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Surface and Internal Temperature versus incident field measurements of Polyurethane based absorbers in the Ku band
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 , 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 : 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 , 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 . 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 . 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
A Triaxial Applicator for the Characterization of Conductor-Backed Absorbing Materials
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.
An Innovative Design of a Size-Reduced Anechoic Chamber for Antenna Measurements at Low Frequencies
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 Temperature Material Measurements Using Refined Perturbation Technique
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.
An Artificial Lossy Dielectric Material Standard for RF Free Space Measurements
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.
Biaxial Permittivity and Permeability Determination for Arbitrarily-Shaped, Electrically-Small Material Specimens Using Shorted Rectangular Waveguides
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.
Square Patch Antenna Design from Equivalent Circuit Models for MIMO Antenna Communications Application
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.
A 200-500 GHz Bi-Static Scattering System for Material Characterization
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.
Express Measurements of the Dielectric Properties of Foam Absorbing Materials
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 . 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.
Characterization of Biaxial Materials using a Partially Filled Rectangular Waveguide
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.
Absorber, Performance, and Advancements in Absorber Technology
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.
A Model-Based Technique With l1 Minimization For Defect Detection And Rcs Interpolation From Limited Data
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 , 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 RF Lens for Far-Field Antenna Measurement
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.
Broadband Free Space Material Measurement System
This paper introduces a broadband free space material measurement system in Temasek Laboratories at National University of Singapore (TL@NUS). The system is designed by TL@NUS and ST Aerospace for measuring permittivity, permeability, reflection and transmission properties of electromagnetic materials and structures from 1 to 40 GHz. The measurement system includes a pair of double convex spot-focusing lenses, horn antennas, a network analyzer and two arms that can be moved along a circular arc. The two arcs of the arms allow measurement to be done with different incident angles. Each of the double convex lenses is made from two plano-convex dielectric lenses of 77 cm in diameter. The plano-convex lenses can collimate the field from the source horn into uniform plane wave thus also allowing both mono-static and bi-static electromagnetic scattering measurement to be done in very limited space. The system is housed in an anechoic chamber of dimension 6.7 m (D) × 6.6 m (W) × 3.8 m (H) to reduce unwanted reflections and interference signals from the surroundings. Typical measurement results are presented in this paper for dielectric materials, magnetic materials, frequency selective surfaces, and metamaterials.
Principles of Operation of Optimized Absorbing Materials at VHF/UHF Frequency Bands
In the paper  the principles of operation of high performance absorbing materials were described and the criterion for absorber performance optimization at UHF/VHF frequency bands was proposed and confirmed experimentally on a number of absorber components optimized for operation at low frequencies such as the VHF/UHF bands. C:\Publishing 2011\AMTA 2011\Papers\Absorbing Material Performance\freq dom 18 24 36 60.jpg The experimentally verified optimization criterion is intended to determine the optimum carbon loading of the absorber components, thus delivering optimal reflectivity of the full absorbing assembly (absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity of the criteria was confirmed by measurements of the reflectivity of pyramidal absorbing components of various heights, (3’, 5’, 6’ and 8’ ) in a 40’ long coaxial line terminated in a metallic back wall [2,4]. In this paper, more details are highlighted explaining how the criterion is delivering the best absorber reflectivity at low frequencies. This is accomplished by implementing time gating post-processing to isolate two primary concurrent peaks corresponding to the reflections from the front surface and metallic backing substrate. It is shown that the improved reflectivity is achieved by a self-cancellation of the two signals delivering the “null” in the frequency domain, which, in turn determines the lowest operating frequency attributed to an absorber of a given height. It is shown that the “null” property of the reflectivity pattern, as well as the properties of the peaks in between “nulls”, can be scaled and, therefore, predicted based on the height of the absorber almost everywhere in the UHF band. Thus, it is possible to optimally choose the grade of the absorber necessary to meet or exceed given reflectivity specifications, or to manufacture the appropriate absorber grade which can deliver the optimum reflectivity at the specified frequency.
