Ferrite Absorber Research Articles

配向性Ｍ‐型バリウムフェライトＢａ１‐ｘＬａｘＺｎｘＦｅ１２‐ｘ‐ｙ（Ｍｅ０．５Ｍｎ０．５）ｙＯ１９における金属元素Ｌａ，Ｍｎ，Ｓｎ，Ｚｎ置換による高マイクロ波吸収特性への効果

Microwave radiation in the GHz band is increasingly being used in many various communication devices. However, this has brought about the problems of electromagnetic interference (EMI). Now M-type Ba ferrite is expected to be used as a powerful countermeasure to EMI. Our recent works have shown that M-type Ba ferrite is promising material for microwave absorber in the GHz range. The electromagnetic resonance absorption properties depend on the complex permeability μ=μ'-jμ", particularly the imaginary part μ"=Ms/2HAα(Ms: saturation magnetization, HA: anisotropy field, a: extinction coefficient). We investigated the effects of substitution composition (x) of Me for Ba (Me: La) and substitution composition (y) of Me ion (Me: Zn, Zr, Sn) for Fe3+ on the electromagnetic absorption properties of the Ba1-xLaxZnxFe12-x-y(Me0.5Mn0.5)yO19 compound. The c-axis aligned samples were formed into toroidal shapes. The magnetic properties (Ms, HA) and the resonance absorption properties (μr', μr" etc) were measured using a VSM and a network analyzer. The aligned Zr-substituted samples (x=0.0, y=1.4) showed values of HA =0.21 MAm-1, and μ'= 7.5 and μ" =7.1 that were higher than those of (TiMn) substituted samples. The matching thickness dm decreased to 0.8 mm at the matching frequency of 7.6 GHz. In the sample of x=0.0 and y=1.5, μ'=7.7 and μ" = 6.7 were obtained, and the matching thickness dm was 0.98 mm at 5.95 GHz. Also, Sn-substituted samples showed high m' and m" values of 6.3 and 6.3, respectively, and dm of 1.02mm. It is therefore concluded that Sn or Zr substitution is effective in enhancing the electromagnetic wave absorption characteristics of M-type Ba ferrite absorber.

Characteristics of cavity‐isolated and cavity‐connected bow‐tie antennas for subsurface pulse radar

AbstractThe characteristics of metal cavity‐attached resistor‐loaded bow‐tie antennas, which are widely employed as antennas for subsurface detection radars using broadband pulses, are investigated theoretically and experimentally in this paper. Two kinds of antennas are considered: isolated antennas having a structure which separates electrically the bow‐tie antenna and the metal cavity, and connected‐type antennas which connect the bow‐tie antenna and the metal cavity via a resistor. The inner cavity surfaces of both types are coated with ferrite. In this paper, the characteristics of both types of antennas are discussed from the point of view of the effects of the ferrite absorber on the above‐ground and subsurface underground radiation power, the coupling between the transmitter and the receiver, and the effects on the transient reception characteristics. The reasonableness of the analysis of the ferrite absorber is verified experimentally. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 85(8): 66–73, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.1119

The use of genetic algorithms to maximize the performance of a partially lined screened room

This paper shows that it is possible to use genetic algorithms to optimize the layout of ferrite tile absorber in a partially lined screened enclosure to produce a "best" performance. The enclosure and absorber are modeled using TLM modeling techniques and the performance is determined by comparison with theoretical normalized site attenuation of free space. The results show that it is possible to cover just 80% of the surface of the enclosure with ferrite absorber and obtain a response which is within /spl plusmn/4 dB of the free space response between 40 and 200 MHz.

Fundamental investigation on a weakly magnetized ferrite absorber

Investigated are the matching characteristics of a magnetized ferrite absorber, which applies a weakly static magnetic field H/sub dc/ perpendicular to the sintered ferrite surface. Applying a static magnetic field to the ferrite enables the matching thickness of the absorber to be reduced from 8 mm in the absence of H/sub dc/ to 3 mm under 750 gauss. The matching frequency can also be changed broadly from 0.1 to 0.9 GHz by controlling ferrite thickness and H/sub dc/, simultaneously. These are well explained in terns of an nonsaturated permeability rather than tensor permeability. A thin magnetized ferrite absorber is realized using a ferrite with a large value of imaginary part of permeability when real part takes the value near 1.0. Matching frequency characteristics, whether broad or narrow, correlates closely with the slant of the curve for real part of permeability taking the value near 1.0 in the present frequency.

