144 BELL SYSTEM TECHNICAL JOURNAL 



Night time transmission, which is the usual case in broadcasting, is 

 in many places commonly marred by fading and sometimes by actual 

 distortion of signals. Often these occur in certain areas not more 

 distant from the transmitting station than other areas which enjoy 

 freedom from such annoyance. Selecting a particular instance of these 

 difficulties in an area near New York City which, in so far as can be 

 judged at present, is probably a typical instance, we have subjected it 

 to an intensive experimental study to determine what is the inherent 

 nature of the troubles and if possible how they may be alleviated. 

 In doing this it has been necessary to employ novel forms of tests 

 especially fitted to bring out in a concrete way the. phenomena being 

 investigated. 



To provide a suitable background for the subject we have started 

 our discussion below with a brief recital of some of the things which a 

 transmission medium is called upon to do. Following this we have 

 described our tests, pointing out in what ways the existing media seem 

 to fall short of doing these things ar^d offering certain speculations as 

 to the reasons for the shortcomings. In conclusion we have analyzed 

 some practical problems in the light of this work. 



Fundamental Considerations 



As the radio art has progressed from spark telegraphy into continu- 

 ous wave telegraphy and into high quality radio telephone broadcast- 

 ing, increasing demands have been made on the transmission medium 

 to deliver at the receiving point a true sample of what was put into it 

 at the transmitting station. The requirements have grown in rigor 

 because in telegraphy the end has been to develop increased reliability 

 of communication at longer ranges and in telephony the medium is 

 called upon to transmit a highly complex form of intelligence. 



Of the requirements placed on the transmission medium by modern 

 uses, those imposed by telephony are far more exacting than those for 

 telegraphy. In telegraphy a single frequency, or at most a narrow 

 band of frequencies sent out intermittently in accordance with a dot 

 and dash code must reach the receiving station in such shape that it 

 may be converted into audible sound for aural interpretation or into 

 current pulses for the operation of relays or recording instruments. 

 Leaving aside noise, the principal requirement is a sufficient freedom 

 from fading so that signals can be interpreted or recorded without 

 interruption. In radio telephony, as at present practiced in broadcast- 

 ing, there is transmitted a modulated high-frequency wave comprising 

 a relatively wide band of frequencies, usually at least 10 kilocycles. 



