Patent GB-773297-A (GB)
Title: Suppression of vertically polarized radiation from an omni-directional range antenna system
IPC: H01Q3/14
Filed: 1955-09-26
Abstract: 773,297. Aerials radio navigation. COLLINS RADIO CO. Sept. 26,1955 [Nov. 16, 1954], No. 27381/55. Class 40(7) An aerial system for radiating a rotating cardioid pattern of horizontally polarized radiation consists of a short rotating horizontal dipole mounted centrally and symmetrically inside a vertical slotted. cylinder, the slots 36 of which are equally spaced round the cylinder and energized so that, by themselves, they would provide omnidirectional radiation. The cylinder is mounted above a circular ground plane 23 and has unperforated upper and lower portions 37 and 38 of axial dimensions A and B respectively chosen so that B is less than A and vertically polarized radiation is suppressed in and to either side of a horizontal plane. The aerial is preferably mounted on a building 20 in which the transmitting equipment may be housed and a spacer 32 of insulating material introduced between the slotted cylinder and the ground plane. The slots 36 are energized by a coaxial feeder 41 which is led up inside the cylinder between two slots and has its outer conductor 43 connected to the metallic domed end 34 of the cylinder and its inner conductor 44 divided at 46 and each of the feed wires 47 led in insulated manner half-way down one side of a slot and then connected to the centre of the opposite side of the slot. The dipole antenna 51 is mounted for rotation at the top end of the shaft of a synchronous motor 53. An enclosure 54 between the motor and the dipole contains an unbalance to balance transformer, a pair of rotary coupling condensers, and series loading inductances, coupling the coaxial feed cable 56 to the dipole. As the diameter of the cylinder is less than half a wavelength, the dipole is considerably less than this in length and requires the inductive loading. Rods 24 are preferably mounted round the ground plane and made to project an' adjustable amount (Fig. 5. not shown) so that the effective diameter of the ground plane can be accurately controlled. It is pointed out that there is a certain amount of coupling between the conductive slotted cylinder and the dipole which results in the latter inducing currents in the former such as are shown at 93, 94. These result in the radiation of vertically polarized energy in a figure-of-eight pattern at right-angles to the main horizontal polarized figure-ofeight pattern of the dipole. To avoid radiation of vertically polarized energy over a considerable vertical angle about the horizontal plane the axial dimensions A and B of the unslotted parts at the top and bottom of the cylinder are so chosen as to cause, in conjunction with the ground plane, cancellation of the energy from the upper and lower parts. For this purpose A has to be made greater than B by a small amount. The aerial system may be used in a rotating cardioid pattern radio beacon, energy being fed to the dipole and slots from the same transmitter with suitable phase adjustment. The energy radiated by the slots is usually frequency modulated with a reference signal which when compared at a receiver with the amplitude modulation caused by the rotation of the cardioid pattern enables the receiver bearing from the beacon to be determined.