Implementation and Analysis of an Improved Accuracy Microwave Measurement Method for Low Loss Dielectric Materials
A free space transmission line measurement method for dielectric constant and loss tangent determination in low-loss dielectric materials has been analyzed and implemented. This method utilizes dielectric materials with thicknesses greater than half the wavelength in the material to obtain greater sensitivity for determining intrinsic dielectric properties. An analysis of the process sensitivities and experimental measurements has been utilized to estimate the accuracy and lower limits of the dielectric property extractions from the reflection loss magnitude.
Bandwidth Enhancement for U-Slot Stacked Patch Antenna by Using Appropriate Dielectric Materials
In this paper, a coaxially fed broadband U-slot stacked rectangular microstrip patch antenna in corporating a high and low dielectric material combination is presented. The antenna essentially consists of two commercially available microwave substrates (Rogers TMM3 and Rohacell HF71 foam). Dielectric constants of materials are 3.27 and 1.07 respectively. Foam material doesn’t include copper surface thus a third dielectric substrate with thickness of 0.254 mm and dielectric constant of 2.2 is added over the foam material to ease of fabrication. The antenna return loss bandwidth is about 52.94%, centered about 3.4 GHz. The effect of the parameters, such as u slot length and width, on the antenna performance are determined, experimentally verified and discussed.
Measurement of Complex Permittivity Using Artificial Neural Networks
In this paper, a Neural Network based methodology is presented to measure the complex permittivity of materials using monopole probes. A multilayered Arti.cial Neural Network, using the Levenberg Marquardt back propagation algorithm is used to back solve the complex permittivity of the medium. The proposed network can be trained using an analytical model, numerical model, or measurement data spread over the complete range of parameters of interest. The input training data for the non linear inverse problem of reconstructing the complex permittivity comprises the complex re.ection coef.cient of the monopole probe. For the results presented in this paper, the network is trained using the analytical model for impedances of monopole antennas in a half space by Gooch et al. . In addition to computational ef.ciency, the proposed approach gives 99% accurate results in the frequency range of 2.55 GHz, with the values of permittivity varying across a range of 3-10 for the real part, and 0 -0.5 for the imaginary part. The accuracy and the effective range of real and imaginary components of the complex permittivity that can be reconstructed using this approach, depends upon the accuracy and robustness of the model / system used to generate the training data. The analytical model used in this paper has a limited range for the values of loss tangent that it can model accurately. However, the performance of the back solving algorithm remains independent from any speci.c model, and the scheme can be successfully applied using any reliable analytical or numerical model, or re.ection coef.cient training data generated through a series of measurements. The methodology is likely to be employed for experimental measurements of complex permittivity of dissipative media.
A Comparison of Methods for Measuring Dielectric Properties of Thin-Film Materials
RF measurement of the dielectric properties of very thin films (less than 1/100 wavelength thick) presents a challenge using traditional techniques. Many techniques, such as conventional transmission line-type measurements, are not sensitive enough to measure a single thin sheet of material. Moreover, in the case of waveguide, the method of mechanically fastening the material in place properly is challenging. In this paper, we explore several different strategies for measuring thin films and compare the merits of each. In particular, coaxial line measurements with stacked layers, waveguide measurements, and cavity measurements are discussed. The methods will be compared in terms of their accuracy and sensitivity. Measurements are carried out using the various methods on several low-loss thin-film materials. The measurements are then compared and validated using known reference materials.
Optimization Criterion and Optimal Loading of High Performance Absorbing Materials at VHF/UHF Frequency Bands Optimization Criterion and Optimal Loading of High Performance Absorbing Materials at VHF/UHF Frequency Bands
This paper describes the principles of operation of high performance absorbing materials and the criterion for its performance optimization at UHF/VHF frequency bands. The optimization criterion is intended to determine the optimum carbon loading of the foam based absorber components, thus delivering optimal reflectivity of the full absorbing assembly (foam based absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity is confirmed by measurements of the reflectivity of pyramidal absorbing components of varying heights, (3’, 5’, 6’ and 8’) in a 40’ long coaxial line terminated in a metallic back wall. In addition, it is shown that the “aging” process of the absorbing components can be characterized by the change of the effective reflectivity in the time-domain of the components as a function of aging time. It is possible to determine whether the absorber performance is stabilized and the “aging“ process is complete, and whether the loading of the absorber carbon mix is optimum, or is otherwise under-loaded or over-loaded. In particular, it is possible to determine prior to the time when the “aging” process is stabilized whether the loading is excessive.
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