. Le journal technique de Bell System . s point le labyrinthe de trolley et de fils de support apparatlyconstitué un écran assez eficient pour ces vagues. Les observationsréalisées au cours des essais ont montré que parfois le champ était réduit de façon con-siderable sur le côté éloigné des collines comme cela a été indiqué par les cartes de contour. Une forte réduction de la force du champ a entraîné une hausse derrière une colline basse sur la rue Saratoga sur l'île de races. L'exemple le plus frappant de cet éfifect a été le lieu de mesure Seaverns Street et l'émetteur mobile conduit de Huntington Avenue vers South Huntin
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. The Bell System technical journal . s point the maze of overhead trolley and support wires apparentlyconstituted a fairly efificient screen for these waves. Observationsmade during the tests showed that sometimes the field was con-siderably reduced on the far side of hills as has been brought out bythe contour maps. A large reduction in field strength resulted upongoing behind a low hill on Saratoga Street on Breeds Island. Themost striking example of this efifect occurred with the measuring setat Seaverns Street and the mobile transmitter being driven fromHuntington Avenue onto South Huntington Avenue (C of Fig. 11).Soon after rounding the corner at the foot of Parker Hill, which is 200feet high, the average field dropped about 15 db. The field was 20 db lower under Funeral Bridge than on eitherside of it. This is a stone and earth bridge appearing as a shorttunnel to the road beneath it. The field was usually reduced uponpassing underneath bridges of this general type of construction. 268 BELL SYSTEM TECHNICAL JOURNAL. 5 KILOMETERS CONTOUR INTERVAL 20 FEETDATUM IS MEAN SEA LEVEL Fig. 11—Field strength contour map of Boston, Mass. Receiver located on S^eaverns Street ne.iCenter Street as shown by the solid circle. Frequency = 34.6 mc. Field strength in decibels abovone microvolt per meter. ULTRA-SHORT-WAVE PROPAGATION 269 A separation of the field strengths into those obtaining in the high-building area (indicated by open circles in Fig. 2) from those in thelower-building area (indicated by solid dots) shows that the attenuationis somawhat greater in the former. No effect of the elevated railway structures on the average field was observed. Noise Measurements For reception in the car, by far the greatest interference is thatcaused by the electrical systems of passing automobiles. Special teststo determine whether or not street cars produced any noise gave anegative result. That is, under conditions where automobile noisewas a limitation to reception, trolley no
