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As millimeter-wave applications become more widely available technologies, there is a demand to know material properties for design and application purposes. However, many mass produced materials are either not specified at these frequencies or the price materials can be costly. Therefore the easiest method for characterization is by measurement. Traditional methods of this measurement type involve the reflectivity of a fabric sample placed on a flat metallic reference plate. However, this method has some major difficulties at these high frequencies. For example, the surface of the reference plate must be very flat and smooth and must be carefully oriented such that their surface is precisely facing the transmitting and receive and antennas. Furthermore the electrically large size of the reference plate of this setup makes it difficult to measure in far-field and anechoic range time is expensive. Resistive and conductive fabrics have applications such as shielding, anti-static, and radio wave absorption. Radio wave absorption and radar cross section engineering is currently of high interest to the automotive industry for testing newly emerging automotive radar systems. Such fabric measurement has already been utilized to accurately characterize artificial skin for radar mannequins to recreate the backscattering of human targets at 77 GHz. This paper presents a new and convenient method for measuring the reflective properties of conductive and resistive materials at millimeter wave frequencies by wrapping fabrics around a metallic reference cylinder. This new approach to fabric characterization method is able to obtain higher accuracy and repeatability despite the difficulties of measuring at high frequency.
Rapid advances in material fabrication capability, such as 3D printing, have made the realization of engineered complex media (i.e., anisotropic and bianisotropic materials) possible. One of the primary aspects prompting the interest in complex media is the added control over scattered electromagnetic fields due to the increase in the number of constitutive parameters. Isotropic media are characterized by the 2 well-known scalar parameters of permittivity and permeability. However, in general, it requires 18 and 36 parameters to describe anisotropic and bianisotropic media, respectively. Although the increase in parameter space provides more control over electromagnetic response, the penalty to pay is the added complexity in theoretical analysis when compared to isotropic media. One method that has been developed for the analysis of complex media is the six-vector field formalism which casts Maxwell’s equations into matrix form for ease of manipulation. Although this formalism handles fully populated permittivity and permeability tensors, inversion of a block 3x3 (i.e., 6x6) matrix is required which is mathematically intensive and physical insight can be obscured since a cofactor-based inversion is often employed in the solution process. The goal of this work is to develop a scalar potential formulation capable of handling gyrotropic media. Advantages and limitations of the formulation will be discussed and relevant examples will be provided to demonstrate the simplicity and physically-intuitive nature of the technique. Future work involving the use of the scalar potential formulation in the analysis of antenna, guided wave structures and material characterization of complex media will also be discussed to demonstrate the promising aspects of the technique.
Crystallographic sample design of complex media influences material tensor properties. These properties offer amplitude, phase and polarization control of the electromagnetic (EM) fields. Previous works have evaluated crystallographic sample designs for isotropic, uniaxial and biaxial anisotropic media, each respective design offering more ways to control the fields. The tensor elements for these designs are all aligned along the main diagonal of the permittivity and/or permeability tensors. Additional field control can be obtained by producing materials that have off-diagonal tensor elements in addition to the aforementioned main diagonal elements. A monoclinic crystal sample design supports the existence of two off-diagonal elements and offers more field control than biaxial anisotropic media. In this work, field analysis is performed on media that possesses a monoclinic tensor element arrangement, demonstrating the additional control over EM fields as compared to biaxial anisotropic media. A monoclinic sample is then constituted using crystallographic symmetry. Future work will yield the development and analysis of a monoclinic sample material measurement capability.
