320 hull: transmitter of modulated waves 



reaches a safe maximum when the load is thrown on. If suffi- 

 cient power is available from the generator it is advisable to 

 over-compensate the transformer, making it possible to heat the 

 filament at reduced current except when the key is pressed. 



The transmitter described in this paper was designed to 

 operate at short wave-lengths. The performance of such a 

 system at short waves is limited by two factors; first, the elec- 

 trostatic capacity between elements of the electron tube, which 

 may provide a reactive shunt for the oscillatory circuit; second, 

 the approximate linear relation between power output, resistance, 

 and capacity. Consider any short portion of the wave train 

 when the amplitude of the supply voltage may be considered 

 constant so far as the radio frequency oscillations are concerned. 

 It has been shown^ that the output power is given for any tube 

 by 



where R, L and C are the resistance, inductance and capacity 

 of the antenna and / is a function which depends upon the char- 

 acteristics of the tube and upon the plate and grid coupling. 

 Over a certain range of operation the function /, which involves 

 the oscillating grid voltage as dependent upon the antenna ctu-- 

 rent and coupling is found to be a direct linear function. Then 

 the output power varies inversely with the antenna capacity 

 and with the antenna resistance. Assuming constant L, since 

 a change in L involves a change in the function /, it is evident 

 that if C is made small, as is the case at short wave-lengths, 

 R must be increased, in order to obtain maximum output. It 

 may be impossible to obtain maximum output from a tube in 

 a given antenna of low resistance at short wave-lengths, par- 

 ticularly in view of the fact that the total effective resistance 

 decreases with increasing frequency. 



When supplying an antenna having a capacity of approxi- 

 mately 0.004 microfarads and a resistance of 10 ohms, this set 



^ Hull, L. M. Determination of output characteristics of electron-tube generators. 

 Bur. Standards Sci. Paper 355. 1919. 



