Propagation modes and dispersion characteristics of coplanar waveguides

The coplanar waveguide (CPW) structure is subdivided into five classes based on substrate thickness, backside metallization, and ground plane width. Radiation and guided modes are studied in each class, and their effects on loss and dispersion are described. For coplanar waveguides, dispersion due to interaction with surface wave parallel-plate modes in the substrate strongly depends on the ground-to-ground spacing (W+2G). If this spacing is small compared to dielectric wavelength and substrate thickness, both dispersion and radiation losses are minimized. In finite ground plane cases, small ground-to-ground spacing compared to substrate thickness is necessary to avoid the excitation of the microstrip mode and to minimize the deviation of the CPW mode from the odd mode of the ideal line. Limitations on the reduction of (W+2G) come from conductor loss, which imposes a minimum width requirement on the center conductor. If substrate thickness is to be kept large compared with this dimension, it is inevitable that, at high frequencies, the operation of the CPW will not be below the cutoff frequency of surface wave modes. However, the interaction of the CPW mode with surface wave modes is negligible for integrated circuit applications when ground-to-ground spacing is small compared with dielectric wavelength.<
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