FIRE-CONTROL RADARS FOR NAVAL VESSELS 5 



was apparent to all of the early experimenters since physical limitations 

 restricted the size of antenna which could be installed conveniently aboard 

 ship. However, development effort along these lines had previously been 

 hampered by lack of suitable vacuum tubes. 



In spite of the vacuum tube dilTiculties the Laboratories work was started 

 in the range from 500 to 700 mcs, a region several times that then in use at 

 the Army and Nav}^ laboratories. The best tubes available were those of 

 the doorknob type which have been described in the literature by A. L. 

 Samuel- and are illustrated in Fig. 2. The smallest of these was used in 

 the receiver input circuits and two of the middle sized ones were used in 

 the tramsmitter oscillator. These triodes operated at quite high frequencies 

 by virtue of the \ery small spacing between their electrodes, a feature which 

 made them fragile and demanded the development of plate modulation. 

 Earlier radars had generally used grid keyed oscillators, i.e., the plate 

 voltage was applied to the oscillator continuously together with a high grid 

 bias voltage. The bias was removed momentarily by the keyer to emit a 

 pulse. In order to obtain a useful pulse output from the doorknob oscillator 

 tubes it was found essential to remove all stress from them e.xcept during 

 the pulse. This was accomplished by using a direct coupled pulse am- 

 plifier or modulator, eflfectively in series with the oscillator and the power 

 supply. Here again in 1938 no really suitable tubes were available for 

 the modulator service since it also demanded a highly intermittent duty. 

 However, since the modulator duty did not require the tubes to operate at 

 ver}' high frequency it was possible to use rugged high-voltage triodes which 

 had been designed for continuous service, and to obtain the required pulse 

 current capacity by paralleling a number of tubes. The earliest radar 

 modulators used in the Laboratories employed a group of Eimac 100-TH 

 tubes. Later, in the CXAS and Mark 1 Radars, sLx tubes similar to the 

 W. E. 356A were used in parallel. 



After a great deal of laboratory' work an experimental equipment was 

 assembled and demonstrated to the Army and Navy in July 1939. This 

 early radar was notable in that it operated at what was then considered a 

 ver}' high frequency and also in that it employed a single antenna only 

 about 6 ft. square. The transmitter and receiver were connected to the 

 common antenna by a duplexing technique to be described later, which had 

 been applied at lower frequencies by engineers at the Xaval Research 

 Laboratory. The results of these first field tests were encouraging and 

 both the Army and the Navy ordered one prototype model equipment to 

 be known as the CXAS. This radar was to operate at 500 or 700 mcs and 

 was to incorporate a number of new features which were designed to make it 



^Proceedings of I. R. E., Vol. 25, page 1243, 19W— "Negative Grid Triode Oscillator 

 and Amplifier for Ultra High Frequencies." Digest in Oct. 1937 B. S. T. J. 



