EXPERIMENTAL AND ANALYTICAL APPROACHES FOR THE DEVELOPMENT OF MICROWAVE ABSORBERS

Abstract

The rapid incorporation of wireless technology in electronic devices tends to create the interfering electromagnetic (EM) waves in the environment. These interfering EM waves interact with the human head, especially during usage of a cell phone, Wi-Fi and other communication devices. As a result, the human head absorbs the EM waves that leads to create thermal and non-thermal effects in tissues. These effects might build defect in tissues and hence, create adverse health issues. Hence in recent years, various research efforts have been put for the development of microwave absorbers that can reduce the EM wave exposure to the human head. In recent years, microwave absorbing materials (MAMs), such as polymer, dielectric, and magnetic materials along with its composites have been widely used to absorb the EM wave due to its tunable electrical (i.e., complex permittivity) and magnetic properties (i.e., complex permeability). Moreover, permittivity (#), permeability (m), along with the physical dimension of absorber can control the impedance value of MAM’s layer. Higher the impedance mismatch between the free space and MAM’s layer represents the minimum absorption of the incident EM wave. So, in order to tune the # and m of MAMs, researchers came up with the idea of fabricating composites. Till now, these composites have been fabricated using trial and error method which is time-consuming and might be overlooked for the best possible combination. Thus, it is desired to develop some analytical approach which could provide optimized parameter of composite to obtain microwave absorption properties defined at the user end. Further, the demand of thin and broadband absorber is increasing day by day as an electronic device can serve multiple applications which operate at different frequency bands. For this purpose, researchers are using various multilayer techniques and frequency selective surfaces (FSSs) which is generally used for the reduction of thickness and the enhancement of -10 dB bandwidth (BW) of the absorbers, respectively. In multilayer technique, MAM’s layers are stacked with multiple layers which comprise many interfaces. These interfaces create multiple reflections which provide enhanced microwave absorption. Hence in multilayer absorber, overall coating thickness (t) reduces as compared to single layer absorber. FSS is a conducting xv Abstract surface which transmits or reflects the incident EM wave. The FSS is represented by an equivalent circuit which comprises series/parallel of resistance (R), inductance (L), and capacitance (C). The value of R, L and C components define the equivalent impedance of FSS. Hence, embedding of FSS over MAM absorber contributes to the impedance match for broad range of frequency. As a result, the BW of absorber can be improved. Thus, an efficient absorber can be developed by utilizing the aforementioned techniques. Further, for the quality assessment of the developed absorber, the comparison of EM wave absorption using developed absorber and the conventional absorbers can be performed in the human head. The problem of EM wave absorption in the human head is taken under consideration because the human head gets the highest exposure of EM wave. Human head tissue is a lossy dielectric material, thus absorbs the EM wave. To compute the effective EM wave absorption, several human head models based on multilayer and MRI based FDTD/FEM techniques have been developed. The human head is a complex structure made up of multiple tissues with intermixing of many layers; thus, the accurate measurement of permittivity (#0) and conductivity (s) of the tissues for the human head is still a challenge. Therefore, we need to explore an algorithm which could provide the effective electrical and thermal properties of the human head model through which the value of effective E-field can be computed. Ferrite sheet, metamaterials, split ring resonator (SRR), and rod reflector based absorbers have been reported to analyze the EM wave reduction in human head. This analysis has been carried out by keeping dipole, monopole, and helical antennas besides the human head. The overall modelling of the antenna exposure and human head has been carried out using FDTD, FEM and other simulation software. Till now, EM wave absorption reduction by the conventional absorbers has been reported in single frequency band. However, the electronic device like cell phone support different frequency bands, such as GSM, UMTS, and PCS etc. Thus, reported absorbers cover single application of the cell phone. Therefore, the development of an absorber which could attenuate the EM wave for broad range of frequency is required. Moreover, for quality assessment, the testing of developed absorber in real time exposure (e.g., testing the exposure of cell phone in its operating condition) is needed to be carried out. Thesis is divided into seven chapters which are briefly described as follows. Chapter 1 presents the introduction of the work carried out in the thesis and provides a brief idea about the aim and scope of the work with the proposed formulation of the problems. It gives an introduction of EM wave propagation through different kind of media; reflection, and transmission from single layer and multilayer structure at normal incidence; microwave absorbers and their absorption mechanism. Moreover, absorption enhancement technique, such as multilayer techniques and FSSs