In this paper, a patch antenna has been designed based on the complementary split ring resonator (CSRRs), complementary rose curve resonators (CRCRs) and without using these inclusions. Complementary rose curve resonators (CRCRs) are used in design of patch antenna. The Patch antenna based on the complementary rose curve resonators (CRCRs) are achieved by patterning the ground plane under the conductor trace. The perimeter of the Rose curve can be adjusted by tuning the amplitude of the sine function and the radius of the base circle. With the order of CRCRs, the loading effect of the complementary resonators on the patch antenna is controlled. This works demonstrated that higher order CRCRs allows more compactness of the design and higher miniaturization factor. We proposed a compact patch antenna based on the complementary split ring resonator (CSRRs) and the complementary Rose curve resonator (CRCRs). The proposed patch antenna shows good performances which is designed to operate at 2.4 GHz. The results demonstrate the configurability of the design for a specific size. The results show the effectiveness of using metamaterials in microwave circuit can obtain from n to n+1 of the CRCRs order will result in 0.3 % miniaturization. IndexTerms: Patch Antenna, Metamaterial, Size Reduction, split ring Resonators, Rose Curve Resonators
Several instances in antenna design are known where an anisotropic material is useful ; however, finding a naturally occurring anisotropic material with the required dielectric tensor is often an impossibility. Therefore, artificial anisotropic dielectric materials must be designed, tested, and implemented. In this paper we shall present a layered artificial anisotropic dielectric material with a biaxial permittivity tensor. This material is designed to be used in conjunction with an antenna in order to improve antenna bandwidth. The design motivation behind this material shall be discussed, along with its implementation, the measurement of its permittivity tensor, and testing characterization with a prototype antenna. Results from CST Microwave Studio® simulations and the mixing rules from dielectric material science will be compared with the measured data. Test fixture design and instrumentation will also be presented. Predictions on various types of artificial anisotropic dielectrics suitable for future applications will also be discussed.
Constitutive parameter characterization of a biaxial anisotropic material using a rectangular waveguide requires three separate samples; each one a different orientation of the parent biaxial anisotropic sample. The Waveguide Rectangular to Waveguide Square (WRWS) characterization method is an alternative, more efficient method, to the rectangular waveguide method because the WRWS method requires only one cube sample of biaxial anisotropic material to perform complete parameter extraction. This cube sample fits uniformly without gaps in the waveguide sample holder and can be indexed to accommodate all orientations required for characterization. The WRWS waveguide transitions insure that only single (TE10) modes are present and thus leads to closed form solutions for the material properties - an advantage over other existing techniques requiring higher-order modal analysis and subsequent numerical root search for extraction. Each WRWS transition mounts to the sample holder and the waveguide test ports of a Vector Network Analyzer and is calibrated using a TRL technique. A biaxial anisotropic test sample was designed based upon crystallographic symmetry, mixing theory and verified in rectangular waveguide measurements. WRWS test data is collected and constitutive parameters are extracted from each orientation of the biaxial anisotropic cube. This method of extracting biaxial anisotropic constitutive parameters using the WRWS system is evaluated in both experiment, simulation and validates the WRWS method. Theory, experimental and simulated results are presented to show that a cubic sample and WRWS measurement system can be efficiently and effectively used to measure biaxial anisotropic materials.
A popular method for microwave characterization of materials is the free-space focused beam technique, which uses lenses or shaped reflectors to focus energy onto a confined region of a material specimen. In the 2-18 GHz band, 60 cm diameter lenses are typically spaced 30 to 90 cm from the specimen under test to form a Gaussian focused beam with plane-wave like characteristics at the focal point. This method has proved popular because of its accuracy and flexibility. Another free-space measurement technique that has been employed by some is the use of dielectrically loaded antennas that are placed in close proximity to a specimen. In this alternate technique, the dielectrically loaded antennas are smaller than lenses, making the hardware more compact and lower cost, however this is done at the expense of potentially reduced accuracy. This paper directly compares a standard laboratory focused beam system to a measurement system based on some recently developed RF spot probes. The spot probes are specially designed antennas that are encapsulated in a dielectric and optimized to provide a small illumination spot 3 to 8 cm in front of the probe. Experimental measurements of several dielectric, magnetic, and resistive specimens were measured by both systems for direct comparison. With these data, uncertainty analysis comparisons were made for both fixtures to establish measurement limits and capability differences between the two methods. Understanding these uncertainties and measurement limits are key to implementing compact spot probes in a manufacturing setting for quality assurance purposes.
