RF Absorber Research Articles

The effects of RF absorbers on exposure levels at 100 MHz

The study presented in this article was designed to complete a dosimetry protocol required to establish the RF exposure levels at 100 MHz for measurement of the effect on cognition in human volunteers near the resonant frequency in seated positions. The results are compared with those reported previously using the same experimental procedures, except with the vertically radiating dipole antenna and corner reflector raised by .30 m to the vertical center of the anechoic chamber. The average whole body SAR for the high and low SAR conditions used in the previous study was achieved with a 12% increase in transmitter forward power. However, the incident power density averaged over the body was increased by 42%, from 40 and 80 W/m(2) in the previous study to 57 and 113 W/m(2) in the current study to achieve the same whole body average SAR used in the previous study. The differences in field patterns and field intensities between the previous and current studies were introduced by interactions between the E-field and RF absorbers in the floor and ceiling, which represent resonant structures at 100 MHz.

RF electromagnetic wave absorbing properties of ferrite polymer composite materials

The frequency dispersion of complex initial (relative) permeability ( μ * = μ ′ - j μ ″ ) and the electromagnetic wave absorbing properties of composite materials based on NiZn sintered ferrite and a polyvinylchloride (PVC) polymer matrix have been studied in frequency range from 1 MHz to 1 GHz. The complex permeability of the composites was found to increase as the ferrite content increased, and was characterized by frequency dispersion localized above 50 MHz. The variation of return loss (RL) of single-layer RF absorbers using the prepared composite materials has been investigated as a function of frequency, ferrite content and the thickness of the absorbers.

A Study of RF Absorber for Anechoic Chambers Used in the Frequency Range for Power Line Communication System

Power line communication (PLC) system is spreading as a new communication system without providing new infrastructure for network communication. However, It is needed to investigate the EMI problem caused by PLC, and it is often tested in an anechoic chamber and an open area test site (OATS). The semi anechoic chamber lined ferrite tiles used for EMC testing is not generally designed in the frequency range from 2MHz to 30MHz used for PLC system. This paper presents characteristics of conventional ferrite absorber which are used for a semi anechoic chamber (SAC) and site attenuation of the semi anechoic chamber in the frequency range used for PLC system.

New higher-order-mode damping scheme for L-band superconducting cavities using a radial transmission line

For energy recovery linacs, excellent damping of higher-order modes (HOMs) in superconducting cavities is required for preventing beam breakup. As a promising method for this purpose, we propose to damp the HOMs using a radial transmission line attached to the sides of the cavity. The HOM powers are dissipated in RF absorbers while an accelerating field is rejected with a choke filter. An initial design of this radial-line HOM damper was carried out with simulations. Then, we measured the damping of HOMs by means of this method using a model cavity of the TESLA type. Investigations on a single-cell cavity showed a promising performance of HOM damping.

MODELING OF RF ABSORBER FOR APPLICATION IN THE DESIGN OF ANECHOIC CHAMBER - Abstract

A proper model of RF absorber must be developed based on information such as absorber reflectivity, in magnitude and phase, for various angles of incidence, and for parallel and perpendicular polarizations. Unfortunately, these data are not available due to the practical limitations of the test fixtures to measure the RF absorber performance. Manufacturer data sheets normally specify only the magnitude of the absorber reflectivity for normal incidence. A model has been developed in this paper for pyramidal RF absorber with pyramid length shorter than a quarter wavelength and poor reflectivity performance. Since the reflection from the metal backing would be much higher than the reflection and scattering from the pyramid tips, the metal boundary may be modeled as a lossy dielectric with certain effective dielectric constant, ε eff , and effective conductivity, σ eff , and the thickness extends to infinity. The appropriate values of ε eff and σ eff can be derived based on the reflectivity information given by the manufacturer's data sheet. The reflectivity at oblique incidence is calculated and compared with the results of method of homogenization and moment method. A reasonable match between the different models is obtained. The plane-boundary dielectric model can be used to evaluate the degradation of reflectivity level with respect to angle of incidence. It can be used in a simulation tool for design of anechoic chamber.

