STUDIES IN RADIO BROADCAST TRANSMISSION 187 



of obstruction enter into the final field distribution. Relative atten- 

 uation of water and land appear to influence the distribution con- 

 siderably though not as definitely as do steel structures close to the 

 transmitter. Distinct minima appear both on the Hudson and on 

 the Sound along radial lines extending from the transmitter. 



Probably refraction of the wave front in passing across shore lines 

 also enters into the shaping of this pattern. 



Perhaps as good an elementary picture as any of the phenomena 

 causing these patterns is that of a "dent" produced in the wave 

 front by an encounter with a portion of New York City's impressive 

 skyline. Since radio waves travel in a direction perpendicular to 

 the plane containing the electric and magnetic fields, opposite sides 

 of this "dent" would cross over one another with the result that an 

 interference pattern would appear beyond the obstruction. An 

 analogous situation exists when a water ripple passes a cluster of 

 marsh grass which, damping its motion and retarding its progress 

 causes part of the advancing front to converge and cross beyond the 

 obstruction. 



There is evidently a relation between day patterns such as have 

 been discussed and night-time conditions. Just what this relation 

 is offers some further opportunity for conjecture. In the first place 

 quality distortion in transmission at night was, as previously ex- 

 plained, observed over parts of the region covered by the pattern 

 shown in Fig. 1. The worst distortion seemed to be somewhat 

 associated with the low field strength regions in this daylight survey. 

 The distortion seemed also to be worse along the low channel ex- 

 tending in the direction of New Canaan, Conn., and beyond the 

 100- wave-length circle. It was particularly bad at a distance of 

 some 140 wave-lengths from the station along this low channel where 

 the field strength became so low in the daytime as to be unmeasurable 

 with the set employed for the work. Accompanying the poor quality 

 were fading and marked directional shifts. 



Quality distortion though not so consistently severe at the River- 

 head station as in the vicinity of Stamford was at times easily detect- 

 able by audible tests. Due to rapid attenuation of the radio waves 

 traveling from the site of 2XB across Manhattan and the length of 

 Long Island the field strength around Riverhead is generally low 

 with higher levels north and south on the open waters of the Sound 

 and Ocean respectively. Night-time fading at this point was rep- 

 resentative of the variety which is usually found at distances of 

 approximately one hundred miles from a broadcast transmitter. 



The situation at Riverhead appears to be somewhat the same as 



