534 BELL SYSTEM TECHNICAL JOURNAL 



and the sealed-in plate lead of the tube. The plate impedance of the tube 

 is transformed by the resonant cavity to a very low resistance (a frac- 

 tion of an ohm) on the plate lead just outside the glass seal. The quarter- 

 wave coa.xial transformer serves to match this low impedance to the surge 

 impedance of the coaxial line {AS ohms). Coarse tuning is accomplished 

 by moving the slug of the outer conductor; fine tuning by moving the 

 inner conductor. The coaxial line is supported at its end by a dielectric 

 washer. Plate voltage is applied to the tube through a high impedance 

 quarter-wave wire brought out to the low impedance probe through the 

 side wall of the waveguide. Both the modulator and the amplifier used 

 this type of circuit, which we call model eleven. 



Fine tuning of the plate cavity is obtained by sliding the inner con- 

 ductor of the coaxial transformer up and down on the plate lead. This 

 movement is derived through a low-loss plastic screwdriver inserted 

 through the hollow probe transducer; the driving mechanism is housed 

 inside the inner conductor of the transformer, thus isolating the mechani- 

 cal design problem from the electrical design problem effectively. The 

 hollow stud at the top of the structure serves two purposes: screwing it 

 into the waveguide introduces a variable capacitive discontinuity which 

 serves to improve the match between the cavity and the waveguide. The 

 length of the hollow plug provides a length of waveguide beyond cutoff 

 which keeps the r.f. energy from leaking out through the plastic tuning 

 screwdriver. 



The heater and cathode leads from the tube are housed in a cylindrical 

 metal can and are brought out through by-pass condensers to a standard 

 connector. The photograph, Fig. 3, illustrates these features. 



The input face of the circuit is illustrated in Fig. 4. The long narrow 

 slot near the base of the rectangular block is the iris opening which 

 couples the input waveguide to the cathode-grid cavity. The single tuning 

 screw provided at the input iris is not adequate to match all of the tubes 

 over the whole frequency band of 500 megacycles; an auxiliary tuner 

 shown at the right of the circuit provides the necessary flexibility. This 

 tuner, described by Mr. C. F. Edwards of the Bell Telephone Labora- 

 tories^ is, in effect, two variable shunt tuned circuits about an eighth of a 

 wavelength ai)art in the waveguide. Each variable tuned circuit is made up 

 of a fixed inductive post (located off center in the waveguide) and a 

 variable capacitive screw. It is capable of tuning out a mismatch corre- 

 sponding to four db standing wave ratio of any phase. 



As shown in Fig. 5, the tube slides into the bottom of the circuit and 

 the grid flange is soldered lo the wall of the cavity with low melting point 