I. INTRUDUCTION In the heating process of microwave absorbers under incident electromagnetic waves, two disciplines of physics are intertwined, i.e., electromagnetic waves behavior governed by Maxwell’s equations and heat transfer process dictated by laws of thermodynamics. The power density in the absorbers due to the electromagnetic .eld is given by p= s|E|2 =2po0 o ' f|E|2 (1) where, E is the total electric .eld (V/m) in the material, s is electrical conductivity of the material (S/m), o0 is the free space permittivity (8.854 × 10-12 F/m), o' is the imaginary part of the relative dielectric constant, and f is the frequency in Hz. This is point form of the Joule’s law, and is well understood by RF engineers. The EM behavior of the polyurethane absorbers can be numerically computed. The EM .eld acts as the heating source, and its distribution in the absorber can provide a good indication on the locations of hot spots. Polyurethane foam is an excellent insulator, so the conductive heat loss may be minimal. The heat exchanges can be reasonably described by radiation and convection transfers. Radiation takes place in the form of EM wave, mainly in the infrared region. The net power transferred from a body to the surroundings is described by Stefan-Boltzmann’s law [1], prad = osA(T4 -T04 ) (2) where A is the surface area, T is the surface temperature of the radiation body in K, and T0 is the ambient temperature in K. Unfortunately, the conventional symbols used in heat transfer s and o are not the same as those in Eq. (1). s here is the emissivity or emission coef.cient, and is de.ned as the ratio of the actual radiation emitted and the radiation that would be from a black body. o in Eq. (2) is the Stefan-Boltzmann constant (5.67 × 10-8 W/m2 K4 ). The context in the paper should make it clear which symbols the authors are referring to. Otherwise, we will make explicit references. The convective heat transfer is due to the motion of air surrounding the absorbers. Two forms can take place, naturally or by forced air. The relationship is described by Newton’s law of cooling [1]: pconv = hA(T -T0 ) (3) where h is the convection heat transfer coef.cient in (W/m-2 K-1 ). h is often treated as a constant, although it can be a function of the temperature. Eq. (3) assumes that the ambient air is abundant, and is taken to be constant. This is a reasonable assumption, because the heating is typically con.ned to a small localized area in a relatively large anechoic chamber. Combining the two mechanisms of heat transfer, the total heat loss is given by p= osA(T4 -T4 )+ hA(T -T0 ) (4) 0 It is possible to solve for the temperatures from coupled Maxwell’s and heat transfer equations. Realistic results require accurate electrical and thermal properties of the materials. It is often a non-trivial process to obtain the material properties in and of itself. Careful validation is warranted before we can have full con.dence in the results. In this paper, we adopt a measurement approach instead. We conduct a series of experiments to measure the temperature both on the surface of the absorbers using an infrared imaging camera, and internally using thermocouple probes inserted into the absorbers. Temperature pro.les versus applied E .eld are experimentally established. From the measured data, we curve .t to Eq. (4) or other mathematical functions. These functions are useful to calculate results at other .eld levels, e.g., extrapolating to a higher .eld where measurement results cannot be readily obtained. II. FIELD DISTRIBUTION INSIDE THE ABSORBERS Numerical analysis was performed using Ansys HFSS, a commercially available Finite Elements software package. As it was described in [2], symmetry is taken advantage of, so only one quarter of the pyramidal absorber is solved. The quarter pyramid is located inside a square cross section prism that bounds the computational domain. The structure is fed using a port located on the top of the geometry and the side boundaries of the domain are set as perfect electric conductor (PEC) or perfect magnetic conductor (PMC). The base is modeled as PEC. This is exactly the same approach taken in [2]. The structure of a CRV-23PCL-4 is analyzed at 12.4 GHz, the same frequency as used in the measurements. The resulting .eld is extracted at one plane. The plane is one of the two orthogonal planes that cut the pyramid in 4 sections. Fig. 1 shows the .eld distribution at 12.4 GHz. The curvature of the absorber pro.le has been added for clarity. The results are an approximation. The permittivity of the material is assumed to be fairly constant from 6 GHz to 12 GHz. The purpose of the numerical analysis is to check the expected .eld