Abstract :
The fading-free range of a medium-wave transmitting station is determined by interference between the ground wave and waves reflected from the ionosphere. In order to achieve as great a range as possible, it is common practice to reduce the strength of the reflected waves by using vertical aerials between 0.5 and 0.6 wavelengths high. The degree of success achieved is controlled by four factors: the current distribution on the aerial, the conductivity of the ground, the flatness of its surface, and the nature of reflection at the ionosphere. An experimental investigation into these has included measurements on small-scale models and on a mast-radiator now in service. In the absence of an exact derivation of the current distribution, a semi-empirical solution has been used in computing theoretical radiation patterns. These are in reasonably good agreement with the experimental results, both for perfectly-conducting and imperfectly-conducting ground. Unevenness of the surface of the ground was found to have a more serious effect than had generally been realized. A theoretical treatment of this problem has been verified experimentally in an idealized case, but further investigation is desirable. Another effect that has hitherto received insufficient attention is the diffuseness of ionospheric reflection. This may degrade the performance of an anti-fading aerial, particularly if an attempt is made to achieve a vertical radiation pattern with a sharp minimum.
