TRANSATLANTIC RADIO TELEPHONY 133 



Study of Transatlantic Transmission 



We come now to a consideration of the second major part of the 

 investigation, namely, that having to do with the transmission of 

 the waves across the Atlantic. It will be evident, from what has been 

 said earlier, that the transmission question is essentially one of how 

 best to deliver, through the variable conditions of the ether to the 

 receiving station, speech-carrying waves sufficiently free from inter- 

 ference to be readily interpretable in the receiving telephone. The 

 transmission efficiency of the medium varies with time of day and 

 year, and is different for different wave lengths. The interference 

 conditions are also influenced by these same factors. 



Now we can study this transmission medium in much the same way 

 we would a physical telephone circuit, by putting into it, at the 

 sending end, electromagnetic waves of a known amount of power 

 and measuring the power delivered at the receiving end. The inter- 

 ference at the receiving station likewise may be measured and the 

 ratio of the strength of the signal waves to the interfering waves may 

 be taken as a measure of freedom from interference; this in turn 

 being directly related to the readiness with which the messages are 

 understood. Accordingly, there has been included as an integral part 

 of the investigation of transatlantic radio telephony, the development 

 of suitable methods and apparatus for measuring the strength of the 

 signal waves and of the interfering waves, as they arrive at the receiv- 

 ing station. The apparatus 5 employed in measuring the field strength 

 of the received signals has been outlined above under Receiving 

 System and need not be gone into further. However, a word of 

 explanation about the method which is employed in making the 

 measurement may be helpful. It will be recalled that the specially 

 designed receiving set is provided with a local source of high fre- 

 quency from which can be originated signals of predetermined strength. 

 A measurement of the field strength of a signal received from the 

 distant transmitter is made by listening first to the distant signal and 

 then to the locally produced signal, shifting back and forth between 

 these signals and adjusting the strength of the local signal until the 

 two are substantially of the same strength. Then, knowing the 

 power delivered by the local source, the power received from the 

 distant station is likewise known. The relation between the power 

 in the input of the radio receiving circuit to the field strength required 

 to deliver that power is known through the geometry of the receiving 



6 It is described in detail in the paper entitled, "Radio Transmission Measurements" 

 by Bown, Englund, and Friis, Proc. Institute of Radio Engrs., April, 1923. 