MODELING OF ANECHOIC CHAMBER USING A BEAM-TRACING TECHNIQUE

Building an anechoic chamber involves a substantial investment both financially and in physical space. Hence, there is much interest in trying to reduce the required investment while still maintaining adequate performance. The performance of an anechoic chamber depends on the type, size, and array configuration of the absorber elements as well as the geometry of the screened room on which the inner surfaces are covered with RF absorbers. If the room geometry is designed such that an electromagnetic ray from the transmitter will only reach the receiver antenna after a few reflections, the wave energy may be sufficiently damped after a few bounces off the absorbing walls and ceiling. Hence, lower cost RF absorbers can be used to make the anechoic chamber design more economical. In this paper, a variant of beam-tracing technique is used for modeling of anechoic chamber to study the normalized site attenuation (NSA) performance of the anechoic chamber. This allows the chamber performance to be predicted prior to the actual construction. The major advantage of beam-tracing over ray tracing is the path loss information at multiple receiver locations can be determined simultaneously as opposed to running a ray tracing simulation for each receiver location one at a time. As a result, the computing time is greatly reduced. This feature is particularly useful in calculating the field strength at different heights of the receiving antenna in EMC site calibration procedure. The efficient modeling tool has given rise to the successful design and construction of an asymmetrical shape anechoic chamber that supports various measurement needs including EMC tests at the Multimedia University, Malaysia. 24 Chung, Tech, and Chuah

High power operation of 110 GHz gyrotron at 1.2 MW on the JT-60 ECRF system

A high power operation of 1.2 MW for 4 s was obtained by a 110 GHz gyrotron with depressed collector on the JT-60 ECRF system, where the cathode voltage is −58 kV without voltage regulation and the acceleration voltage is +85 kV with regulation. The gyrotron is featured by a diamond output window and a RF absorber in its beam tunnel. The RF absorber suppresses the parasitic oscillation, which limited the performance of the gyrotron without the RF absorber less than 1 MW×2 s. The operation was done with the JT-60 tokamak plasma as a dummy load, where the EC wave was transmitted through a diamond antenna window on the corrugated waveguide. The edge temperature of the antenna window without cooling was in proportion to the pulse duration. The maximum efficiency of the gyrotron was about 42% in the depressed collector operation on the JT-60 ECRF system.

Modeling of RF Absorber for Application in the Design of Anechoic Chamber

A proper model of RF absorber must be developed based on information such as absorber reflectivity, in magnitude and phase, for various angles of incidence, and for parallel and perpendicular polarizations. Unfortunately, these data are not available due to the practical limitations of the test fixtures to measure the RF absorber performance. Manufacturer data sheets normally specify only the magnitude of the absorber reflectivity for normal incidence. A model has been developed in this paper for pyramidal RF absorber with pyramid length shorter than a quarter wavelength and poor reflectivity performance. Since the reflection from the metal backing would be much higher than the reflection and scattering from the pyramid tips, the metal boundary may be modeled as a lossy dielectric with certain effective dielectric constant, e eff , and effective conductivity, σ eff , and the thickness extends to infinity. The appropriate values of e eff and σ eff can be derived based on the reflectivity information given by the manufacturer's data sheet. The reflectivity at oblique incidence is calculated and compared with the results of method of homogenization and moment method. A reasonable match between the different models is obtained. The plane-boundary dielectric model can be used to evaluate the degradation of reflectivity level with respect to angle of incidence. It can be used in a simulation tool for design of anechoic chamber.

