10-15] 



MISCELLANEOUS MICROWAVE COMPONENTS 



547 



twists are feasible in surprisingly short lengths. A typical 3-face, 90° step 

 twist for X-band is only 1 inch long, yet has a very low VSWR over a 12 

 per cent band. 



The step-twist principle has been applied to an interesting variable twist 

 useful as an oscillating rotary joint having both ports on the axis of rotation. 

 The commercial unit shown in Fig. 10-19 will rotate ±90°, with a VSWR 

 of under 1.15 over the full 40 per cent waveguide band. 



Fig. 10-19 Step-Twist Rotary Joint Permits ±90° Rotation of a Waveguide 

 About Its Own Axis. (Courtesy of Lieco Incorporated) 



The more conventional waveguide rotary joints, capable of continuous 

 rotation, involve a mode conversion from rectangular waveguide to an 

 axially symmetrical mode and back to rectangular waveguide again. Modes 

 commonly used are the TEM coaxial mode, the TMoi circular magnetic 

 mode, and the TEoi circular electric mode (Fig. 10-1 la, f, g). Such rotary 

 joints commonly have both rectangular waveguide ports extending radially, 

 or one radially and the other axially. Although rotary joints vary consider- 

 ably in bandwidth and performance, units having a VSWR under 1.15 over 

 a 12 per cent band are now common. Disturbing resonances, occasionally 

 found to occur in rotary joints, are best detected with a swept-frequency 

 reflectometer while slowly rotating the joint through its full range. Spot 

 frequency checks of VSWR seldom reveal such resonances. 



