June 4, 1921 dellinger and whittemore : radio fading 247 



EXPERIMENTAL AND OBSERVATIONAL DATA 



Intensity of signals in day. (Variation with distance, place, and 



wave-length.) — The current in a receiving antenna is given by the 



following expression : 



T 1 -a d 



/, ^ ^ e vx . (1) 



where X is the wave-length, d is the distance from the transmitting 

 station, e = 2. 718. ...and a is a constant known as the absorption 

 coefficient. Quantitative measurements by L. W. Austin and others 

 have shown that signals transmitted in the daytime follow this law 

 within about 50 per cent particularly for the longer waves (over 1000 

 meters) . Variations in received current are associated with variations 

 of the absorption coefficient a. It is a minimum for transmission 

 over ocean water and varies greatly over land, apparently depend- 

 ing largely upon the nature of the ground over which the waves travel. 



It has been observed that city buildings obstruct short waves. 

 There is some evidence that waves tend to follow water, thus travel- 

 ing greater distances along the coast than inland, and following rivers 

 very readily. The Alps and other mountains are said by Schwartz- 

 haupt to obstruct signals greatly by day, but little by night. 



The intensity of signals received from a given transmitting station 

 varies greatly from day to day. Very long w^aves, such as those from 

 the Lafayette station in France, vary as much from day to day as they 

 do between day and night. Greater distances are covered by stations 

 of a given power when transmission is along the Mississippi Valley 

 than in other directions in the United States. 



For small distances, the factor containing the absorption coefficient 

 in equation (1) is very nearly unity. Within these distances (up 

 to 200 miles in many cases), the intensities of signals during day and 

 during night are about the same, over sea-water at any rate, according 

 to Austin. In transmission entirely in mid-ocean, very little fading 

 has been observ^ed. 



The absorption factor of equation (1) also approaches unity as 

 the wave-length is increased. Thus for a given distance long waves 

 are absorbed less than short waves and the falling off in intensity with 

 distance is less marked for long than for short waves. 



Intensity and fading of signals at night. (Variation with distance, 

 place, wave-length and kind of wave.) — The average signal intensity 

 is much greater at night than by day, and the fluctuation or fading 



