The ionospheric propagation of radio waves with frequencies near 100kc/s over distances up to 1000 km

Abstract

In a companion paper1 the authors have presented the results of a scries of measurements of the characteristics of the downcoming wave observed at a frequency near 100 kc/s at a distance of about 100 km from the sender. In this paper, the results of a complementary series of experiments at greater distances from the sender are discussed. The measurements have been made at frequencies between 70 and 128 kc/s. From measurements of the Hollingworth interference pattern, the apparent height of reflection at oblique incidence (for distances greater than 400 km) has been found to be approximately 70 km at midday in summer, and no definite variation of height with frequency has been detected. The reflection coefficient deduced from the same measurements was found to increase steadily with distance, from 0.02 at 300 km to 0.09 at 800 km from the sender for a frequency of 70 kc/s. The reflection coefficient is found to be less on the higher frequencies. In winter the reflection coefficients at oblique incidence are found to be roughly double the summer values. Up to 350 km from the sender, the diurnal and seasonal variations of apparent height and of absorption appear to be very similar to those reported in the companion paper for 100 km from the sender. The absorption increases about 1 hour before ground sunrise and the apparent height does not begin to fall until very near ground sunrise, when it decreases by about 8 km; this is in good correlation with the observations at shorter distances. When observations are made at distances of 850?950 km from the senders, it is found that both the absorption and the apparent height change about one hour before ground sunrise at the mid-point of the path and that the decrease of apparent height is of the order of 20 km. During a sudden ionospheric disturbance it is found that, at distances of 850?950 km from the sender, the amplitude of the sky wave increases by a factor up to 5 and sometimes reaches its nighttime value. This behaviour is in marked contrast with the observations at short distances, when the amplitude of the downcoming wave is very much reduced. The apparent height of reflection decreases by about the same amount as is observed at shorter distances.