Zubair Akhter

Time domain microwave technique for dielectric imaging of multi-layered media

In this paper, a novel time domain practical scheme is proposed to reconstruct the dielectric image of the inhomogeneous multi-layered media. The overall process is noninvasive and noncontacting, which employs the measured scattering data of the test media in equivalent time domain. The measurement system consists of a modern vector network analyzer, a wide-band lens horn antenna, and the associated microwave components. The main focus here is to determine both the relative permittivity and the thickness of each layer in terms of the partial reflection coefficient calculated for the corresponding interface. It is also demonstrated here that the proposed approach can be used to characterize the lossy media under some approximation. The proposed approach is quite useful under situations such as the through-wall imaging, where only reflection measurements can be carried out due to restricted access of the object from the other side. The proposed scheme is validated here by determining the permittivity, the thickness, and the effective conductivity of a number of multi-layered media obtained by stacking few reference dielectric samples whose data are available in literature.

Free-Space Time Domain Position Insensitive Technique for Simultaneous Measurement of Complex Permittivity and Thickness of Lossy Dielectric Samples

A novel time domain measurement technique is proposed to facilitate the simultaneous measurement of electrical properties (complex relative permittivity) and geometrical parameters (thickness) of the material under test (MUT). The overall process is noninvasive and noncontacting, which uses the measured scattering data of the MUT in the equivalent time domain or spatial domain. The effective time domain scattering data are employed to detect the primary and secondary peaks of the overall reflection and transmission coefficients. To this end, a novel algorithm is proposed to obtain the complex permittivity and thickness of the MUT in terms of extracted reflection and transmission power peaks. From the practical point of view, the main advantage of the proposed scheme is that one avoids the complicated calibration procedure normally required to define the reference plane. For increasing the accuracy of the overall reconstruction process, an automated optimization procedure based on parameter sensitivity analysis is proposed, which uses standard time gating procedure to implement the corresponding direct problem. The proposed technique is validated by extracting the relative permittivity, the dielectric loss (effective conductivity), and the thickness of various standard materials, such as polyethylene, Plexiglas, PVC, mortar, nylon, and so on, and comparing the extracted data with their values available in the literature.

Hemisphere lens-loaded Vivaldi antenna for time domain microwave imaging of concealed objects

Abstract The hemisphere lens-loaded Vivaldi antenna for the microwave imaging applications is designed and tested in this paper. The proposed antenna is designed to work in the wide frequency band of 1–14 GHz, and is fabricated on the FR-4 substrate. The directivity of the proposed Vivaldi antenna is enhanced using a hemispherical shape dielectric lens, which is fixed on the end-fire direction of the antenna. The proposed antenna is well suited for the microwave imaging applications because of the wide frequency range and high directivity. The design of the antenna is carried out using the CST microwave studio, and various parameters such as the return loss, the radiation pattern, the directivity, and input impedance are optimized. The maximum improvement of 4.19 dB in the directivity is observed with the designed hemisphere lens. The antenna design is validated by fabricating and testing it in an anechoic environment. Finally, the designed antenna is utilized to establish a setup for measuring the scattering coefficients of various objects and structures in the frequency band of 1–14 GHz. The two-dimensional (2D) microwave images of these objects are successfully obtained in terms of the measured wide band scattering data using a novel time domain inverse scattering approach, which shows the applicability of the proposed antenna.

Directivity enhancement of double slot Vivaldi antenna using anisotropic zero-index metamaterials

In this paper, a compact design of the ultra-wideband (UWB) highly directive double slot Vivaldi antenna (DSVA) is presented for microwave application in the frequency range of 2-10 GHz. In order to improve the directivity of the proposed DSVA, the zero index metamaterial (ZIM) unit cells are loaded on the E-plane of the antenna. It is found that the directivity of the ZIM loaded DSVA is increased by 5.5 dB as compared with the reference tapered slot Vivaldi antenna. The proposed antenna is fabricated on the FR-4 substrate and the return loss is measured using the vector network analyzer in the non-anechoic environment. Preliminary results show that the directivity of the proposed antenna is significantly increased in the frequency range of 5 to 9 GHz.

