A Negative Index Metamaterial to Enhance the Performance of Miniaturized UWB Antenna for Microwave Imaging Applications  
[PERSON]  
1 Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Malaysia; [EMAIL_ADDRESS] (N.M.); [EMAIL_ADDRESS] (M.J.S.); [EMAIL_ADDRESS] (K.M.)  
[PERSON]  
1  
[PERSON]  
1 Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Malaysia; baha  
the control of wave beams, permittivity, permeability, etc. These features make metamaterials (MTM) antennas of great interest for medical applications. The study of metamaterials has been expanded by adopting various techniques such as split-ring-resonators (SRRs) [6], structures based on transmission lines [7], resonators of double-bowknot shape [8], and complementary electric field-coupled resonators [9]. Due to miniaturization, cost effectiveness and the capabilities of label-free detection, metamaterial-based antenna have been focused on in the design of MWI systems [10]. An approach that increases the gain and bandwidth of simple patch antenna by using planar metamaterials on the upper patch and bottom ground of the substrate is presented in [11].  
The number of metamaterials using SRRs, capacitance-loaded strip (CLS), artificial magnetic materials with stacks of SRRs and some other planar structures [12; 13; 14; 15] were proposed in antenna fabrication to minimize size and enhance the impedance bandwidth. By using those structures, it was found that the resonant frequency of the original patch antenna can be remarkably reduced. Therefore, the narrow bandwidth and high loss of metamaterial antennas are still issues of a challenge to achieve higher efficiency and directivity. This drawback becomes especially serious when the SRR and other inclusion types of metamaterials are used as the substrate of the patch antenna. A compact geometry structure with high gain, directive and improved radiation characteristics is needed to implement microwave imaging applications.  
In this paper, a new, compact, low-cost metamaterial antenna is presented for UWB applications leading to microwave imaging. The proposed antenna consists of a novel left-handed, planar, wideband, negative index, metamaterial unit cell array (2 \(\times\) 4) on the radiating patch and the ground plane, which improves the antenna bandwidth and radiation efficiency. The metamaterial unit cells are constructed with a modified-square, split-ring resonator (SRR) resulting in negative permeability and permittivity with a stable negative refractive index. Compared to antenna without left-handed metamaterial (LHM), it is shown that the bandwidth is significantly broadened up to a few megahertz and becomes more convergent leading to the achievement of desired properties for the microwave imaging applications leading to breast imaging. The size of the proposed MTM antenna structure is 26 \(\times\) 22 \(\times\) 1.6 mm\({}^{3}\), fabricated on commercially available flame retardant (FR4) material of 4.6 dielectric constants. The simulation was carried out using a computer simulation technology (CST) microwave studio and measured with an Agilent E8362C vector network analyzer and a Satimo near-field anechoic chamber (UKM StarLab). The antenna achieved 3.1 GHz to 10.71 GHz of impedance bandwidth (\(-\)10 dB), which covers the full UWB band. The double-layer, negative-index, MTM-unit cell array is applied to enhance UWB performance, so that the patch antenna can have excellent performance. The near field directivity factor, radiation efficiency with a breast model and coupling efficiency are computed to proof the antenna suitability for UWB-based MIS application.