. The notch frequency response characteristic allows testing over the dynamic range of the measurement system. When fabricated in metal, the filter provides a predictable frequency response, has mechanical and temporal stability, and is reproducible using standard machining techniques. However, manufacturing errors introduce uncertainty in the measured S-parameters. Determining the sensitivity of S-parameter uncertainty as a function of manufacturing errors is important in assessing the appropriateness of the notch filter as a metallic standard for use throughout the material measurements community. This paper presents the characteristics of the filter, showing both calculated and measured S-parameter values, and provides an analysis that demonstrates the relationship between dimensional manufacturing tolerances and the resulting S-parameter uncertainty.
High frequency (up through X-band) magnetic materials are gaining in importance across a wide range of applications such as microwave components, electromagnetic shielding, and antenna substrates. Development of new magnetic materials and alloys requires convenient and accurate measurement methods with well-understood uncertainties. For this reason, a finite difference time domain (FDTD) model was developed of a shorted microstrip (single coil) permeameter, appropriate for measuring small samples or thin films. Simulating the response to various magnetic materials, this model was used to analyze the prevailing semi-empirical inversion methods and a new, more accurate inversion method was developed to correct deficiencies in existing techniques.
This paper describes how Radio Frequency Identification (RFID) utilizes typically backscattering communication between reader unit and transponder. In passive UHF RFID system the transmitted electromagnetic wave must propagate two times thru the medium. The level of moisture changes electromagnetic properties of medium and the electromagnetic properties of medium have effects on quality of backscattered signal [1]. The well-known fact is that the demand for item level RFID tag will face challenge of different packing materials. However, environment where some parameter such as humidity and temperature changes a lot, will be a great operational challenge for future RFID systems as well. In this paper we have used one application and present measured result how different moisture levels effects for passive UHF RFID operational main parameters such as threshold power level and level of backscattered signal.
Advantages of Far-Field (FF) anechoic chambers utilized for antenna measurements, as compared to conventional outdoor ranges, such as security, interference-free radiation, and immunity to weather conditions allowing broadband antenna measurements on a 24/7 basis, are well known. The dimensions of an anechoic chamber are primarily determined by the lowest operating frequency and are, therefore, significantly increased if operation is required down to VHF and UHF frequency bands. As a result, the advantages of indoor chambers are often disputed when considering low frequency applications. The main counter-argument is the real estate required for chamber construction. In addition, such chambers require the use of high performance absorbing materials, and consequently, chamber certification is always a challenging task. Therefore, rigorous and accurate 3D EM analysis of the chamber is an important procedure to increase confidence, reduce the risk associated with achieving the required test zone performance, and to make the design more efficient. Thus, an accurate simulation of the chamber is even more important these days due to a dramatically growing number of antenna manufacturers supplying products at VHF and UHF bands. Such analysis is a standard procedure at ORBIT/FR, and is described below for the example of a chamber with dimensions of 6m (W) x 6m (H) x 10m (L), operating down to 150 MHz.
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 [1] 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 [2].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.
Method of moments (MoM) codes have become have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In particular, for simple structures like canonical shapes or antenna / RCS test fixtures, especially those with material treatments, the primary source of disagreement between measurements and predictions is often due to differences between the “as-designed” and “as-built” material parameters rather than to the underlying MoM code itself. This paper describes an algorithm that uses a MoM model combined with backscatter measurements to estimate the “as-built” materials parameters for the case where the treatments can be modeled using an equivalent boundary condition. The algorithm is a variant of the network model technique described in [1]-[3]. The paper presents a brief formulation of the network model materials characterization algorithm, along with numerical simulations of its performance for a simple canonical RCS shape using the CARLOS-3D™ MoM code [4]. The convergence properties of the algorithm are also discussed.
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-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).
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.