DEVELOPMENT & CHARACTERIZATION OF MICROWAVE ABSORBERS

Abstract

Microwaves span over a wide frequency spectrum, but in this particular subject of study, material-composites in the form of planar thin sheets, have been analyzed and designed to absorb microwave energy in the frequency range of 2-18 GHz. Various design approaches have been formulated for the design and analysis of thin planar absorbers. An impedance matching technique has been formulated to match thin planar absorbers to free space, without increasing the overall thickness of the absorber. A generalized design approach is formulated with Cobalt-substituted Barium Hexa Ferrite (Co-BHF) as the lossy material for the design of broad band absorbers. Broad banding is achieved by combining the material and thickness absorption effects in the absorber. The design formulation has been practically verified by fabricating of an absorber for broad band operation over Ku-band, with a constraint of 2mm on the thickness of the absorber. The close agreement between the theoretically predicted and experimentally obtained absorption characteristics has validated the formulation. A mathematical model for the theoretical analysis and design of microwave absorbers has been formulated, based on the evaluation of the overall reflection coefficient of the absorber. The design criteria for two types namely, matched and resonant absorbers are established. The design criteria for the matched absorber indicates that it is best suited for broad band operation and a predominantly magnetic type of material will have to be employed to realize matched absorber in practice. The design criteria for the resonant absorber implies that, resonant absorbers are narrow band absorbers.

Absorbers one each for matched (broad band operation over Ku-Band) and resonant (narrow band operation in X-Band) criteria with an added constraint on thickness of 2mm were designed and fabricated. Co-BHF and carbon have been used as the lossy materials along with silicone/neoprene rubber as the binder in the design. A very close agreement between the theoretically predicted and exper imentally determined performance for both types of absorbers justifies the proposed mathematical model. Waterman's T-matrix approach has been utilized in a multiple scattering formu lation, to determine the bulk or effective propagation constant for the composite absorbing medium, as a function of particle (scatterer) size and shape, volume concentration of the lossy material, orientation and position of the scatterer in the binder or host matrix. In the multiple scattering model, T-matrix is used to represent the effect of size and shape of individual lossy particles and the random nature of orientation of the particles in the host matrix. An average over all possible position is performed using the two-point joint probability function, which in turn is defined by the radial distribution function obtained under the self consistent approximation. Aclosed form solution of the dispersion equation for a ferrite polymer composite is obtained under the long wavelength approximat ion. The designed and fabricated matched absorber performance is verified using this approach. This model describes more closely the practical behavior of the absorber than the earlier method, thus proving that bulk properties have to be considered, when high loss materials are employed in the design. Employing the proposed model, the optimum configuration for a ferrite-rubber composite, such as size of the spherical particles (scatterer), volume

concentration of the ferrite and thickness of the absorber were determined for broad band operation over X-Band within 3mm thickness limitation. The experimentally determined absorption characteristics for the designed and fabricated absorber of thickness 2.3mm, displays good agreement with the theoretically predicted curve as a broad band absorber, except for the shift in the absorption peak. An impedance matching technique was formulated to overcome the mismatching (at the free space-absorber interface) problem associated with thin planar absorbers. A thin conducting screen perforated with an array of structures has been employed as the Impedance Matching Layer (IMS). Jerusalem cross and double square loop array structures were found to be promising candidates for IMS application from a number of structures that were experimentally examined. A simple equivalent circuit appro ach was formulated for the design analysis of the composite structure formed by incorporating the IMS at the front end of the absorber. Equivalent circuit models available in literature, to analyze the array of Jerusalem cross and double square loops have been utilized in the design of the IML's. The effect of the various par ameters of the array on the overall performance of the absorber have been analyzed and critical parameters identified. Using the proposed equivalent circuit model, with Jerusalem cross as the IMS structure, the Co-BHF rubber absorber designed using the first technique discussed above, which practically provided around 8dB absorption over 12-18 GHz., was impedance matched into (i) operating as a narrow band absorber with a peak absorption of over 30dB; and (ii) operating as broad band absorber with bandwidth over 8 to 18 GHz., with a marked increase in the level of absorption through out the band. The close agreement between the theoretically predicted absorption

characteristics and the experimentally determined values for both the cases consolidates the proposed impedance technique. With double square loop array as the IMS structure, the absorber was impedance matched to operate as broad band absorber to provide 10 GHz (8-18 GHz) bandwidth. An established approach to overcome the mismatching (at the free spaceabsorber interface) problem associated with planar absorbers is to construct an impedance taper employing a multilayered structure. This approach has been extended to the case of thin planar absorbers. Generalized design equations have been arrived at, for a plane wave at arbitrary angle of incidence, as a function of number of layers, concentration of the lossy material in each layer, individual layer thickness, and the overall thickness of the absorber. As a special case of multilayered structure when the number of layers is one, a single layer absorber was designed using the multilayered approach, to operate as a broad band absorber (X-Band) over ±45° incidence angle. A limitation on the overall thickness of the absorber of 3mm was also met. Iron powder has been used in the design as the lossy material along with silicone rubber as the binder. The designed and fabricated absorber of 2.3mm thick, displays a practical absorption characteristics which matches very closely with the theoretically predicted curves. One of the major problems faced in translating the theoretical design into practical absorbers is dispersing the lossy particles uniformly in the host medium. In this context, Secondary Electron and Optical microscopic analysis were done on samples fabricated with different dispersing aids. The type and critical concentration of the dispersing aid, and the method for dispersion, for particular applications have been identified.