232 BELL SYSTEM TECHNICAL JOURNAL 



to the airplane battery and to the meter and range switch on the 

 instrument panel. The transmitting antenna is shown below the 

 wing to the left of the engine nacelle and the receiving antenna to the 

 right of the other engine nacelle. Coaxial transmission lines connect- 

 ing the antennas to the transmitter and receiver, respectively, are 

 indicated by the lines extending through the wings from the antennas. 

 It was necessary to exaggerate the size of some of the units in order to 

 make them large enough to see in the diagram. 



The installation with apparatus as pictured in Fig. 3 weighs com- 

 plete with all cables and connections about seventy pounds. Since 

 the equipment shown in the pictures represents a working model built 

 with the idea of attaining performance rather than minimum weight, 

 undoubtedly some reduction in weight will be obtained in future 

 models. 



The antenna installation shown utilizing half-wave dipole type 

 antennas approximately a quarter wave-length below the reflecting 

 surface of the wing is not particularly directional. The signal is 

 radiated over approximately the whole hemisphere below the wing 

 centered on the transmitting antenna. The strength of the signal 

 is greatest in the downward direction but does not fall off rapidly in 

 other directions. The advantage of this antenna arrangement is that 

 the distance to the nearest reflecting surface is measured regardless of 

 whether it is directly beneath, or to the front or side. As a result very 

 little change in reading occurs when the airplane banks steeply. Some 

 advance indication also is given when the airplane in level flight 

 approaches higher terrain. 



Performance 



The terrain clearance indicator in its present experimental form 

 indicates altitudes between approximately twenty and five thousand 

 feet. When over smooth water or land, it is subject to errors as 

 indicated by a consideration of the fundamental equation upon which 

 the altimeter is based, 



F, = AAFFmH/C. 



Since Fd is directly proportional to both AF and Fm, any variation of a 

 given percentage in either will result in a corresponding percentage 

 error in the reading of the meter. It is believed from the data available 

 that the errors due to variation of either AF or Fm do not exceed ±1 

 per cent. 



Additional errors can also occur in the frequency meter circuit. 

 These errors are believed to be less than ±7 per cent, so that a total 

 error of ±9 per cent might occur if all the errors were simultaneously 



