TK.IXS.II L.tXriC RADIO IT.I.lil'llOXIi IK.IXSAflSSION AM 



Major Rkgional Source of Static Noise 



A l)r().ultr conception as to the causes imclerlyinR the (hiiriial and 

 seasonal variation is obtained by considerini; tlie time of sunset and 

 sunrise over a considerable area of the earth's surface. Fig. 19 shows 

 a series of day and night conditions for three representative parts. 3f 

 tlie diurnal noise characteristic at Kngland for January. It will l)c 

 seen that the rise to high night values does not l)egin until practically 

 the time of sunset in ICngl.\nd with over half of .Africa still in daylight. 

 My the time the high night-time values are reached, as indicated in 

 the .second phase, darkness has pervaded all of the e(|ualorial belt to 

 the south of Kngland. Incidentally at this time sunset occurs between 

 the I'nited States anil Kngland, resulting in very poor signal trans- 

 mission. The third phase of this series shows the noise luuing just 

 reached the low daytime value and, although the sun is just rising 

 in Knglanil, the African equatorial belt is in sunlight, subjecting the 

 static transmission path to high daylight attenuation. 



The sunset conditions which existed for the afternoon and evening 

 of the day upon which the diurnal measurements of Fig. 14 were 

 taken are shown in Fig. 20. The hourly positions of the sunset line 

 ,irc shown in relation to the evening rise of st.ilic in London. The 

 coincidence between the arrival of sunset in l.niiilon and the sUirl 

 of the high night-time noise on the higher freciuencies is evident. 

 By the time the high night-time values are reached, about 7 o'clock 

 G.M.T., the equatorial belt to the south of London is in darkness. 

 Fig. 21 shows the sunrise conditions in relation to the decrease in 

 static froin the high night-time values to the lower daylight values. 

 The decline starts about 5 or 6 o'clock an hour or two before sunrise, 

 and is not completetl until several hours later, at which time daylight 

 has extended over practically the entire tropical belt to the south of 

 England which corresponds in general to equatorial Africa. 



Another fact presented in the previous figures which appears to be 

 significant in shedding light upon the source of static, is that noise 

 on the lower frecjuencies rises earlier in the afternoon and persists 

 later into the morning than does the noise on the higher frequencies. 

 This could be accounted for on the basis that the limits of the area 

 from which the received longer wave static originates, extend farther 

 along the eciuatorial zone than they do for the higher frequencies. 



The inclination of the shadow line on the earth's surface, which is 

 indicated in the previous figure for October 28, shifts to a maximum 

 at the winter solstice, recedes to a vertical position at the equinox 

 and then inclines in the opjx)site direction. These several positions 