Progress of Electron Cyclotron Heating/Current Drive Technology in JT-60U

A key issue in attaining high plasma performance in the Tokamaks is the optimization of current profile, involving such functions as stabilizing the neo-classical tearing mode (NTM) and control of the internal transport barrier (ITB). In the JT-60U, a high power electron cyclotron heating (ECH) system at 110 GHz has been developed for local plasma heating and current drive. We have demonstrated the technical feasibility of an antenna which can scan EC beam in both the toroidal and poloidal directions. The toroidal scan enabled co- and counter- current drive and also pure plasma heating. Automatic stabilization of the NTM was demonstrated by means of current drive at the magnetic island employing feedback control of the poloidal beam angle. The total injected power has been extended to 2.8 MW for 3.6 sec and each gyrotron delivers ˜ 1 MW for 5 sec. This world record regarding the injection energy was attained by an upgrade of the gyrotron using an RF absorber in the beam tunnel to suppress the parasitic oscillation and to improve the transmission efficiency of the waveguide system.

Electromagnetic interference shielding by using conductive polypyrrole and metal compound coated on fabrics

AbstractElectromagnetic interference (EMI) shielding materials of complex type of conductive polypyrrole (PPy) as an intrinsically conducting polymer and silver‐palladium (AgPd) metal compound coated on woven or non‐woven fabrics are synthesized. From dc conductivity and SEM photographs of PPy/fabric complexes, we discuss charge transport mechanism and the homogeneity of coating on the fabrics. The EMI shielding efficiency of PPy/fabric and AgPd/fabric complexes is in the range of 8 ∼ 80 dB depending on the conductivity and the additional Ag vacuum evaporation. The highest EMI shielding efficiency of PPy/fabric complexes vacuum‐evaporated by Ag is ∼80 dB, indicating potential materials for military uses. We propose that PPy/fabrics are excellent RF and microwave absorber because of the relatively high absorbance and low reflectance of the materials. Copyright © 2002 John Wiley & Sons, Ltd.

Effects of RF absorbers on measurements of small antennas in small anechoic chambers

In this work, the influence of resistive absorbers in the vicinity of small antennas was investigated, in order to define a lower limit for the size of a small anechoic chamber for antenna calibration and radiation pattern measurements, with respect to the distance between the antenna under test (AUT) and the RF absorbers lining the walls. Furthermore, the influence of a relatively narrow transmission channel, formed by the absorbers on the side walls between the measurement antenna and the AUT was investigated. It was found by measurements that absorbers with the length of 610 mm do not have a measurable effect on the radiation characteristics of a small antenna at around 1.8 GHz, as long as they are kept more than 100 mm from the AUT, and the transmission channel formed by the absorbers between the antennas is wider than 100 mm.

Mode selection in high power microwave sources

In designing a microwave generator or an amplifier of enhanced power, we always risk encountering self-excitation of unwanted oscillations. To suppress parasitic oscillation modes, we may implement a selective rf absorber, organize a selective feedback, taper the rf structure profile to admit only the main operation mode into the wave-electron synchronism band, etc. At a proper switching-on scenario, the main operation mode of the device can suppress its rivals at the nonlinear interaction stage. This brief review is aimed to systematize numerous methods efficiently used to keep the coherence in high power vacuum devices based on the stimulated radiation by electron beams.

Novel material with narrow-band transparency window in the bulk

This paper presents the theoretical design of an artificial dielectric exhibiting narrowband frequency selective properties in the bulk without relying on periodic placement of elements. In this manner, it initiates a novel approach that bypasses the drawbacks of the traditional frequency selective surfaces (FSS), namely, unwanted passbands, dependence on excitation angle and polarization, and difficulties in conversion from planar to curved geometries. The key design elements are the concentric geometry of the inclusions and the use of Lorentzian resonant media. A discussion of physical resonant materials is presented, substantiating the credibility of the theoretical design. To illustrate the approach, a novel complex medium is synthesized as an ensemble of spherical particles composed of a lossy core coated with a highly resonant dielectric layer and embedded into a dielectric host. The resulting structure is an amorphous substance, lossy over its entire spectrum except for two narrow-band transparency windows, where it may become as lossless as desired. The parameter space of the system is thoroughly analyzed which determines the type of constitutive materials and geometries for tailor-designing the windows according to specifications (shape, positioning and overall normalization). In this sense, the lossy concentric structure forms an ideal candidate for thin absorbing films (TAFs) with extensive applications in antenna systems, RF absorbers, and anechoic chambers.