Effect of gap between ferrite tiles on ferrite electromagnetic wave absorbers

Ferrite electromagnetic wave absorbers are widely used for electromagnetic half reflectionless rooms for EMC measurement and anechoic chambers. If gaps exist between the ferrite tiles, the return loss characteristics are substantially degraded in the low-frequency region between 30 and 200 MHz. In this article, the effects of the gap in ferrite tiles on the return loss characteristics are studied for grid-type ferrite absorbers and planar ferrite absorbers. From the numerical simulations, it is found that the degradation of the characteristics due to the gaps is smaller in the grid-type ferrite absorbers than in the planar ferrite absorbers at low frequencies. This finding is also confirmed by model experiments. Further, filling of the gaps with ferrite powder is studied as a means of reducing the degradation of the characteristics by the gaps. © 2000 Scripta Technica, Electron Comm Jpn Pt 2, 83(2): 52–60, 2000

複合電波吸収体材料の開発および評価

We developed a composite conductor-backed single layered electromagnetic wave absorbers of composite materials (spinel-type Ni-Zn ferrite, SiO2) which can operate in the frequency region between 1 and 10 GHz. Both the complex permittivity and the complex permeability for the composite ferrite obeyed Lichtenecker's logarithm mixed law. A composite ferrite with a suitable thickness have the higher absorption frequency range than both Ni-Zn ferrites and Ba ferrite absorbers. The center absorbing frequency can be changed by changing the mixing ratio of SiO2, the thickness, or the composition of Ni and Zn. The absorption band width is almost independent on the thickness of the composite ferrite in the wide frequency range. The range of absorbable thickness becomes wider than that of Ni-Zn ferrite. The density and the hardness is improved by adding B2O3. The conductance value, however, increases and the center absorbing frequency is shifted to the lower frequency in proportional to the density. The conductance value can be reduced by adding a small amount of CoO.

Three-dimensional calculations for a 4 kA, 3.5 MV, 2 microsecond injector with asymmetric power feed

The DARHT-2 accelerator under construction at Los Alamos National Laboratory requires a long flattop ($2\ensuremath{\mu}\mathrm{s}$) 2--4 kA, 3.5 MV, low-emittance electron beam source. The injector is being constructed at Lawrence Berkeley National Laboratory and consists of a large-area thermionic cathode mounted atop a vertical column. The 90\ifmmode^\circ\else\textdegree\fi{} bend between the horizontally emitted beam and the column produces dipole and higher-pole fields which must be corrected. In addition, the fast rise of the current flowing into the vacuum tank excites rf modes which cause transverse oscillations of the beam centroid. We have modeled these effects with the 3D electromagnetic code LSP. The code has models for pulsed power transmission lines, space-charge-limited emission and transport of charged particles, externally applied magnetic fields, and frequency-dependent absorption of rf. We calculate the transverse displacement of the beam as a function of time during the current pulse, and the positioning and thickness of ferrite absorber needed to damp the rf modes. The numerical results are compared to analytic calculations.

FDTD analysis of resistor-loaded bow-tie antennas covered with ferrite-coated conducting cavity for subsurface radar

This paper presents a full-wave analysis of a ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method. The antenna treated here consists of a resistor-loaded bow-tie antenna, which is covered with a rectangular conducting cavity of which inner walls are coated partially or fully with ferrite absorber. Some techniques are introduced into the FDTD analysis to obtain the accurate results and to save the computer resources. The validity of the FDTD analysis is confirmed experimentally. Furthermore, the effects of the ferrite absorber on the GPR characteristics are theoretically investigated in detail. The FDTD results indicate that the remarkable improvement of the antenna characteristics for the GPR system cannot be attained by the ferrite absorber.