distribution in the pyramid, which we can use to compare with the infrared (IR) images of the absorbers taken during the measurements. Fig. 1. Electric Field distribution at 12.4 GHz The .eld distribution data shows that most of the .eld exists on the upper third of the pyramid. It also shows that there is a region of high .eld existing in the valleys between the pyramids. The surface temperature pro.le from the IR pictures shows that this is an real phenomena. On the other hand, the .eld is higher at the very tip of the absorber. Measurements from the IR images seem to contradict this result. This can be explained. Since the tip is smaller, it cools faster to the surrounding ambient temperature. III. EXPERIMENTAL SETUP AND DATA Experiments were performed on ETS-Lindgren CRV-23PCL-8, and CRV-23PCL-4 absorbers at 12.4 GHz. Both types are 23” long from tips to bases. A piece has a base size of 2’ × 2’. A CRV-23PCL-8 piece consists of 8×8=64 pyramids, whereas a CRV-23PCL-4 piece consists of 4×4=16 pyramids. The two types are designed to have similar RF performances, but the CRV-23PCL-8 is made of slender pyramids to facilitate better heat transfers to the surroundings [2]. The absorbers are mounted on a particle board with metallic backings, and are placed in front a Ku band horn antenna with a circular aperture (the gain is approximately 20 dBi). A 300W ampli.er is used, and the power to the antenna is monitored through a 40 dB directional coupler connected to a power meter. The test setup is shown in Fig. 2. The ambient temperature is at 23.C. Fig. 2. Test setup using a conical horn antenna to illuminate the absorbers As a .rst step, a 200 V/m .eld is generated by leveling to a calibrated electric .eld probe. The distance from the probe to the antenna is 30”. At this distance, near .eld coupling is assumed neglegible, and the incident wave uniform (numerical simulation also validated these assumptions). The power needed to generate 200 V/m .eld is recorded. Next, the .eld probe is replaced with the absorbers under test. The tips of the absorbers are placed at the same distance (30”) from the antenna. Other .eld strengths can be leveled by scaling from the power for 200 V/m. A. Surface Temperature Figs. 3 and 4 show two examples of the infrared images taken after the temperature reached equilibrium under a constant 700 V/m CW at f=12.4 GHz for the two types of absorbers described earlier. There is no forced air.ow during the measurement. Table 1 summarizes the resulting temperatures on the absorber surfaces at different .eld levels. Tests were performed on two .nishes of otherwise identical CRV-23PCL-8 absorbers, i.e., fully covered with rubberized paint, or with latex paint. The data indicates that the paint has minimal effects on absorber temperatures. Table 1 also lists data for the wider CRV-23PCL-4 absorbers (with latex paint). B. Internal Temperature of the Absorber recorded by Thermocouples Three thermocouples are inserted in the CRV-23PCL-8 which are painted with rubberized coating. They are inserted at distances of 4”, 6”, and 8” from the tip of the pyramid, as illustrated in Fig. 5. Fig. 6 shows the temperatures measured by the three sensors. The temperatures at 8” from the tip are consistently higher than at other locations. There is a gap in the data at 700 V/m because RF power was turned off brie.y. Internal temperature reached 115 .C under 1.7 kW/m2 Fig. 3. Infrared camera image for incident electric .eld of 700 V/m. The absorber is the slender CRV-23PCL-8. Fig. 4. Infrared camera image for incident electric .eld of 700 V/m. The absorber is the wider CRV-23PCL-4. (800 V/m). Since the maximum allowed temperature for the polyurethane foam material is 125 .C, the incident power density is recommended to stay less than 1.7 kW/m2 for CRV-23PCL-8 absorbers mounted vertically and with natural convection in a 23.C room. After the temperature reached equilibrium under 800 V/m, additional air.ow was introduced by turning on a 6” diameter fan at 45” in front of the absorbers. The air.ow rate was measured to be approximately 80 ft/min at this distance. Note that this is a rather moderate air.ow, which can arise naturally from air-conditioning vents in a chamber. As shown in Fig. 6, the internal temperature quickly dropped to 102.C from 115.C. TABLE I MAXIMUM SURFACE TEMPERATURE RECORDED BY THE IR CAMERA (AT EQUILIBRIUM). T0 =23. C. E Power CRV-23PCL-8 CRV-23PCL-8 CRV-23PCL-4 (V/m) Density rubberized latex (. C) rubberized (kW/m2 ) (. C) (. C) 200 0.11 24 300 0.24 28 360 0.34 30 400 0.42 35 36 43 500 0.66 41 50 600 0.95 54 67 700 1.30 63 82
