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BELL SYSTEM TECHNICAL JOURNAL 



percentage variation is made small by the use of an average frequency 

 of approximately 450 megacycles. The use of this ultra-high fre- 

 quency has the additional advantage that the antennas can be both 

 small and efficient and cause little drag upon the airplane. 



Apparatus 



Figure 3 is a photograph of all the units of the altimeter, with the 

 exception of the transmission lines used to connect the antennas to 

 their respective units. The units are as follows: left to right, receiver, 

 power unit, and transmitter, with a junction box in the upper right. 

 In the foreground are the two dipole antennas and the indicating meter 

 with its range-shift switch. The meter and one of these antennas are 

 shown in larger scale in Fig, 4. The meter has two scales, the upper 



^ t ^^- j ,£iK- 



Fig. 4 — Antenna, meter, and range switch. 



extending from to 5000 feet and the lower to 1000 feet. The 

 position of the range switch determines the scale to be used in reading 

 the meter. 



Figure 5 shows an assembly of the various units located approxi- 

 mately as they would be installed in an air transport. The trans- 

 mitter, power unit, receiver and a junction box are installed in the 

 baggage compartment just aft of the cockpit with cable connections 



