WIDE RANGE MICROWAVE SWEEPING OSCILLATOR 555 



nant frequency is lowest when the two surfaces are nearly in contact, and 

 highest when the cup is fully extracted. 



The recessed end of the cup fits over a protuberance on the center con- 

 ductor when they are nearly in contact. This special shape was designed 

 to give a reasonably straight curve of frequency vs. displacement. With 

 planar surfaces, the frequency would change more rapidly with displace- 

 ment at the low than at the high frequency end of the band. 



The grid disk of the tube is separated from the wall of the cavity by 

 a narrow annular space, and contact is made across the gap by a number 

 of small screws. These screws act as an inductive reactance in series with 

 the circulating currents of the resonant cavity. The voltage developed 

 across this reactance is applied between the grid and the main envelope 

 of the tube, and in this way energy is fed into the grid-cathode space to 

 provide feedback. 



The mechanical tuning device was adapted from an inexpensive per- 

 manent magnet loudspeaker of the type used in small home radios. The 

 construction is shown in Figs. 1 and 5. The speaker cone was removed 

 and the voice coil was attached to a thin-walled paper cylinder which 

 supports the tuning cup inside the cavity. Two sheet fiber springs support 

 the paper cylinder and maintain the axial alignment in the magnet and 

 cavity. These springs are cut with a number of incomplete circular slits to 

 reduce the stiffness for axial motion. With the voice coil actuated from a 

 small filament transformer, peak to peak motion | of inch is obtainable. 



The heater and cathode connections are made at the base of the 

 tube which protrudes from the cavity. The grid is internally connected 

 to the main body of the cavity. The anode lead is brought out through a 

 quarter-wave choke and mica button condenser. 



To prevent overheating of the anode of the tube, air must be blown 

 through the cavity. This is done by connecting a low pressure air hose to 

 the air inlet shown in Fig. 1. Excessive air flow must be avoided, as it will 

 cause erratic vibrations of the tuning plunger. 



3. Adjustment and Operation 



The degree of feedback is adjustable by changing the number and rela- 

 tive positions of the feedback screws which connect the cavity to the 

 grid ring of the tube. There are 16 possible screw positions, but only 

 about 5 or 6 are needed to obtain optimum feedback. Reducing the num- 

 ber of screws increases the amount of feedback. 



Care should be taken that the spring which contacts the anode for dc 

 connection is not of such a length to have resonances within the band. 

 When such resonances exist, "holes" or other irregularities will be found 

 in the output spectrum. This spring can act as a helical line, and when it 



