An investigation of short waves

Abstract

In this paper an account is given of investigations of shortwave transmission effects made during the past two years. These include, firstly an investigation of scattering, multiple signals and signal mutilation, which supplies perhaps the main bulk of the paper, secondly the results of a nine months' direction-finding interception of short-wave commercial stations, and finally a discussion, in the light of these results, of the problems of short-wave transmission and such cognate subjects as the nature and constitution of the Heaviside layer, fading and polarization effects.An opportunity is afforded for the revision of conclusions arrived at in the author's previous paper.* The chief alteration is in the estimated height of the lowest levels of the Heaviside layer in daytime. The accumulation of evidence from the time-lag experiments of Breit and Tuve, and the interference experiments of Appleton and those of Hollingworth (which incidentally both give the group time and consequently an over-estimate of the height) have led the author to a revision of the mathematical result by which he obtained from long-wave measurement the effective height of the daylight ionized layer, and in doing so he has found an error which makes the estimated height just double that previously obtained, for inseatd of the relationd0 = h2/λ† we have d0 = h2/λ, making the estimated height lie between 80 km for summer and 97 to 100 km for winter.Previously the value of τ, the mean time between collisions, was found to lie between the limits 0.5 × 10−6 and 0.5 × 10−7 (on the basis of magnetic storm effects). Calculations based on the kinetic theory would imply a height of between 76 and 92 km for these values of τ in fairly close agreement with the above, and there is no necessity to make the rather forced assumption previously made that the electronic mean free path was some 10 to 50 times greater than that calculated from the normal kinetic theory on the assumption that the electron may penetrate the outer rings of the molecules without being appreciably deflected. Apparently this effect occurs only in inert gases, such as helium, and is not likely to occur in the upper atmosphere.With regard to scattering, the glancing angle for transmission, great-circle bearings of the main rays, and the daylight attenuation, the author has found the previous conclusions to be on the whole well-founded.Some modification of the lower wave-length limit for night transmission is required, and the author finds that this lower limit depends on the time of the night and season. Late-night regions behave very differently from early-night regions, thus confirming Appleton's results showing a progressive change in the Heaviside layer during the hours of darkness. The short-wave daylight limit appears to be close to 10m, but sporadic long-distance transmissions have been observed on wave-lengths shorter than this.The interpretation of many observations of short-wave transmission depends upon a previous experimental knowledge of scattering. A discussion of scattering will therefore be given first, and later the general results of long-distance and short-distance direction -finding will be considered. Fading and magnetic storms will next be discussed, then “skip effects”, and finally a general theoretical discussion in the light of the results obtained will be given.