The effect of RF absorbers in the vicinity of small antennas

The influence of RF absorbers in the vicinity of small antennas was investigated. Absorbers placed more than 100 mm from an antenna have no considerable effect on its radiation characteristics at 1.8 GHz. Results were confirmed by measurements and simulations, indicating the possibility of using small anechoic chambers for small antenna measurements. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 328–330, 1999.

The effect of RF absorbers in the vicinity of small antennas

The effect of RF absorbers in the vicinity of small antennas

Muon collection channel

A preliminary analysis of the collection channel for a muon collider is presented. Pions, produced in a target by 8–30 GeV protons, are captured and transported in a solenoid where they decay into muons. Muon yield and phase space characteristics at the end of the channel can be improved by placing RF cavities and absorbers along the decay channel as well as by providing a supplemental toroidal field in conjunction with the absorbers. Results of Monte Carlo simulations are presented for a few such scenarios.

Untersuchung biologisch-zerebraler Effekte in niederfrequent gepulsten Hochfrequenzfeldern

Possible cerebro-biological effects of low-frequency pulsed RF fields were investigated using a commercial mobile GSM telephone. As test parameters the EEG and neuropsychological tests were evaluated. The examinations were carried out in a room equipped with RF absorbers. The field distribution in the room was simulated with the help of a numerical field calculation tool. Based on measurements of the electric field strength dependent on the distance from the transmitting antenna the distance between the head of the test person and the antenna was adjusted to approximately 45 cm so that the limit value according to DIN/VDE 0848 could not be exceeded. In the study with 52 test persons no significant impact of the transmitting field on the human EEG as well as on the cognitive performance could be observed.

ELECTROMAGNETIC SCATTERING BY A PERIODIC SURFACE WITH A WEDGE PROFILE

ABSTRACT Two-dimensional scattering from a periodic surface with a wedge profile is treated for both TE and TM polarization. The Interface can be either perfectly conducting or penetrable, and the extended boundary condition (EBC) is used in both cases. The method results in simple matrix elements and is easy to apply. Comparison with a perturbation solution shows that the EBC method has greater accuracy and a greater range of validity. Even so, EBC becomes numerically ill conditioned for surface heights that are a significant fraction of the period or the wavelength. Numerical results are presented for conservation of power, and some comparisons are made with previous results for sinusoidal surfaces. Some applications to rf absorber are also presented.

Generation of EM Susceptibility Test Fields Using a Large Absorber-Loaded TEM Cell

This paper discusses the development of an electromagnetic simulator for accurate generation of broad-band suspectibility test fields within a shielded environment. The simulator consists of a large, 3 m X 3 m X 6 m, rectangular transverse electromagnetic (TEM) transmission cell that is loaded with RF absorber to suppress multimoding at frequencies above the cell's waveguide cutoff or resonant frequencies. The paper describes the measurement facility and technique, and the experimental verification of pertinent test paramenters such as system VSWR, insertion loss, and test field uniformity. The measurement system is anticipated to provide swept, automated susceptibility measurements of electronic equipment to CW, pulsed, and EMP fields within the frequency band, 10 kHz to 1 GHz.

Standing Wave Reduction in an RFI Laboratory

After review of some general properties of RF absorber materials, several of the standard and semistandard techniques for measuring absorber and anechoic chamber quality are discussed. A general purpose RFI laboratory is described together with a simple technique for evaluating the performance of absorber material as it is actually used. An effective background reflectivity of about 4% from 1 - 10 Gc was measured and is believed to be adequate for many applications.