HIGH Q RESONANT CAVITIES 431 



levelling is a practical solution. Tilt adjustments in the order of O.C)01 inch 

 at the edge of the plate (about 3-inh diameter) are recjuired in the 9 kmc 

 band. Experience in the 25 kmc band shows evidence of the need for even 

 liner control. 



Plating 



In addition, adequate control of the conductivity of the interior surfaces 

 of the cavity is necessary to achieve a uniform manufactured product. This 

 requires attention not only to the thickness and uniformity of the plating 

 but also to the purity of the plating baths and the avoidance of introduc- 

 tion of foreign matter during buffing processes. 



Cotiplings 



The t}pe and location of the coupling means can be used to discriminate 

 between wanted and unwanted modes. Hence, this is a fertile field for mode 

 suppression techniques. For example, since TM modes have Hz = 0, 

 orifice coupUng to the main mode at the side wall of the types shown in 

 Table IV, cases lA and 2A, will not couple to any TM modes. Again, if end 

 coupling is used in a cavity which will support both the TE 01 and TE 02 

 modes, by locating this coupling at the point where Hp = for the TE 02 

 mode (about 54% of the way out from the center), it will not be excited and 

 coupling to the TE 01 will be only slightly below maximum. 



For echo box test sets the magnitude of the input coupling to the wanted 

 mode is a compromise between the incomplete buildup of the fields within 

 the cavity during the charging interval and the loading of the cavity Q on 

 discharge. This is carried on by varying the coupling and observing the 

 "ringtime" (the echo box indication on the radar scope). Optimum coupling 

 is achieved when ringtime is made a maximum. 



Output couplings for echo boxes are made so that just enough energy is 

 withdrawn from the cavity to give an adequate meter reading. 



Drive Mechanism 



The objective of the design of the tuning mechanism is to provide a 

 smooth, fine control with a minimum of backlash. An illustration of the 

 mechanical perfection required can be cited in a 9 kmc band design where 

 J inch of tra^'el covered 200 megacycles in frequency. Hence, for frequency 

 settings to be reproducible to within j mc the mechanical backlash of the 

 moving parts had to be held to about 0.0003 inches or 0.3 mil. To realize 

 this in commercial manufacture and to maintain it after adverse operating 

 conditions such as vibration and shock was a major mechanical design 

 problem. 



In the design of this drive mechanism it should be recognized that equal 



