Patent GB-1415599-A (GB)
Title: Radio frequency antenna system
IPC: G01S1/50
Filed: 1973-02-09
Abstract: 1415599 Aerials; radio-navigation E-SYSTEMS Inc 9 Feb 1973 [9 Feb 1972] 6452/73 Headings H4A and H4D An aerial system comprises first and second radiators spaced apart along an axis and coupling means between them and a feed point, whereby they are fed with respective currents between which there is a phase difference. The system also comprises a first plurality of spaced apart parasitic elements positioned radially from the axis of the first radiator and a second plurality of spaced apart parasitic elements positioned radially from the axis of the second radiator. The radius on which the second plurality of parasitic elements is spaced is the same as the radius on which the first plurality of parasitic elements is spaced. In use, there is a phase difference between the currents generated in the two pluralities of parasitic elements. As described, a TACAN beacon aerial comprises a central fixed array of two co-linear vertically disposed half-wavelength dipoles 82, 84, Fig. 3, which are fed from an RF input connector through a power divider/phaser (94), Fig. 4 (not shown) in such a manner that the peak (96) of the main radiation lobe is tilted up, for example by 30 degrees, from the horizon. Two adjacent split vertical parasitic elements 98 are mounted to rotate around the dipoles 82, 82 at 900 r.p.m., and provide the 15 Hz modulation. Nine equally-spaced split vertical parasitic elements 100 are mounted to rotate with the parasitic elements 98, and they provide the 135 Hz modulation. In oder to further improve the uptilt of the main radiation lobe, each of the parasitic elements 98, 100 comprises an upper part and a shorter lower part, so that the current flowing in the former is retarded in phase with respect to that flowing in the latter. Also rotating with the parasitic elements is a metal disc 102 having sets of radial slots open on three diameters. Light emitting diodes 104, 106, 108 illuminate photosensitive transistors 110, 112, 114 to produce trigger signals for the 15 Hz and 135 Hz modulations and a 1350 Hz signal for generating a beacon identification pulse train. As shown in Fig. 2, amplified trigger signals are distributed via a control/transfer unit 16 to dual receiver/transmitter units 24, 26. The 15 Hz and 135 Hz trigger signals are additionally distributed to dual monitors 28, 30, which also receive signals from a monitoring receiving aerial 18 via a divider circuit 70. Each monitor 28, 30 includes a test interrogation generator 76, a signal analysis and mode stepping module 78, and a display module 80. The monitors are used together, and if both sense a failure of the receiver/transmitter unit in use, a transfer switch 52 is operated to substitute the other unit. Each receiver/transmitter unit contains a duplexer 60, a function module 58, an encode module 62, and a transmit module 66. Received interrogating signals are applied to the module 58, which provides output pulses to the encode module 62. The trigger signals are also received by the module 62, which develops pulse trains for modulating the signals from the transmit module 66. In one embodiment an assembly comprising a central support tube 156, Figs. 6, 7 and a drum 154 is rotated by a motor 44 which is contained within a fixed housing 134. The tube 156 and the drum 154 are made of thin dielectric material. The parasitic elements 100 are wires which are fixed by an adhesive to the outside of the drum. Alternatively, they may be mounted on nylon or other dielectric material filaments which are stretched between spacers projecting from the drum 154 (Fig. 9, not shown). A two-part fixed radome 126a, 126b encloses the rotating assembly. The parasitic elements 98 are secured inside the tube 156 by means of filaments 159 extending between collars 160. The fixed aerial array assembly extends centrally within the tube 156 and comprises two dipoles with respective radiating skirts 176, 178 and 180, 182 which are contained within a dielectric material tube 172, and which are supplied by a co-axial transmission line 166 through RF chokes 168, 170. A secondary transmission line 174 is arranged about the main transmission line for feeding said radiating skirts. At the top of the tube 172 a cavity 188 is filled with RF enrgy absorbing material.