. The Bell System technical journal . nes Flight Research Airplane. Operation and TheoryThe fundamental parts of the altimeter in relation to their applica-tion are shown in Fig. L An ultra-high frequency oscillator is pro-vided, whose frequency is varied up and down by a modulator whichconsists of a small rotating variable condenser driven by a oscillator is connected through a coaxial transmission line to atransmitting antenna which is located on one of the lower surfaces ofthe airplane. The signal is radiated downward by this antenna. A 226 BELL SYSTEM TECHNICAL JOURNAL radio rece
. The Bell System technical journal . nes Flight Research Airplane. Operation and TheoryThe fundamental parts of the altimeter in relation to their applica-tion are shown in Fig. L An ultra-high frequency oscillator is pro-vided, whose frequency is varied up and down by a modulator whichconsists of a small rotating variable condenser driven by a oscillator is connected through a coaxial transmission line to atransmitting antenna which is located on one of the lower surfaces ofthe airplane. The signal is radiated downward by this antenna. A 226 BELL SYSTEM TECHNICAL JOURNAL radio receiver is connected through a similar coaxial line to a secondantenna similarly located but arranged in such a way that a minimumof direct signal is received from the transmitting antenna and as muchecho as possible from the ground. The direct and reflected signals are TRANSMITTER FREQUENCY(MODULATOR I OSCILLATOR f (ultra-high FR£Q.)l FREQUENCY- I MODULATED J SIGNAL 1 RECEIVER (? METER ^ CIRCUIT t ] AMPLIFIER ^ ] DETECTOR. Fig. 1—Overall system. applied to a detector circuit in the receiver. The output of this de-tector is a signal of a frequency equal to the instantaneous differenceexisting between the direct and the reflected signals and is proportionalto the height of the plane above the terrain. This signal is ampHfiedby the receiver and applied to a frequency meter or counter circuitwhich is so designed that a current proportional to the frequency and,hence, to the height flows through a meter calibrated in feet andlocated on the airplanes instrument panel. A number of types of A TERRAIN CLEARANCE INDICATOR 227 indicating frequency meter circuits of the condenser charge and dis-charge variety have been described in the technical Hterature. The operation of the system can be understood more easily byreference to Fig. 2. The variation of the transmitter frequency with signals:direct reflected MAX RADIOFREQ,
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