Design of anisotropic zero-index metamaterial loaded tapered slot vivaldi antenna for microwave imaging

In this paper, a compact design of conventional tapered slot antenna (TSA) loaded with the anisotropic zero-index metamaterial (ZIM) for the frequency range of 1 to 10 GHz is presented. The proposed TSA loaded with the ZIM exhibits an increase in the directivity up to 2.6 dB. The designed antenna is fabricated on the FR-4 substrate and tested. A good agreement between the simulated and the measured results is obtained.

A novel half space time-domain measurement technique for one-dimensional microwave imaging

In this paper, the moisture content for cement based materials having different water to cement (w/c) ratios is measured using the microwave free space technique. The overall process is non-destructive and employs the time-domain measurement data. The measurement system consists of a vector network analyzer, a wide-band lens horn antenna, cement based material of having different water to cement (w/c) ratio and associated microwave components. The main focus here is to determine the permittivity, the effective conductivity and the thickness of test specimens under the situation, where only reflection measurements can be carried out due to the restricted access of the object from the other side. It is observed that the effective conductivity is directly proportional with the moisture content or the w/c ratios of these test samples.

Generalized RF Time-Domain Imaging Technique for Moving Objects on Conveyor Belts in Real Time

In this paper, an attractive time-domain RF imaging technique is proposed for online monitoring of moving objects over conveyor belts. The proposed technique combines the strength of the time-domain approach and the Riccati equation-based ultrawideband reconstruction method in order to image the stratified lossy dielectric medium. The applicability of the proposed method is first tested under stationary condition by reconstructing the permittivity and conductivity image of the target area comprising of various standard samples placed in free space. For moving objects under real-world scenario, two antenna arrays comprised of ultra-wideband Vivaldi elements are designed and tested. These arrays are then employed to produce the 2-D microwave image of various wooden-based samples and human mannequins carrying metal object concealed behind the cloth. The obtained real-time microwave image of the test medium shows that the proposed RF imaging technique is best suited for online monitoring of stationary as well as moving targets, where the structural as well as the electrical properties of the test medium can be obtained through a nondestructive process without using any iterative scheme.

Qualitative analysis of moisture content in cement based material using microwave non-destructive testing

In this paper, the moisture content for cement based structures having different water to cement (w/c) ratio is carried out using the microwave free space technique. The measurement system consists of a vector network analyzer, a wide-band lens horn antenna and associated microwave components. The main focus here is to determine the loss factor or the conductivity of cement based samples, which can directly be related with the moisture content in the structure.

Simplified two‐dimensional microwave imaging scheme using metamaterial‐loaded Vivaldi antenna

In this paper, a highly efficient, low-cost scheme for two-dimensional microwave imaging is proposed. To this end, the AZIM (anisotropic zero index metamaterial) cell-loaded Vivaldi antenna is designed and tested as effective electromagnetic radiation beam source required in the microwave imaging scheme. The designed antenna is first individually tested in the anechoic chamber, and its directivity along with the radiation pattern is obtained. The measurement setup for the imaging here involves a vector network analyzer, the AZIM cell-loaded ultra-wideband Vivaldi antenna, and other associated microwave components. The potential of the designed antenna for the microwave imaging is tested by first obtaining the two-dimensional reflectivity images of metallic samples of different shapes placed in front of the antenna, using the proposed scheme. In the next step, these sets of samples are hidden behind wooden blocks of different thicknesses and the reflectivity image of the test media is reconstructed by using the proposed scheme. Finally, the reflectivity images of various dielectric samples (Teflon, Plexiglas, permanent magnet moving coil) along with the copper sheet placed on a piece of cardboard are reconstructed by using the proposed setup. The images obtained for each case are plotted and compared with the actual objects, and a close match is observed which shows the applicability of the proposed scheme for through-wall imaging and the detection of concealed objects.

Microwave imaging of lossy dielectric stratified media using quasi-numerical optimization technique

A quasi-numerical optimization technique is presented to obtain the depth-dependent microwave dielectric image of the lossy inhomogeneous media. The proposed optimization scheme involves inverse solution of the corresponding microwave imaging problem in an efficient way. The initial condition for the optimization routine is obtained using an analytical approach, which leads to faster convergence and stable solution. It is shown that the permittivity as well as the conductivity profile of the media under test can be estimated using the proposed optimization technique.