HUIylvl TRANSMITTER OF MODULATED WAVES 317 



to zero occurring at a suitable audiofrequency. Radio-fre- 

 quency harmonics, manifested by distortions in the shape of 

 the radiofrequency or carrier wave, Hmit the total power radiated 

 by a transmitter at a single frequency to which the receiver is 

 tuned. Audio-frequency harmonics, manifested by distortion 

 in the envelope of the radio-frequency oscillations from sinu- 

 soidal form, determine the response of any amplifying and rec- 

 tifying detector. 



There are two possible methods of operating an electron- 

 tube generating system so as to furnish a completely modulated 

 output: (i) The use of a direct supply voltage in connection 

 with a mechanical interrupter or "chopper," which periodically 

 breaks the supply circuit, causing the antenna current to be 

 reduced to zero; (2) the use of an alternating audio-frequency 

 supply voltage. If the frequency of the supply voltage be F 

 and the peak value Et, then the plate is positive with respect 

 to the filament F times per second while the supply voltage 

 rises from o to Eb volts, and negative F times per second while 

 the supply voltage falls from o to —Ei, volts. The antenna 

 current is maintained for a half cycle when the plate is positive 

 and is reduced to zero a greater part of the half cycle when the 

 plate is negative. 



The first method requires a source of high direct voltage 

 which may be inconvenient if high power tubes are used. With 

 the second method the whole system can be operated from any 

 audio-frequency generator with suitable transformers for the 

 high-voltage plate and the relatively low-voltage filament. 

 The note produced by telephone receivers actuated by the rec- 

 tified output from the transmitter corresponds to the frequency 

 F. Consequently a desirable value for F would be 800 cycles 

 per second since most audio-frequency receiving apparatus is 

 designed for best operation at about that frequency. If no 800- 

 cycle generator is available, a 500-cycle machine can be used. 



A transmitter of this description has been designed and built 

 at the Bureau of Standards. The set fulfills the following re- 

 quirements: (i) Use of a single, type "P" pliotron, with 

 500-cycle, 150-volt alternator; (2) power output exceeding 200 



