Wide-band aerials and transmission lines for 20 to 85 Mc/s

Abstract

Wide-band aerial and transmission-line systems are described covering the frequency bands 20 to 30 Mc/s, 40 to 50 Mc/s and 50 to 85 Mc/s. These systems are capable of dealing with pulse transmissions of 600 kW peak power. The aerials have vertical polar diagrams such that the major part of the energy is radiated at low angles of elevation, and horizontal polar diagrams either circular or with reduced backward radiation. While the theoretical background of such aerials is well established, the problems of electrical and mechanical design have hitherto received little attention, and some interesting solutions to the special problems encountered are described. The aerial arrays are built up of full-wave centre-fed dipoles of wire-cage construction, with an input impedance of 600 ohms. Openwire transmission lines are used throughout. When more than one dipole is used, branch feeding is employed and 300-ohm 4-wire transmission lines are introduced to eliminate frequency-sensitive matching devices. Wire-mesh reflectors are used to obtain the horizontal polar diagram required, and their effect on dipole impedance is discussed. The problem of the mismatch introduced by the reactance of supporting insulators on transmission lines has been investigated, and a solution is described which is attractive both from the electrical and mechanical viewpoints. Other problems arising from the use of 4-wire transmission lines are reviewed. An exponential transformer is described which simplifies the construction of arrays consisting of four full-wave dipoles, its function being to transform an impedance of 300 ohms to 600 ohms. Reference is made to the use of compensating stubs for increasing the bandwidth of the aerials.