Planar helix slow-wave structure with straight-edge connections in the presence of coplanar ground planes

A planar helix slow-wave structure which has straight-edge connections has been studied for traveling-wave tube applications. The effects of coplanar ground planes, on the dielectric surfaces which also contain the top and bottom inclined helix strips, are examined. The phase velocity and interaction impedance are studied for two possible planar helix configurations. The coplanar ground planes reduce the phase velocity near the cut-off frequency and hence provide a relatively flat phase velocity versus frequency curve. A planar helix with coplanar ground planes has been designed and it produces 118% of bandwidth, for ±5% phase velocity variation, with significant interaction impedance. All results are simulated using CST Microwave Studio eigenmode solver.