wound coils should be satisfied with the practice in this transmitter, for the two types are found in equal proportions. The plate tank condenser is mounted immediately above the inductance and is in two sections one on each side.

Each section consists of a series of slightly dished aluminum discs. The discs are about 30 inches in diameter and are each equipped with three projecting lugs spaced 120 degrees apart around the circumference for mounting. The plates are fixed on pipe supports which in turn are fastened to the frame of the transmitter by insulating pillars. The supports are spaced 60 degrees apart and alternate plates are mounted on and connected to each set of three pipes. The two sections of the condenser are in series across the plate inductance. Each pair of plates has a capacitance of approximately 75 fd, the total capacity of the condenser being 750 fd. The tank capacitance is varied by the addition or removal of plates and adjustment of the spacing between them. This type of air-dielectric condenser construction has practical application in amateur transmitters; the plates might be aluminum "pie-plates" or discs cut from sheet aluminum and the supports could be threaded rods. Sufficient capacity variation for tuning purposes could be obtained by mounting one end plate on a screw rotated by a knob or small crank. Fine tuning adjustment of the plate circuit is by means of an aluminum disc "flipper" inside the inductance, operated from the panel.

___The neutralizing condensers for this stage are most unique. An aluminum disc supported by a large threaded screw and similar to those used in the tank condenser but without mounting lugs, is capacitively coupled to the front end plate of each section of the tank condenser. The spacing between these plates and the end plates of the tank condenser is variable, the rotation of the screws moving the plates toward or away from the stationary plates. The movable plates are connected to the respective grids of the UV-862 tubes. The neutralizing circuit is identical with that of the 5-kw. stage.

___As in the preceding stage, excitation is controlled by input cooling and adjustment of the grid-shunt resistor. Since the output of the preceding stage is 5 kw. and but 2 kw. are required for proper excitation, it is obvious that the grid-shunt resistor is called upon to dissipate a considerable amount of power. While the dissipation of 3 kw. in resistor may seem to be a wanton waste of good power, it is essential to linear amplification. Good grid-regulation is absolutely necessary and it can be obtained only by making the input resistance of the grid circuit comparatively low and supplying sufficient power, in excess of that dissipated in the resistor, to insure "grid saturation".

THE ANTENNA SYSTEM

___Although the construction of a directive antenna system has been considered, that in use at present is of the "T" type, operated against ground and fed by two-wire transmission line. The schematic plan of the antenna and feed-line is shown in Fig. 3. The input to the transmission

line is inductively coupled to the plate coil of the 50-kw. amplifier. The output terminal equipment is such that the surge impedance of the line is matched to the antenna resistance. The antenna resistance is 65 ohms and the natural frequency is 1140 kc. (380 meters). The value of the radio frequency current at the base of the antenna is 27.8 amperes at normal carrier power output.
___The ground system consists of 200 feet lengths of heavy bare copper wire buried in shallow trenches radiating from a point immediately below the center of the antenna. All wires are connected to a large copper sheet beneath the tuning house. The line terminal equipment and antenna ammeter are in the tuning house.
___The possibility of even harmonic radiation is eliminated by use of the antenna-and-ground combination and also by making the line terminal circuit high-C with ground at the nodal point.

PLATE, FILAMENT AND GRID BIAS SUPPLY

___Plate supply for the tubes preceding the UV-863 stage is obtained from rectifier units using the UX-866 type rectifier tubes and that for the UV-863

 
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