GB724795A - Improvements in and relating to antenna structures - Google Patents

Abstract

724,795. Aerials. GENERAL ELECTRIC CO. Feb. 5, 1953 [Feb: 13, 1952], No. 3274/53. Class 40 (7). An antenna comprises two helical conductors 3 and 4 extending from a common energizing point in opposite axial directions but with similar rotational directions about a substantially straight hollow conductor 2 so as to be right- and left-hand helices when viewed in the same axial direction. A predetermined radial spacing is provided between the helical and straight conductors and the antenna is fed between the common energizing point and the conductor 2. To give an omnidirectional coverage perpendicular to the axis and a directive pattern in planes including the axis, each turn of the helical conductors is made two or more integral wavelengths round. It is pre. ferred to make the axial length of each turn a half-wavelength although this may be varied if it is desired to tilt the direction of maximum radiation in the axial planes. The helical conductors may be maintained in position by insulating brackets 6 spaced at any suitable distance apart but preferably every threequarters of a wavelength along the aerial. They form with the cylindrical conductor a leaky transmission line and are made of sufficient length that there is negligible reflection at their ends. The radial spacing between the conductors 3 and 4 and the hollow conductor 2 largely determines the characteristic impedance of the line and it is pointed out that a large number of turns on the helices will adversely affect the naturally good broad-band characteristics of the aerial. The radial spacing between the helical conductors and the cylindrical conductor 2 need not be constant but may be varied along the length of the antenna to change the current distribution and so modify the radiation pattern. A change of impedance is also obtained by such variation. The antenna shown is mounted on a supporting plate 5 and fed by means of the transmission line formed by conductor 7 and the inside of conductor 2. Coaxial feeder 16 feeds this line at one end and is matched to it by tuning probes 17 and 18 while the conductors 3 and 4 are capacitatively coupled to the line by probe 24 supported in insulator 26 and coupled to plate 25 to which the conductors 3 and 4 are secured. The transmission line 7-2 is matched to the antenna and the section beyond the point of insertion of probe 24 by quarter-wave matching transformer 28 and the amount of energy fed to the antenna adjusted by varying the position of probe 24. Stub 27 is provided so that the impedance of the aerial may be made resistive. To increase the directivity in the axial planes a number of similar antennµ may be stacked to form an array with their conductors 2 either joined together or integral with one another. Each may be energized by a probe from the line 7-2 which may be terminated a quarter-wavelength above the point of energization of the end antenna. The helical conductors may be coupled together as indicated at 23 at the top of Fig. 1 and the uppermost end connected to the tube 2. A heater current for de-icing can then be passed through all the helical conductors in series; with its return path through tube 2. If conductor 7 is made hollow a central conductor 40 may be provided to carry lighting current to a signal lamp on top of the aerial; the return path for the current may also be through tube 2. Rotation of some elements of the array through 360 degrees divided by the number of wavelengths in a helix turn as compared with other elements may be used to compensate for small variations in the omnidirectional field plane pattern.