The Magnetic Properties and Microwave Absorbing Characteristics of Ni-Zn Ferrite

The relations between magnetic properties and microwave absorbing characteristics of Ni-Zn ferrites were examined. As the initial permeability of Ni-Zn ferrite material increased, the microwave absorbing performance of ferrite absorber was improved in the MHz region. From the results of these experiments, we could develop the ferrite absorber having superior microwave absorbing performance in the 10MHz-400MHz region.

Thin Wave Absorber Containing Carbonyl-Iron Particles Surface-Treated with a Coupling Agent

Magnetic polymer compounds containing carbonyl-iron particles surface-treated with a silane coupling agent were used to fabricate electromagnetic wave absorbers for the quasi-microwave band (1−3 GHz). The coupling treatment changed the surface property of the particles from hydrophilic to lipophilic, and simultaneously decreased the electrical conductivity on the surface of the particles. These polymer compounds can thus contain a large volume (up to 59 vol%) of the particles, and as a result show high permeability and permittivity (7 and 45, respectively, at 2 GHz) without generating much conductivity. These magnetic and electrical conditions—high permeability and permittivity with low conductivity—result in a wave absorber that is thinner (about 2 mm thick) than a conventional spinel-type ferrite absorber (about 7 mm thick).

Electromagnetic wave absorbing characteristics of Ni-Zn ferrite grid absorber

The reflection loss of Ni-Zn ferrite grid absorber is studied in the low frequency limit which the period of the array is small compared with wavelength. The relative equivalent complex permeability and permittivity with air volume fraction is obtained using the Hashin-Shtrikman (1962) upper-bound formula. The impedance matching conditions for maximum electromagnetic wave absorption are obtained by the "contour map method." Two impedance matching thicknesses and frequencies are compared with theoretical matching relationships derived under the limiting case of impedance matching formula. As the air volume fraction increases, the first matching frequency, f/sub m1/ remains nearly constant, while the second one, f/sub m2/ decreases and approaches f/sub m1/. Therefore, the absorption band of a ferrite grid absorber is broader than that of the single layered ferrite plate absorber.

空孔フェライトによる同軸管用整合終端負荷材の整合特性

空孔フェライトによる同軸管用整合終端負荷材の整合特性

Design of super wideband electromagnetic wave absorber and its characteristics

AbstractBy combining a ferrite‐type electromagnetic wave absorber and a lossy dielectric layer, an electromagnetic wave absorber with a super wideband characteristic can be realized. In this paper, a grid‐type ferrite absorber and a rubber ferrite loaded ferrite multilayered absorber are treated as a ferrite‐type absorber. When a lossy dielectric material is mounted in front of these ferrite‐type wave absorbers, the thickness of a lossy dielectric layer necessary for realization of a return loss of more than 20 dB at any frequency above 30 MHz was computed. It was found that the thickness is much less than that for the lossy dielectric attached to a single layer ferrite. Further, by means of a model experiment in the coaxial tube, the thickness of the lossy dielectric layer was proven to be smaller if a lossy dielectric is combined with the grid‐type ferrite absorber loaded with rubber ferrite.

Characteristics of grid ferrite electromagnetic wave absorber

AbstractA novel ferrite grid electromagnetic wave absorber composed of sintered ferrite is proposed. Based on the condition that the return loss should be above 20 dB at 30 MHz, its wideband characteristics for suppressing reflections compared to a single‐layer absorber are demonstrated by computer simulation.Calculating the return loss by varying the geometrical parameters of the ferrite grid, an optimum structure is determined which gives the maximum frequency band. In addition, it is found that the return loss of the grid absorber is impaired by an air gap between the ferrite and conductor. Moreover, the operating characteristics of the grid absorber are demonstrated by experimental models.

