Variations in Nocturnal Transmission

Experiments on night transmission, on wave lengths of 500, 850 and 1,500 meters between two inland stations 1,250 kilometers apart are described. The bibliography of wave absorption is first considered critically. The average transmitting current was 12 amperes and the decrements varied from 0.09 to 0.18. The effective height of the transmitting antenna was 27 meters. Receiving was done with an audion tested against a constant exciting circuit. In making audibility measurements by the shunt-to-telephone method the true impedance at the audio frequency in question was used. Observations were made hourly during night, with fifteen-minute periods near sunset and sunrise. The curves of signal strength are not symmetrical about the solar midnight. In general, the fluctuations of the 500 meter-wave follow those of the 1,500 meter-wave, but not necessarily vice versa. The 850 meter-wave does not seem to fluctuate in marked synchronism with either of these. This is explained on the basis of definite wave interference. Short waves are superior by night, long waves by day, generally. The disappearance of the ice sheet between the stations and the breaking up of the soil seems to account for a spring drop in transmission. Cloudy days markedly favor the following night transmission. Fading and "freak" effects are more numerous on short waves, thus favoring the interference explanation. It is suggested that future observations should be almost continuous, of long wave length range, of high power, reciprocal between stations, and in various directions.