Experimental investigations of very long waves reflected from the ionosphere

Abstract

This paper is an account of experiments which have been carried out to determine the characteristic features (amplitude, height of reflexion, and polarization) of waves of very great length (18·8 km) reflected from the ionosphere at fairly small angles of incidence. The transmission characteristics of long waves have previously been studied by Holling-worth, Naismith, and Namba. In Hollingworth's pioneer experiment measurement were made on the space characteristics of the interference pattern produced at the ground by the superposition of the direct and the downcoming wave. the observations were made within the range 300-1000 bin from the sender, during the daytime, and one minimum and one maximum of the interference pattern were located. With the large distances to be covered the measurements extended over three months, and it was necessary to assume constancy of the conditions from day to day. The result demonstrate the presence of the interference system in a very beautiful manner, but cannot be used for an accurate determination of the height of reflexion at any one time. In §2 of the present paper we describe experiments, of the same type as Holling-worth's, carried out at shorter distances (70-140 km). The advantage of using shorter transmission distances are twofold. Firstly, it is possible to make sufficient measurements in the course of a single day (or night) to determine the reflexion height and the reflexion coefficient for a single day (or night), and, secondly, the information derived applies to the conditions of nearly vertical incidence, and so is more directly comparable with the detailed information which is now available concerning short waves. In the papers mentioned above muck attention has been given to the observation and explanation of the effects observed near sunset. It is pointed out that if the resultant signal strength on a single aerial system is alone observed, it is impossible to decide whether the changes are due to changes of amplitude, phase, or polarization of the downcoming wave. Assuming that the sunset variations are due entirely to phase variations, Hollingworth first deduced a change of reflexion height from 75 km to 90 km during sunset, but later believed that the variations were almost entirely explained by a rotation of the plane of polarization of the wave, and that the change of reflexion height was only about 2 km. Naismith states that no sunset variations are observable at short distances (100 km) from the sender, and suggests that at vertical incidence the waves are reflected from a higher level which does not exhibit changes at sunset.