Circular Electric Wave Propagation in Periodic Structures

Low-pitch helix waveguide closely wound from insulated wire has been shown to be a good transmission medium for circular electric waves. 1 Likewise, spaced-ring or spaced-disk guides have been considered for TEm transmission. 2 All these structures are periodic along the axis of propagation. In analyzing them, however, the period has always been assumed short compared to the wavelength of propagation and the periodic structure then replaced by an anisotropic but homogeneous model.3*6 Recently, measurements have indicated that the TE 0 i loss is low 755 756 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , MARCH 1964 enough at very high frequencies -- perhaps up to 300 gc -- for the helix waveguide to be operated there. 7 At these frequencies the period could no longer be assumed to be short compared to the wavelength. Furthermore, a detailed study of the optimum jacket for most efficient unwanted mode absorption over wide frequency bands has indicated that the helix wire diameter and spacing should be nearly a third of the wavelength at the upper band limit. 8 Both these results point out the need for a more accurate analysis of circular electric wave propagation in periodic structures, taking into account a period comparable to the wavelength of propagation. It is to be expected that the distribution of electric surface currents around the helix wires or rings and disks will depend on frequency. The losses associated with these eddy currents will therefore also depend on frequency.