RADIO BROADCASTING TRANSMITTERS 



133 



made to compensate for minor variations in antenna conditions with- 

 out leaving the operating room. 



Another feature of interest is the artificial antenna shown in Fig. 7. 

 This unit is essentially a 600-ohm non-inductive resistance capable of 

 dissipating approximately 75 kw. which can be connected to the 

 output circuit of the final power amplifier stage in place of the trans- 

 mission line. The heat dissipating elements consist of a series of 

 woven wire grids mounted in the units at the top of the framework. 

 The resistance of these grids is substantially independent of frequency, 

 but the combination presents a slight inductive reactance which is 

 compensated for by means of the condenser and coil combination 

 shown. These elements are inserted into the circuit symmetrically 



Fig. 8 — Piezo-electric crystal mounting and temperature control apparatus. 



in order to maintain an approximate balance to ground. The struc- 

 ture is completely shielded and is fitted with safety door and ground- 

 ing switches similar to those already described. 



The piezo-electric crystal mounting and temperature-control appa- 

 ratus which is a part of the oscillator-modulator unit is shown in 

 Fig. 8, dismantled to facilitate inspection. The quartz plates em- 

 ployed are approximately one and a quarter inches square and are 

 cut parallel to one of the faces of the natural rock crystal. This plate 

 is mounted between two lapped metal plates and covered with a 

 porcelain cap carrying a terminal to which the upper electrode is 

 connected by means of a short section of metal foil. The mounted 

 crystal is supported by a brass block, through the center of which 

 extends a spiral bimetallic thermostat. The top of the block is also 

 lapped and the crystal mounting is secured to it by means of the four 