which improve the absorption properties have also been discussed in this chapter. xvi Abstract Moreover, various characterization, measurement, and fabrication techniques have also been discussed, which is used in the proposed task. Chapter 2 includes a brief literature review of different types of microwave absorbers, such as MAM and FSS, Embedding of FSS over MAM, optimization techniques, mixing model technique in order to identify the current state of art for the development of absorber which works in microwave frequency range. EM wave interaction with human head model for computation of effective EM wave absorption is reviewed. Further, available solutions to reduce the EM wave radiation have also been reviewed. Chapter 3 discusses the critical analysis of FSS embedded MAM based absorber to enhance the -10 dB reflection loss bandwidth (RLBW10). For this purpose, different MAMs have been chosen from a database of MISTAL lab. In literature, hard-, softferrites and their composites have majorly been used as MAMs because of their good absorption properties. The electrical and magnetic properties of these MAMs have been measured using waveguide transmission/reflection method. It is observed that, dielectric constants of these ferrites based MAMs are mostly obtained in the range of 1-20. Thus, the ferrite based MAMs have been categorized on the basis of their dielectric value, such as class I (dielectric is varied from 1-5), class II (dielectric is varied from 5.1-10), class III (dielectric is varied from 10.1-15) and class IV (dielectric is varied from 15.1-20). The RLBW10 of these classified MAMs is generally obtained at greater than 2 mm coating thickness. Absorbers of t 2 mm are generally quite thick and not suitable for commercial purposes. To enhance the RLBW10 at t 2mm, FSSs is embedded on MAMs. Conventionally, the common FSS structures, such as square, ring, and cross were often implemented. Later on, these canonical structures became less prevalent because of their limitation towards unit cell size. However, embedding of fractal frequency selective surfaces (FFSSs) on MAMs is another approach. Embedding of FFSSs on MAMs improves RLBW10 responses with miniaturized unit cell size. Hence, to incorporate these advantages, we have critically analyzed FFSS embedded over MAM to observe the effect on thickness and RLBW10. The commonly used FFSSs are Sierpinski carpet, Sierpinski gasket, T-loop and Minkowski loop. The simulated result is optimal FFSSs dimensions to achieve maximum RLBW10 and minimum t for all the classes of considered MAMs. The result of this work is a compendium for selecting the appropriate FFSSs structure for MAM of specific permittivity value. In these results, the maximum RLBW10 is increased from 0 to 4.01 GHz in class I absorber (dielectric of 4.9) with T-loop FFSS under a normally incident EM wave. Moreover, the maximum RLBW10 of 4.2 GHz (i.e., 8.2-12.4 GHz) in class II absorber having dielectric of 8.6 with Minkowski loop FFSS. All the proposed absorbers (i.e., FFSS embedded MAMs) exhibit ultra-thin layer (i.e., t is optimized to the range from 0.11l to 0.5l mm) and low profile (i.e., unit cell dixvii Abstract mensions of FFSSs optimized to the range (l) =0.1ll to 0.55l mm, and gap (g) =0.27l to 0.41l mm) geometrical properties. The value of RLBW10 of Minkowski loop FFSS impacted MAM is found to be 4.2 GHz (4.2 to 12.4 GHz) which is better as compared to other FFSS geometries. Therefore, only Minkowski loop FFSS impacted MAM has been fabricated and measured for the validation. The obtained measured value of RLBW10 is in good agreement with the simulated results. Therefore, applying FFSSs over MAM coatings has proved to be an efficient technique and resulted in the eventual formation of an effectively performing MA. Chapter 4 focuses on the development of the analytical approach for MAM based absorber to achieve the user-defined absorption properties, i.e., reflection loss (RL) and RLBW10 at a defined operating frequency. To develop this approach, absorption dependency of MAM’s layer over its #, m, and thickness has been explained using advance EM theory. The complex permittivity and permeability of a mixture depend on its constituents (i.e., host and inclusion). Mixing model can compute the effective # and m of the composite which relies on the shape, particle size, and composition of each constituent. Besides that, mixing model theory explains the dependency of physical parameters such as shape, particle size, and proportion of each MAM over absorption properties. Therefore, in this chapter, a mathematical relation is proposed. This relation computes the RL for known morphology, #, m, proportion of constituents, and thickness of MAM’s layer in the defined range of frequency. The proposed relation has been incorporated with genetic algorithm (GA) to optimize the proportion of constituents (Vf ), selection of material (mi), and coating thickness of MAM’s layer (t) to compute the user-defined RL and RLBW10. The developed model has also been tested on the nano-structured NiFe2O4 (NF) and TiO2 (T) composite. The absorption properties of their composites have been analyzed in both C and X bands (i.e., 0.5 to 12.4 GHz). Maxwell-Garnett (MG) mixing model is successfully implemented to analyze the value of #e f f and me f f while assuming particle shape of inclusion as a spherical using heterogeneous mixture of NF and T. Moreover, multi-objective genetic algorithm (GA) is applied for optimization of mi, Vf and t at the user-defined RL ( -10 or -20 dB) and RLBW10. Proposed methodology has been implemented for different frequency regime (6.0125 GHz, and 10.0375 GHz) and corresponding values of mi, Vf and t are optimized for user-defined value of RL and RLBW10. A fairly good match between the simulated and the fabricated composite result justifies the validity of the proposed methodology. Hence, the implementation of the proposed method is a fruitful step towards the prior computation of the mi, Vf and t of the composite at the user-defined RL and RLBW10, without undergoing trial and error experimental methods. For the additional validation purpose, the proposed methodology has also been implemented on other heterogeneous composite systems like nickel ferrite (NF) - nickel zinc ferrite (NZF) and cobalt. xviii Abstract Chapter 5 presents algorithm to compute effective dielectric (#e f f ) and thermal (heat capacity (ce f f ) and mass density (se f f )) properties of human head in order to assess the EM wave absorption in the frequency range of 0.3 to 3.0 GHz. For this purpose, 14 major tissues of human head have been taken under consideration, which are dura, bone (cortex), bone (cortical), lens (cortex), lens (cortical), retina, aq. humor, cornea, gray matter, white matter, cerebellum, brain stem, cartilage and tongue. In literature, #e f f of a heterogeneous mixture has been implemented by the various mixing models. The #e f f (#0-j#00) of any mixture depends on the volume fraction ratio (Vf ), shape (morphology), size, and dielectric values (#) of its constituents. Vf is the volume percentage of a tissue in the entire group of tissues. To execute #e f f and ce f f for the human head model, the volume of each tissue has been estimated from the well-established techniques, such as magnetic resonance imaging (MRI), X-ray computed tomography (CT), stereology, and microtomy. These techniques give the volume of tissue which varies corresponding to the age and gender of the human. In this analysis, we have considered the adult male human head. Further, the dielectric properties of each tissue are taken from the Cole Cole model. Using De-Loor mixing model, the #e f f of human head model has been computed in the frequency range from 0.3 to 3.0 GHz. Because the other reported mixing models had a drawback that it neglects the influence of the host surrounding to the inclusion. Similarly, to define the thermal effect in the tissue, thermal properties of human head have also been computed using the dielectric mixing method. Later, the effective dielectric and thermal properties of the equivalent human head model is compared with the IEEE Std. 1528. The obtained frequency dependent effective Permittivity (#0) of the equivalent human head model is underestimated up to 1.0 GHz after that value is overestimated up to 3.0 GHz in comparison with IEEE Std. 1528. On the other hand, conductivity (s) values are underestimated as compared to IEEE Std. 1528 in the entire frequency range of 0.3 to 3.0 GHz. Similarly, the effective density of the human head is overestimated and effective heat capacity has been underestimated as compared to IEEE Std. Aforesaid approach has further been implemented to assess the EM wave reduction from any developed absorber. Chapter 6 explores an efficient approach to design the EM wave reduction box (EMRB) for frequency range from 0.5 to 3.0 GHz. For this purpose, different kinds of commercially used cell phone has been chosen. These cellphones use GSM (890- 960 MHz), DCS (1710-1880 MHz), PCS (1850-1990 MHz), UMTS (1920-2170 MHz), and Wi-Fi (2400-2480 MHz) communication bands. Thus, the EM wave exposure of cellphone is generally found in frequency range of 0.5 to 3 GHz which covers most of the applications like calling, messaging, and Internet usage etc., EM radiations of cell phone is measured using the EMSCAN RFxpert at both transmitting and receiving modes. EMSCAN RFxpert is a patented scanner which measures the radiation xix Abstract in terms of magnetic field strength (H-field). To reduce the EM wave exposure from the cell phone while maintaining its signal strength, a broadband frequency selective surface (FSS) based EM field reduction box (EMRB) is designed. The performance of the EMRB has been evaluated in terms of transmission coefficient (S21 in dB) through simulation using high frequency structure simulator (HFSS) software. The EM wave exposure of cell phone in terms of H-field was measured with and without EMRB using EMSCAN’s RFxpert scanner. It was observed that, in presence of EMRB, a significant amount of reduction in the EM wave exposure of cellphone is attained. Thus, proposed novel approach can measure EM wave exposure of commercially used cell phone in the presence as well as absence of EMRB. The proposed methodology may be further modified as per requirement of the user. These results might be helpful in designing the safety compliance for cell phones and other wireless devices. Chapter 7 presents the concluding remarks which highlights the significance of the work towards improvement of MAM’s absorption properties by embedding the FFSS, and optimizing the critical parameters of MAM’s composite. Moreover, work like “development of algorithm to compute electrical and thermal parameter of human head”, and “development of an approach to design the EMRB” are carried out to reduce the EM wave exposure in communication band. In the future, more experimental and analytical study can be carried out to assess the validity of the product for the welfare of society.