Abstract—A new technique is presented to measure the permittivity and permeability of conductor-backed magnetic absorbing materials using a triaxial probe. The probe consists of two coaxial transmission lines that share an aperture in a conducting flange, which is placed against the sample creating a two-port network. By measuring the reflection coefficients at each port and the transmission between the ports, the material parameters may be determined. This paper describes the technique and provides a theoretical method for computing the S-parameters of the triaxial system. Experimental implementation of the system is still under study.
The core technology to this innovative chamber design is the invention of a new feed structure which integrates the design of the chamber’s wall, and reduces the multipath effects from the walls. In this design, the absorbing materials are integrated as a part of its feeding wall thereof to produce a homogeneous property on the plane, i.e., the plane parallel to the feeding wall. The material attached to the other walls has a non-homogeneous property on the plane parallel to its corresponding attached wall, which allows the scattering of incident field in a widely spread fashion.
High performance materials are often used in extreme temperature environments to enable advanced microwave frequency designs in both commercial and military applications. Accurate knowledge of microwave material properties as a function of temperature is key to ensure product or mission success. Robust designs must accommodate intrinsic material property changes with temperature or else the design may become unstable or fail. Researchers at the Georgia Tech Research Institute have recently developed a refined methodology suitable for high temperature testing of microwave materials based on the ASTM D2520 perturbation measurement technique. This paper presents the system design and examines the measured system response as a function of temperature to study the relationship of system dynamics and measurement uncertainty. Lessons learned from laboratory experiments are provided and measured data for several commonly available materials is presented to illustrate typical system performance for medium and low loss materials. The paper concludes with suggestions for further system improvements.
A new material validation and verification standard is designed to imitate the behavior of a lossy dielectric absorber. This standard is constructed from well-characterized, low-loss materials in a manner that ensures manufacturing repeatability. The performance of this standard is verified with S-parameter and permittivity measurements in a free space focused beam system and with finite difference time domain simulations. A sensitivity analysis, based on a series of simulations, is presented to quantify the uncertainty in the measured S-parameters due to dimensional and alignment variations from the ideal design values.
A method for determining the anisotropic permittivity for arbitrarily-shaped, physically and electrically small material specimens with diagonal biaxial dielectric and magnetic anisotropy is described and representative measured results are presented. The method permits the extraction of the six complex tensor permittivity and permeability components from six or more independent reflection measurements on a single specimen in a shorted rectangular waveguide. The specimen need not fill either dimension of the waveguide cross-section and is permitted to be electrically short in the propagation direction. Extracted material parameters from a known specimen were used to demonstrate the method.
Although the square patch antenna is a well known printed circuit antenna, there are gaps in the publications that prevented accurate design for practical dual polarization patch antennas. This paper describes (without gaps) the steps that allow rapid design of the dual polarized square patch antenna with typical commercial RF materials. Given a patch laminate material, the design process proceeds by using the Matlab program which is given in Appendix A. Typical values for a 5 GHz patch antenna are given. Dual polarization square patch antennas were constructed. Measurements show the two ports are well isolated, and they provide polarization diversity which is useful in our MIMO array development program. The scattering matrix of the two port antenna was measured with an Agilent PNA network analyzer. The antenna patterns were measured in our anechoic chamber and on our far field range. The pattern widths provide hemispherical coverage. The results which are given imply good efficiency for the antenna ports. When combined with the other patch elements in the MIMO array, robust communications are achieved for all look angles.
