The ionospheric propagation of radio waves of frequency 16 kc/s over distances of about 540 km

Abstract

The paper describes an investigation of the ionospheric propagation of waves of frequency 16 kc/s over a distance of 535 km. It represents an extension of other investigations in which the propagation over distances of 90 and 200 km was studied.1, 3, 4, 13A long series of measurements of signal amplitude covering the period 1940–44 has been used to study regular diurnal and seasonal variations, and anomalous effects associated with sudden ionospheric disturbances and severe magnetic disturbances. In another series of experiments from May to October, 1949, measurements of phase were made by using the method, previously employed at 200 km, of sending a reference signal from transmitter to receiver by land line. In this way the phase and amplitude of the downcoming wave could be deduced separately.The observations showed that reflection took place from an apparent height of about 74 km by day and 92 km at night; the downcoming wave was of approximately linear polarization during morning twilight, the plane of polarization being rotated clockwise from the normal through an angle of 40°, as seen by an observer looking in the direction of propagation; the effective reflection coefficient in the summer of 1949 was about 0.27 by day and 0.55 at night; the sunrise effects on the height of reflection and the amplitude of the downcoming wave regularly occurred when the zenith distance of the sun was 97° 50′. The effects of sudden ionospheric disturbances on the signal amplitude were explicable in terms of a change of reflection height with little change in the amplitude of the downcoming wave. During and after severe magnetic storms the diurnal variation of the signal amplitude. was abnormal, the abnormality persisting for as long as 38 days.It is clearly evident that there is a marked change in the propagation characteristics of the reflected wave in passing from short (90–200 km) to long (535 km) distances from the transmitter. At short distances the downcoming wave is of approximately circular polarization with a left-hand sense of rotation, while the reflection coefficient in summer is about 0.15 by day and 0.5 at night. The most striking evidence of the change in propagation characteristics is given by the markedly different ways in which the height of reflection varies with the inclina tion of the sun's rays. These conclusions concerning the change of reflection conditions confirm the deductions made by Weekes5 from measurements on the ground interference pattern.