ON THE COUPLING CHARACTERISTICS OF COPLANAR WAVE-GUIDES AND MICROSTRIP LINES TO MULTILAYER DIELECTRIC MEDIA

In medical diagnostics and geophysical well-logging applications of electromagnetic (EM) techniques, it is of critical importance to couple the EM energy to the object under interrogation efficiently and with minimum external leakage. Coplanar waveguides and microstrip lines have been found experimentally to be very useful in these applications because of the low external leakage. Recently the spectral-domain technique was utilized to analyze the coupling characteristics of the coplanar waveguides and the microstrip lines to highly lossy multilayer dielectric media. It was observed that a superstrate layer may enhance an axial electric-field component and that the coupling to the lossy dielectric media gets stronger with the increase in the dielectric constant of the lossy medium. In this paper, the spectral-domain technique is utilized to optimize the coupling performance of these structures to lossy dielectric media. It is shown that to enhance the coupling, higher frequencies should be used, the dielectric constant of the substrate should be small (epsilon = 2.56), and the width of the center conductor and gaps should be as large as possible. The dielectric constant of the superstrate should be low if a leaky-wave type of coupling is desired or large if a strong coupling by the transverse-field components is desired. Furthermore, the possibility of exciting surface waves in the coplanar structure is investigated. It is shown that the quasi-static TEM and TM0 modes will be excited if a superstrate is present between a high dielectric lossy medium and the coplanar waveguide.