Widening the bandwidth of ferrite electromagnetic wave absorbers by attaching rubber ferrite

With a view to increasing the bandwidth of ferrite electromagnetic wave absorbers, the reflection characteristics were studied for an absorber made of sintered ferrite loaded with rubber ferrite via an appropriate dielectric layer. First, by means of calculations, it is shown that this structure has a bandwidth wider than that for the sintered ferrite single-layer absorber. Also, the bandwidth of the absorber increases as the permittivity of the dielectric material inserted in between is lower. By loading an appropriately thick rubber ferrite plate via an air layer on a sintered ferrite plate, an absorber can be constructed for which the reflection is less than -20 dB over the frequency range from 30 to 1830 MHz. By loading another air/rubber ferrite pair, an even further increase of the bandwidth is possible. For an existing ferrite single-layer absorber, it is effective for increase of the bandwidth to load the air/rubber ferrite by attaching a sheet resistance on the ferrite surface. Finally, by an experiment, it is shown that a broader bandwidth can be obtained than the ferrite single-layer absorber by loading a rubber ferrite via a low-permittivity layer.

The microwave absorbing and resonance phenomena of Y-type hexagonal ferrite microwave absorbers

The microwave absorbing characteristics and resonance of Y-type hexagonal ferrite–rubber composites were investigated. The complex permeability and permittivity of Ni2−xZnxY ferrite bodies were measured using a network analyzer in the frequency range of 200 MHz–14 GHz. Two types of resonance, the domain wall and the spin rotational resonance, were observed. With a ferrite particle with a diameter of about 1 μm, only spin rotational resonance was observed. The first matching frequency, found in the ferrite–rubber composites, which was higher than that of spin rotational resonance, increased with spin rotational resonance frequency. It was also found that domain wall resonance had no effects on the microwave absorbing characteristics. Based on these findings, it could be concluded that the microwave absorbing characteristics were caused by only one type of resonance, the spin rotational resonance.

The microwave absorbing phenomena of ferrite microwave absorbers

The complex permeability dispersions and the microwave absorbing phenomena are investigated in ferrite microwave absorbers. The complex permeability of hexagonal (Y- and Z-type) and spinel (NiZn ferrite and NiZnCo ferrite) ferrite microwave absorbers were measured in 200 MHz-16 GHz range. The theoretical matching frequencies are determined by plotting the measured complex permeability loci on the impedance matching solution map. One or two impedance matching phenomena are observed in the ferrite absorbers according to their complex permeability loci on the impedance matching solution map. It was discovered that the first matching frequency (f/sub m1/) of the ferrite absorber strongly depends on its spin resonance frequency (f/sub r2/) and the second matching frequency (f/sub m2/) is independent of the resonance frequency. >

Representing ferrite absorbing tiles as frequency dependent boundaries in TLM

A new way of modelling ferrite tile absorbers as frequency dependent reflecting boundaries in the TLM method is described. Results are presented which show a good fit to the manufacturers' reflectivity curves.

Scattering of Electromagnetic Fields by Ferrite Absorber Lined Conducting Plane

Reflectivity of a ferrite lined conducting plane has been determined by considering the scattered fields, for horizontal Hertzian dipole excitation, with and without the ferrite lining. Numerical results for a particular ferrite type reveals that the scattered fields undergo significant attenuation even when the thickness of the ferrite lining is small compared to the wavelength of the incident field.

Analysis of ferrite absorber considering hysteresis and relaxation characteristics by spatial network method

AbstractRecently, various microwave absorbers have been studied and developed for electromagnetic interference problems. One of the important materials for absorbers is ferrite in which the high‐frequency magnetization mechanism consists of a superposition of the motion of the magnetic wall and the rotation of the magnetization. Its complex permeability has a frequency dispersion; therefore, it is difficult to develop a rigorous theoretical analysis as an absorber. If a numerical analysis based on a simulation model is possible, the design of the absorber will be expedited.In this paper, the spatial network method is used for treating the hysteresis characteristics of the shift of the magnetic domain wall and the relaxation characteristics with an absorbing characteristic for the rotation of magnetization. A basic equivalent circuit model is proposed in which both are combined, and then this model is used to analyze the frequency characteristics of the reflection coefficient of a plane wave. The results based only on the basic relaxation model are shown to agree well with the theory. The basic validity of the model is verified by showing that the reflection coefficient is reduced at less than the matching frequency due to the hysteresis loss, and that the matching frequency and the tendency agree with the experimental results by Ishino et al.