Patent GB-924145-A (GB)
Title: Improvements in or relating to antenna structure
IPC: H01Q11/08
Filed: 1959-05-12
Abstract: 924,145. Aerials. GENERAL ELECTRIC CO. May 12, 1959 [June 4, 1958], No. 16205/59. Class 40 (7). An omnidirectional aerial system consists of a linear central conductor 20, a first spiral conductor 42 wrapped round it and extending from a first point 22 along the length of the central conductor towards one end thereof, and a second spiral conductor 44 extending round the central conductor in the same sense from a second point 24 between the point 22 and the other end towards said other end of the central conductor. Energisation of the aerial is at the points 22 and 24 by coaxial lines 32 and 34 whose inner conductors are connected to the respective spiral conductors and whose outer sheaths are connected to the central conductor 20. A tubular insulator 21 surrounds the central conductor and insulates the spiral conductors and the connections thereto. A number of such systems may be stacked end to end to improve the polar diagram in planes containing the axis of the conductors 20. The conductors 42 and 44 preferably each have an axial length of between one-and-a-half and five wavelengths and the helix diameter and pitch is made such that the circumference of each turn is an integral number of operating wavelengths. Their outer ends may be free or in contact with the central conductor as little energy reaches the extremities of the system. The radial spacing between the central conductor and conductors 42 and 44 determines the characteristic impedance and the radiation per unit length. It must be related to the helical pitch in such a way that the coupling between a turn of helical conductor and the central conductor is greater than the coupling between adjacent turns of helical conductor. If this were not so energy would propagate along the aerial as along a coaxial line instead of along a helical path. Typically the radial spacing may be 0.1 #, the helical pitch 0.5 #, the helical turn length 2 #, and the number of turns per helix 5. Energization.-It is explained with the aid of vector and other diagrams (Figs. 4 to 9, all not shown) that energization with equi-strength and equi-phase signals at the feed points 22 and 24 gives radiation of horizontal polarization; energization with equi-strength but oppositely phased signals gives vertically polarized radiation. The aerial may be arranged to radiate independent signals of different frequencies, one vertically and one horizontally polarized, at the same time. This is done by coupling the two transmitters 60, 62 to a hybrid junction 64. Signals from transmitter 60 appear in the same phase over line 74 and lagging by 90 degrees over line 71. Those from transmitter 62 appear with the same phase over line 71 and lagging by 90 degrees over line 74. A 90-degree phase retarder 66 is coupled between line 71 and aerial feeder 72. Thus signals from transmitter 60 are radiated vertically polarized and those from transmitter 62 are horizontally polarized. If the transmitters are combined and signals of the same frequency are fed to the inputs of the hybrid junction, elliptically, or with suitable equality of constants, circularly, polarized radiation may be radiated.