A Broadcast Antenna for "Low Angle" Radiation

The present trend in broadcast engineering is to build transmitting antennas with reduced radiation at high angles of elevation. This increases the coverage of transmitters, within which no appreciable amount of "short range fading" takes place. A novel antenna of this kind is described below. It consists of a vertical radiator combined with a horizontal shield located near its top. In this paper both an experimental and a theoretical investigation of such an antenna are presented. The experiment was performed on a small scale model at a wavelength of five meters. A number of different antennas were tested having shields of various diameters located at various heights above ground. Their vertical plane field patterns were compared with the pattern of a vertical wire, a quarter of a wavelength long. The experiment, as well as the theory, indicates that the current in the shield must be properly controlled in order to obtain the desired field pattern. Practically, this condition is met if the radius of the shield is such that the latter oscillates at its natural wavelength (current node in the center of the shield), and if the shield has a certain height above ground. Both from experiment and theory it is found that good results are obtained, even if the height is less than a quarter of a wavelength, which, from the point of cost, is a very important fact.