Circular Waveguide Taper of Improved Design

900 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1 9 5 8 of normal modes. The normal modes of the circular electric wave family -- the only family of interest here -- are, in the cylindrical guide, cylindrical circular waves with plane equiphase surfaces and, in the conical guide, spherical circular waves with spherical equiphase surfaces. At the junction of a cylindrical guide to a conical guide -- such as occurs twice in a conical transition -- a cylindrical wave from the cylindrical guide excites a series of spherical waves in the conical guide. For instance, an incident TE i wave will excite all the T E waves, thus causing a rather high spurious mode level. To avoid this mode conversion, a transition which transforms the cylindrical wave into the spherical wave must be introduced at the junction. S. P. Morgan has suggested and worked out the design of dielectric inserts placed near the junction which, acting as quasi-optical lenses, transform the cylindrical waves into the spherical waves. However, because of the dispersive character of the lenses and of the waveguide, good broadband performance is difficult to achieve. Another way of making a transition from cylindrical waves to spherical waves -- at least approximately -- is to taper the cone angle from zero at the cylindrical guide to the finite value of the conical guide. If this is done gradually enough, nearly all the power incident in the cylindrical wave will be transformed into the spherical wave, with a very low spurious mode level.