GB693435A - Improvements in or relating to radio antenna arrangements - Google Patents

Abstract

693,435. Aerials. ELECTRIC & MUSICAL INDUSTRIES, Ltd. May 7,1951 [May 6, 1950], No. 11259/50. Class 40 (vii). In a dipole aerial arrangement in which one of the dipole elements is arranged as a sleeve round a coaxial feeder centrally feeding the antenna, dielectric material is included in the space between the sleeve and the outer conductor of the feeder so that the rejector circuit formed by said sleeve and conductor is resonant at the resonant frequency of the dipole elements. The dipole elements may have similar electrical characteristics so that the plane containing the maximum of the aerial polar diagram is symmetrical with respect to the elements. The dipole elements 1, 2 may be tubes of equal diameter and fed by a coaxial line 6, 7 led through the element 2 with the outer conductor 6 attached to element 2 as by a metal insert 8 and the inner conductor connected to the adjacent end of tube 1 by a conducting link. The tube 1 may be hollow and closed by an insulating plug 4 or may be made solid. In operation the inside of the element 2 and outside of the cable 6 should form a quarter wavelength line and present a high impedance to currents which would otherwise flow from the conductor to the outside of the cable. However, the dipole elements need to be less than a free space quarter-wavelength long for resonance and, moreover, the insert 8 may be of substantial length for purposes of rigidity so that the line 2, 6 will physically be less than the required quarter electrical wavelength. To compensate for this a block of dielectric material 9 may be inserted in the open end of the line to produce the required electrical length. If the antenna is designed for use at two slightly differing frequencies the annular space is tuned to one frequency and one or two earthing bars 14 provided tuned to the second frequency.