We present performance results of a bi-directional scattering measurement system in the 200-500 GHz range. The goal is to provide dense-spectrum, bidirectional reflectance distribution function (BRDF) of sample materials and small objects that can be propagated into detection models and used as standard materials to compare performance of various detection and imaging systems. Our system is built upon a commercial frequency-domain, vector network analyzer system. The system is designed to minimize drift due to movement and temperature changes. The initial data, presented here, of reflectance from a variety of standard targets and sample materials show operation from 200-500 GHz and highlight stability, repeatability, and dynamic range of the system.
In order to determine the permittivity of homogeneous dielectrics used in the production of microwave absorbing materials, it is necessary and sufficient to know /measure the complex reflectivity of thick dielectric bricks utilizing the materials [1]. If the permeability of the material is to be sought then, in addition, the knowledge of the complex transmission coefficient is necessary to determine unknown parameters. The coaxial lines like the one described in [2,3], as well as NRL arch systems, are widely used in the industry to characterize the performance ( reflectivity) of absorbing materials over a wide frequency range. In these systems, the conventional S11 or S21 methods of parameter extraction from the absorptive samples are utilized in conjunction with reference measurements from a metallic surface. The difference between the two measurements characterizes the reflectivity of the samples. In this paper, extension of the S11 and S21 methods to measure the permittivity and permeability of absorbing materials in conventional coaxial line and on NRL arches is described. The technique is based on measurements of thick absorptive bricks followed by signal processing with time - gating being utilized. The application is very useful for rapid dielectric property measurements of the raw materials prior to impregnation or full absorbing materials after impregnation prior to cutting the material into pyramidal or other shapes.
A technique is proposed to measure the permittivity and permeability parameters of a sample of biaxial material placed into a rectangular waveguide. By constructing the material as a cube, only a single sample is required to find all six material parameters. The sample is inserted into the waveguide in three different orientations, and the transmission and reflection coefficients of the sample region are measured using a vector network analyzer. The material parameters are then found by equating the measured S-parameters to those determined theoretically using a mode-matching technique. The theoretical details are outlined and the extraction process is described. Comparison of the mode-matching S-parameters with those obtained using the commercial finite element solver HFSS validates the theory.
All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.
Method of moments (MoM) codes have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In an earlier AMTA paper [1], we presented a network model (NM) algorithm that uses a Gauss-Newton iterative nonlinear estimation method in conjunction with a MoM model to estimate the “as-built” materials parameters of a target from a set of backscatter measurements. In this paper, we demonstrate how the NM algorithm, combined with the basis pursuits (BP) l1 minimization technique, can be used to locate unknown defects (dents, cracks, etc.) on a target from a limited set of RCS pattern measurements. The advantage of l1 minimization techniques such as BP is that they are capable of finding sparse solutions to underdetermined problems. As such, they reduce the requirement for a priori information regarding the location of the defects and do not require Nyquist sampling of the input pattern measurements. We will also show how the BP solutions can be used to interpolate RCS pattern data that is undersampled or has gaps.
Large size, light weight, broadband convex RF lens was developed to meet far-field requirements for antenna measurements. The Lens was fabricated from low loss, low density meta-materials and has diameter of D=2 m, focusing distance 2.4m and weight of just 50 kg with operational frequency 0.8 to 6 GHz. The lens is able to produce a plane-wave zone with an approximate size of 0.7D, allowing a 2m diameter lens to test antennas up to 1.4m in relatively small anechoic chamber. Another possible application of large size, lightweight RF lens is RCS measurements that include bi-static measurements. Results of quiet zone measurements for different frequencies are presented.