volume 26 number 11 12 page 350 1965 who starts with a tapered rod in a rectangular waveguide section 16a makes a transition to a circular waveguide section 16b and then to a conical horn section 16c it has been our experience that the vswr in the waveguide can be held below 1 1 throughout the guide bandwidth the transition to circular guide te 11 mode from the rectangular te 01 mode aids in orienting the maximum electric field lines in coincidence with those of the dielectric he 11 mode shown in fig5 the conical horn will be extended with a 20 u00b0 flare angle until its aperature is 25 centimeters diameter to provide a smooth transition and eliminate back radiation the value 25 centimeters was determined by the 99 energy aperature criterion for maximum efficiency of mode excitation yip launching efficiency of the he 11 surface mode on a dielectric rod waveguide ieee transactions mit 18 number 12 page 1033 december 1970 choose 2a u03bb 2 0 5 requiring a rod diameter of 3 7 centimeters an abrupt transition from this diameter to 2 5 centimeters at the radiation end would cause undesired radiation a gradual taper will reduce this undesired radiation since the fields are more loosely coupled at small diameters however it is reasonable to expect that a linear taper in a region of small diameter.