On Some Measurements of the Equivalent Height of the Atmospheric Ionised Layer

In previous communications various experimental methods of examining the characteristics of downcoming wireless waves have been described. Of these methods by far the most useful has proved to be that which involves a small continuous change of transmitter wave-length, by means of which there is produced at a receiving station a succession of interference maxima and minima between ground waves and waves deviated by the upper atmosphere. By using loop and antenna receiving systems in several different ways it has been possible to estimate, from a comparison of the interference “fringe” amplitudes, the relative intensities of the ground and atmospheric waves and also the angle of incidence of the latter at the ground; while a comparison of the phase differences between the two sets of waves, as deduced from the interference “fringes” recorded on different types of aerial systems, has yielded information relating to the polarisation of downcoming waves. In all such investigations, however, the number of “fringes” produced by a known wave-length change has been found, even when the determination of that number was not the primary object of the experiment. In this way the equivalent path difference for the ground and atmospheric wave tracks has been calculated as a routine measurement from which the equivalent height of the ionised layer could be deduced. In the course of such investigations carried out at the Peterborough Radio Research Station, much evidence has therefore accumulated relating to the diurnal variation of the equivalent height of those regions in the atmosphere which are responsible for the deviation of wireless waves. As was pointed out in the first communication dealing with the work of the station, considerable interest is, in the study of wireless wave propagation, attached to the transitional periods of sunrise and sunset. As the experiments under discussion were usually carried out on wave-lengths of from 300 to 600 metres ( i. e ., on frequencies of from 1000 to 500 kilocycles per second) the study of the sunset period was, except on rare occasions, impossible, because of interference from broadcasting transmitters. The majority of the tests therefore took place during the sunrise period when, in addition to the advantage of the low level of artificial interference, there was also that of the early morning minimum of natural atmospheric disturbances. In carrying out such early morning runs it was soon found that, in the few hours before sunrise, the signal records showed the presence of both primary and secondary interference maxima and minima, indicating the simultaneous reception of two or more downcoming rays. After sunrise the phenomena became simpler and smooth “fringes” were recorded. This latter particular period was therefore especially suitable for the determinations of the characteris­tics (intensity, angle of incidence, polarisation, etc.) of downcoming waves already described. The present communication, is in part an attempt to elucidate these pre-sunrise phenomena and in part an introduction to two papers which are to follow, since it was the study of the significance of multiple downcoming rays which led to the investigation of the subject with which these two papers respectively deal, namely, the simultaneous reception of downcoming waves at several receiving stations and the use of shorter wave-lengths.