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BELL SYSTEM TECHNICAL JOURNAL 



an arc of more nearly constant radius. In this connection it may be 

 mentioned that the field is nearly constant within the bulge of this 

 contour to the southwest. 



A striking fact brought out by the map is the crowding of the 

 contours in the business district. There are particular directions for 

 which the attenuation is greater presumably due to the combined 

 eflfects of high buildings, for example to the east-northeast. There are 

 other directions where the field strength is higher than the av^erage. 

 Several such places were noted when salt water extended immediately 

 in front of the measurement location in the direction of the transmitter. 

 The closed contours in the Mystic River to the north illustrate the 

 better reception over salt water as predicted by theory.^ An example 

 of the records taken over bridges upon which these contours are based 

 is shown in Fig. 9. (The route over which these data were taken is 



25 



.TOWARD 



TRANSMITTER 



ON MERIDIAN STREET 



- BRIDGE OVER THE — 



CHELSEA RIVER 



3 4 0.5 



DISTANCE IN MILES 



Fig. 9 — Portion of records showing the variation of field strength on going over water. 



indicated at C of Fig. 8.) The average increase in field when going 

 over bridges was 10 db. This may be explained by the better con- 

 ductivity of the water which results in a receiving directive charac- 

 teristic that is more favorable to low angle reception. In some cases, 

 the absence of buildings and increase in the height of the antenna due 

 to the elevation of the bridge undoubtedly contributed to the greater 

 field. 



Another fact that is illustrated by the map is that the effects of 

 obstacles and type of terrain are in general local. For example, the 

 higher field strengths beyond salt water are soon reduced to normal 

 with the intravention of additional land, illustrated at D in Fig. 8. 



It was found that the field strengths along Columbus Avenue 

 (indicated by E on F'ig. 8) were about 10 db higher than those obtained 

 on either side. This increase probably resulted from the fact that a 



