DESIGN AND ANALYSIS OF A NOVEL LOW LOSS ULTRA-WIDEBAND COPLANAR WAVEGUIDE (CPW) TO COPLANAR STRIPS (CPS) TRANSITION FOR TAPERED SLOT ANTENNAS (TSA) IN GROUND PENETRATING RADAR (GPR) APPLICATION

A novel ultra-wideband CPW to CPS transition for TSA in landmine detection by GPR system is proposed. The structure is constructed on a 140 × 140mm2 FR4 dielectric substrate. It is composed of 2 sections. The first is nonuniform tapered asymmetric coplanar waveguide (TACPW), and the second section is nonuniform Tapered Asymmetric Coplanar Strips (TACPS). Electromagnetic Band Gap (EBG) structure of coplanar circular patches exists near the transition open slot and aligned with the outer edge of the CPW ground to act as a capacitive loading. The design of the proposed transition is given in very simple four design steps. The CPW to CPS transition is analyzed theoretically and experimentally. To characterize this transition, back to back transition is constructed; besides, the equivalent-circuit model that consists of nonuniform transmission lines is established. The equivalent circuit is constructed by dividing both sections TACPW and TACPS into 35 sections and using ABCD parameters to characterize each section, and conversion to S-parameters is done using MATLAB Program. The selection criterion of the section length is to maintain a linear change in the characteristic impedance with the distance. The results based on equivalent-circuit model, CST simulation (CST studio ver. 15), and measurements are compared. Several parameters are studied through simulations and experiments which are used to derive some design guidelines. The operational bandwidth for the CPW to CPS transition covers from 0 (DC) to almost 10 GHz with minimum return loss reaches −50 dB. For the GPR application (landmine detection) which extends from 0.4 to 3 GHz, the insertion loss of the proposed transition reaches almost −0.5 dB which satisfies the design requirements. The back to back transition performance was simulated and measured. Good agreement is found between numerical and experimental results especially for the GPR ranges of frequencies. The proposed transition has the advantages of compact size, ultra-wide bandwidth, and straightforward design procedure.