Design of Broadband Microwave Absorbers using Carbonyl Iron Filled Silicon Rubber Sheets in the Frequency Range of 1 to 8.2 GHz

Abstract

For microwave absorber applications, carbonyl iron (CI) powder is a commonly used filler material in silicon rubber because of the high value of attenuation coefficients (50.56 dB/cm), high curie temperature, good temperature stabilization and the higher specific saturation magnetization intensity. Silicon rubber is preferred as the host material because of many desirable properties such as excellent weathering resistance, resistance to aging, chemical resistance, insulating properties and compatibility with many kinds of fillers. Microwave absorbers using carbonyl iron filled silicone rubber sheets (CISR) are widely used to attenuate electromagnetic interference, eliminating cavity resonances, isolating components via insertion loss, reducing harmonics and termination signals in waveguides. The fabrication accuracies in thicknesses and concentrations of CISR sheets are ± 0.5 mm and ± 2% of CI by volume, respectively. Multilayer microwave absorbers designed in this thesis have two layers because of inaccuracies in fabrication of CISR sheets. Broadband microwave absorbers are designed by using conventional type and pixelated type Frequency Selective Surface (FSS) layers embedded between two different concentrations of CISR sheets in the frequency range of 3.95 GHz to 8.2 GHz which are light in weight and small in thickness. WR-187 (3.95 to 5.85 GHz) and WR-137 (5.85 to 8.2 GHz) rectangular dielectric waveguide (RDWG) systems are designed for non destructive testing of reflectivity, r and μr of broadband microwave absorbers. RDWG systems consist of vector network analyzer (VNA), coaxial cables, coaxial to rectangular waveguide transitions, sections of metallic waveguides, standard gain horn antennas and sections of dielectric waveguides. For 1 to 8.2 GHz, NRL arch method is used for measuring reflectivity of microwave absorbers in the anechoic chamber at normal incidence and 20° in TE/TM polarizations. This method involves measurements with two double ridge horn antennas, two long coaxial cables, wooden board for holding 1 foot × 1 foot microwave absorbers and VNA. Reflectivity of microwave abasorbers are measured in the far field using time domain gating of VNA. Characterization ( r and μr) of CISR sheets are carried out in coaxial air line system (1 to 4 GHz) and RDWG systems (3.95 to 8.2 GHz) for six CISR sheets (0%, 10%, 20%, 30%, 40% and 50% of CI in CISR sheets by volume). In RDWG systems, flexible CISR sheets are sandwich between two Teflon sheets which are quarter wavelength at mid band. Teflon sheets provide impedance matching and avoid sagging of CISR sheets. Coaxial air line system consists of coaxial air line fixture, sections of coaxial cables and the VNA. Toroidal shaped CISR sheets are prepared and used in the coaxial air line fixture for characterization. The Nicolson- Ross-Weir (NRW) method is used to extract r and μr from measured S11 and S21 parameters of CISR sheets. For the design of microwave absorbers, r and μr value are required for arbitrary volume fraction of CI in CISR sheets. The polynomial approximation method is used to get r and μr at intermediate concentration of CI in CISR sheets by volume. Multilayer microwave absorber in frequency range of 2.5 to 8.2 GHz is designed in this thesis using genetic optimization of concentrations and thicknesses of different layers of CISR sheets. Two layer broadband microwave absorber (7.35 mm of silicone rubber as first layer and 1.4 mm of 50% of CI powder in CISR sheet by volume as second layer) provides reflectivity better than -10 dB in the frequency range of 2.5 to 8.2 GHz. For frequency 1.6 GHz to 2.6 GHz range, single layer microwave absorber of thickness 3 mm is designed by 50% of CI powder in CISR sheet by volume. Broadband microwave absorbers using convential type FSS and pixelated type FSS are designed fabricated and tested by embedding them between two CISR sheets (24% and 33% of CI in CISR sheets by volume). Conventional type FSS layer were designed in CST microwave studio after performing large number of simulations. Pixelated FSS layer is designed with genetic algorithm optimization and geometry refinement method. The reflectivity of both broadband FSS microwave absorbers are tested in RDWG systems and found to be better than -11 dB at normal incidence in the frequency range of 3.95 to 8.2 GHz. The two layer microwave absorber (without FSS layer) reflectivity is around -6 dB. But, by embedding FSS layer between two CISR layers increases its reflectivity to -11 dB which is low cost, light in weight. The broadband microwave absorber using pixelated FSS layer has better design methodology as it does not require human intuition, experience and a large number of simulations required in hit and trail metholodgy of conventional type FSS absorber. The analysis is performed at different incidence angle and TE / TM polarizations which shows reflectivity is generally insensitive to the incidence angles from 0° to 20°.