RADIO TRANSMITTER ANTENNAE COMPRISING A PLURALITY OF OPEN-ENDED COAXIAL CAVITIES AND MEANS FOR EXITING THEM WITH PULSED ELECTRON BEAMS

摘要

1,223,845. Aerials; amplitude modulation; radio transmitting apparatus. MINISTER OF TECHNOLOGY. 19 June, 1968 [19 June, 1967], No. 28180/67. Headings H3R, H4A and H4L. A radio transmitter aerial comprises a plurality of tunable quarter wavelength coaxial resonant cavities which are disposed in parallel with their open ends adjacent. Radiating elements mounted on the inner conductors of the cavities protrude from their open ends. Electronic beam excitation means associated with each cavity is pulsed in accordance with the resonant frequency, and the timing of the pulses differs between cavities and is variable by modulation means. As described, two similar resonant cavities 1, 2, Fig. 1, formed of quarter-wavelengths of coaxial line, are arranged close together with their axes vertical, and are short-circuited at their lower ends by a ground plane 24 and by adjustable sets of conductive tapes 15, 16. The upper ends of the cavities are open and radiator elements 3, 4 are mounted on top of their inner conductors 5, 6. In operation, the external surfaces of the outer conductors of the cavities 1, 2 are excited by the said elements like a vertical monopole. A number of electron-beam accelerator-decelerator tubes 7, 8 are mounted radially across the cavities. The electron guns and acceleration regions of the tubes are mounted within the radiator elements 3, 4. High voltage D.C. is supplied through cables within the inner conductors 5, 6, and signals from a modulator 19 are conveyed to beam control units 18 by infra-red rays via mirrors 20, 21. Feed back from subsidary collector electrodes in the tubes is connected by leads to the modulator 19. The radiator elements 3, 4 are contained in a radome 11 which is provided with a series of conductive annuli 12 for suppressing any horizontally polarized radiation. A vertical conductive screen 13 reduces the capacitance between the radiator elements 3, 4. Harmonics of the desired frequency can be reduced by mounting suitable conductive sleeves over the inner conductors 5, 6. Alternatively, the lowest of the conductive annuli 12 (marked 12a) may be connected by an inductance to ground, so that it acts as a tuned short circuit at an harmonic frequency. In another embodiment (Fig. 2, not shown) also comprising two adjacent, vertically disposed, quarter-wavelength, coaxial type resonant cavities (1), (2), sets of electron-beam accelerator-decelerator tubes (7), (8) are arranged vertically in squirrel cage formations between disc-shaped enlargements (30), (31) of central conductors (5), (6) and conductive annuli (34), (35) at the upper ends of said cavities. Modulator units (19a), (19b) are contained within the conductive annuli (34), (35) and beam control units are contained within radiator elements (3), (4). In both embodiments each of electron beam tubes is fed with trains of pulses having a recurrence frequency equal to the frequency of the radio wave to be transmitted. Radio frequency signals of stabilized amplitudes are induced in each cavity which produce a resultant whose amplitude depends on the phase difference between the pulses in one cavity and those in the other. A modulation arrangement for this purpose comprises a pulse generator 51, Fig. 3, supplying two chains of multivibrators 52, 54, 56 and 53, 55, 57 having respective outputs 66, 67 for the two sets of electronbeam tubes. A modulating signal on line 60 controls the phase displacements between the two trains of pulses, and feed-back from the subsidary collector electrodes of the electronbeam tubes on lines 25, 26 is applied through rectifiers and filters 58, 59 to control the durations of the pulses in each train, and hence to stablilize the power in each cavity.