TRANSATLANTIC RADIO TELEPHONY 343 



2. The Output Power of the Receiving Antenna for a Given Signal 

 Strength. From the observed constants of a Houlton wave-antenna 

 and the assumed value of 0.4 microvolts per meter received at zero 

 degrees to the antenna direction as the lowest field, we calculate, using 

 equations (125), (126), and (127) in Appendix 1, that the power sup- 

 plied to the reflection transform.er terminals is 3.716 X 10~^ micro- 

 watts. This power must suffer loss as a result of the transmission 

 back to the receiving station over transmission lines arid as a result 

 of the necessity of providing flexibility in the operation of the appa- 

 ratus used to combine the output of the antenna in question with the 

 output of other antennas before it reaches the input terminals of the 

 radio receiving set. (See Fig. 11.) This loss is such that the power 

 at the input terminals of the radio receiving set from a single antenna 

 and for the minimum signal field is very nearly equal to 3.7 X 10~^ 

 microwatts. With the combining system actually used, the input to 

 the radio receiver from all four antennas will be 12 TU above this 

 value or 5.9 X 10~® microw^atts. 



3. Power Output Required from the Radio Receiver. The value of 

 output power required from the radio receiver is really governed by 

 considering the whole radio circuit as a part of a long-distance tele- 

 phone system. An overall loss of 10 TU has been found satisfactory 

 for long toll circuits. It the telephone lines connecting the circuit 

 terminals to the transmitting and receiving stations have an equivalent 

 of TU then we can place the 10 TU loss in the radio portion of the 

 circuit. If we then supply on a single frequency within the voice- 

 frequency band a power of 1 milliwatt to the input terminals of the 

 radio transmitter, to get a 10 TU equivalent in the radio circuit we 

 must obtain 0.1 milliwatt at the output of the radio receiver. 



In the preceding section we determined that the minimum input 

 would be 3.7 X 10~^ microwatts from a single antenna and hence the 

 maximum gain required in the radio receiver to raise this power to 

 the specified 100 microwatts output is 84 TU. 



Within amplifiers using three-electrode vacuum tubes, noise is 

 generated in two ways: (a) by thermal agitation ^■' in the conductor 

 of the input circuit; and ib) by "Schottky Effect"^" in the \'acuum 



^^ J. B. Johnson, "Thermal Agitation of Electricity in Conductors," Plivs. Rev., 

 32, 97; July, 1928. 



Harry Nyquist, "Thermal Agitation of Electric Charge in Conductors," Phys. 

 Rev., 32, 110; July, 1928. 



J. B. Johnson, "Thermal Agitation of Electricity in Conductors," Nature, 119, 

 50; Jan. 8, 1927. 



-"Walter Schottky, "Atomare Schwankungsvorgange an Gliihkathodenober- 

 flachen," Physik. Zeitschr., 27, 701; Nov. 1, 1926. 



T. C. Fry, "The Theory of the Schroteffekt," Jour. Frank. Inst., 199, 203; Feb., 

 1925. 



