The ionospheric propagation of radio waves with frequencies near 100kc/s over short distances

Abstract

Experiments made in Cambridge1, 2 during 1933–39 on the reflection of very-low-frequency waves (16kc/s) from the ionosphere have shown that ionospheric propagation at these frequencies differs in important particulars from that at higher frequencies (e.g. 2 Mc/s). There is a large downcoming wave even during a summer day, and the apparent height of reflection is estimated to be about 70 km, whereas on 2 Mc/s, the downcoming wave is relatively weak and the apparent height of reflection is about 100 km. The experiments described in the paper have been undertaken as part of a series of investigations designed to study the transition between these two different conditions, and they have revealed the main characteristics of the behaviour of the downcoming wave on frequencies between 70 and 127 kc/s. The observations have been carried out using c.w. senders situated approximately 100 km from the receiving site.The downcoming wave was found to be approximately circularly polarized with a left-handed sense of rotation viewed along the direction of propagation. Most of the observations have been made on the abnormal component of the downcoming wave field, polarized perpendicular to the plane of propagation; the ratio of its amplitude to that of the “normal” component of the incident field is called the conversion coefficient. At a frequency of 71 kc/s, the conversion coefficient was approximately 0.3 at night, 0.06 on a winter day and 0.002 on a summer day, and at the higher frequencies the values were slightly smaller. These figures are much closer to those observed at 2 Mc/s than those measured at 16 kc/s.The apparent height of reflection is found to decrease over the sunrise period by about 7–8 km on the average. On any one day, the change is very similar at all the frequencies studied, but there are considerable differences from one day to another. This is very different from the observations on 16 kc/s, in which the change of apparent height is about 19 km in summer and 14 km in winter and is very constant from day to day.The decrease of apparent height begins within a few minutes of ground sunrise, but the change of conversion coefficient occurs about one hour before sunrise. The effect of a sudden ionospheric disturbance on the downcoming wave is found to be a decrease in the conversion coefficient and a decrease in the apparent height of reflection. For a sudden ionospheric disturbance which decreases the apparent height by 5 km, the conversion coefficient is decreased by a factor about 0.03.