TRANSATLANTIC RADIO TELEPHONY 347 



Houlton, Maine, instead of Riverhead, New York, and this increase 

 in distance from many of the American high-power transmitting 

 stations decreases somewhat their field strengths. In view of these 

 factors the "Assumed Maximum Interference," although it is not 

 greater than any observed station field strength, is, in fact, greater than 

 the interfering signals when the outputs from the actual receiving 

 antennas are used instead of field-strength observations. 



6a. Selectivity Requirements Imposed by the Use of Amplifiers. To 

 operate a vacuum tube as an amplifier with negligible distortion the 

 peak voltage applied to its grid must be less than a limiting value 

 so that the tube always operates over the practically linear portion 

 of its characteristic. If no discrimination were provided against un- 

 wanted signals, we would be placed in the peculiar situation of having 

 to supply ample tube capacity in the radio receiver to care for the 

 combined load produced by perhaps 100 telegraph stations each of 

 which, on the average, may have a received signal strength 1,000 times 

 the assumed minimum signal. An easy way to decrease the load 

 produced by interference is to insert a filter at the input of the receiving 

 set, which will reduce the required capacity of the first tube. Addi- 

 tional selectivity following the first tube still further reduces the load 

 of undesired signals on the following tubes as more of the capacity 

 of those tubes is used for the desired signals. 



Now for design purposes let us assume that the load capacity of 

 each tube is at least 6 TU greater than the capacity required in the 

 tube for the performance of its functions on the desired signal. The 

 undesired signals may then be allowed to produce on the tube grid a 

 voltage equal to that of the desired signal. 



Since each of the undesired signals shown in Fig. 16 are about 60 

 TU stronger than the minimum desired signal, they must be reduced 

 by that amount to make them each no greater than the desired signal. 



It is shown in Fig. 21 of Appendix 4 that, as a result of unit random 

 input voltages from 100 operating radio-telegraph stations, a peak 

 voltage will be produced equal to or greater than 10 such units during 

 less than 0.1 per cent of the time. If the undesired telegraph station 

 signals were all of the same magnitude as the desired signal then the 

 voltage which they would produce would be 20 TU above the voltage 

 of the desired signal. 



From purely load considerations then, the total required suppression 

 of every interference-bearing frequency outside of the desired signal 

 receiving band will be 



60 + 20 - 80 TU. 



