GB600327A - Improvement in directive antennae - Google Patents

Abstract

600,327. Aerials. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 17, 1945. No. 18341. Convention date, Jan. 22, 1944. [Class 40 (vii)] A directive antenna system for operation over a band of frequencies comprises a parabolic reflector having a focal length F substantially equal to one-fifth of the wavelength at the lower limit of the frequency band and a broad-band radiator element with its centre disposed substantially at the focus of the reflector. The width of the mouth of the reflector is preferably about twice the wavelength corresponding to the low-frequency limit of the band. As shown in Fig. 1, a broad-band radiator 5 has its centre located near the focus of a cylindrical parabolic reflector 1 having one end wall 3 and the apex portion 9 formed of perforated sheet metal. The wing portions are formed by parallel wires 11 secured to tubular frame members defining the edges of the reflector. The wires and perforated sheets may be replaced by solid sheets. The broad-band reflector 5, Fig. 2, formed as a conductive cylinder whose length (L) and diameter (D) are approximately equal, is supported by a coupling member 23 having an outer conductor 25 carrying a plate 27 which is secured to the wall 3. The coupling member 23 has a member 19 secured to the bottom wall 17 of the radiator 5 and in contact with an inner conductor 21. The lower end of the coupling member is secured to the end of a coaxial cable (not shown) by a complementary coupling device. For a system designed to operate over a frequency range of 350 to 725 megacycles, the optimum focal length is seven inches, i.e. one-quarter of the wavelength at 423 megacycles and 0.208 of the wavelength at 350 megacycles. Suitable dimensions for the width of the mouth of the reflector and for the broad-band radiator 5 are also quoted.