132 



BELL SYSTEM TECHNICAL JOURNAL 



tended for installation in a small building with a grounded copper 

 roof located at the base of the antenna downlead. It consists of two 

 tuned circuits, each housed in separate shielded compartments. In 

 the photograph the doors and two of the screen panels have been 

 removed to show the interior arrangement. The line is terminated 

 by the parallel tuned circuit on the left which is inductively coupled 

 to the antenna circuit to preserve an approximate balance to ground. 

 The antenna is tuned by means of the series condenser and coil shown 

 on the right. Accurate adjustment of the inductance of the coil is 



Fig. 7 — Artificial antenna for 50-kw radio transmitter. 



provided for by means of a short-circuited single-turn secondary 

 which is located within the coil and arranged so that it can be rotated 

 through approximately 90 deg. by the motor mounted on the floor 

 beneath. The latter may be controlled from the operating room by 

 a reversing switch placed on the right-hand panel of the transmitter 

 assembly. A polyphase position indicator is provided to indicate the 

 angle and movement of the secondary. The direct-current circuit of 

 the thermal ammeter in the antenna circuit is also carried back to a 

 bracket-mounted instrument on the end of the transmitter. These 

 facilities permit the antenna tuning to be checked and adjustments 



