Patent GB-621839-A (GB)
Title: Improvements in and relating to directive antenna systems
IPC: G01S1/02
Filed: 1944-11-01
Abstract: 621,839. Directive radio systems; radio navigation; modulation systems; keying carrier frequencies. SPERRY GYROSCOPE CO., Inc. Nov. 1, 1944, No. 21413. Convention date, Nov. 1, 1943. [Classes 40 (v) and 40 (vii)] A directive antenna system comprises means for radiating two overlapping narrow - angle beams in two directions making a small angle with each other, together with means for radiating two relatively wide - angle beams in directions making a relatively large angle with one another and on opposite sides of the overlap of the narrow beams. The wide-angle beams are not radiated simultaneously with the corresponding narrow-angle beams but have similar frequency compositions. The narrow-angle beams can be made very sharp to accurately define the equisignal course line and the wideangle beams provide a reasonably strong and correct signal at some distance off-course despite any subsidiary lobes on the narrow-angle beam patterns. If the wide-angle beams are cardiods (Fig. 12, not shown), correct indications may be given at all azimuthal angles. The radiating means may comprise a paraboloidal reflector 21 divided by means of a partition 22 into two separate radiators 1 and 2, which may be energized by facing wave-guide mouths alongside the partition. The partition 22 may comprise two portions in parallel planes equidistant on either side of the focus, in which case the feeding wave-guides 25, 26, may pass between the portions so that only their mouths project over the apertures of the parts 1 and 2, (Figs. 17 and 18, not shown), minimizing side-lobes caused by reflections from the exterior of the wave-guides. In other variants, the centre piece of the reflector, equivalent to that part between the portions of the partition in the embodiment just described, may be removed and the radiators 1 and 2 again brought side-by-side, Fig. 16 (not shown); or the lower halves of each portion of the reflector modified in shape in order to direct energy upwardly with respect to a line parallel with the generator of the paraboloid. Radiators 1 and 2 are used to provide the two narrow-angle beams and further auxiliary radiators 3 and 4 of parabolic shape and an aperture equal vertically to the diameter of reflector 21 and a width suitable for wide coverage are utilized for the wideangled beams. These may be set at an angle to reflector 21 and fed directly by wave-guides 41 and 42. If the system is used to define an approach path the whole antenna system may be tilted at an angle corresponding to an associated glide path. Radiators 3 and 4 are not suitable for radiating the cardiod patterns previously mentioned and to radiate these, two vertical stacks of horizontal bent dipoles with half wavelength spacing may be used (Figs. 13 and 15, not shown). These dipoles are fed in alternate phase by means of concentric lines 75 (Fig. 14), whose centre conductors 77 extend as probes into a feeding wave-guide 74. Keying and modulating system. To provide the two modulation frequencies required to characterize the beam radiations, energy from the carrier frequency oscillator is fed through branch wave-guides 46 and 47 to resonant chambers 41, 42, 43 and 44. Coupling between the branches and chambers may be by means of concentric lines, each having one terminal extending into the branch and a coupling loop in the chamber. Each chamber has are-entrant portion 53 which co-operates with the periphery of modulator wheel 54 or 55, both of which are driven at constant speed by motor 57. Teeth are provided on a part of these wheels and it is arranged that the chambers resonate when the troughs of the teeth are opposite the portion 53 and are detuned at a peak. Probes 66 connect the chambers with waveguides 61, 62, 63, 64, feeding radiators 1 to 4 inclusive. If wheels 54 and 55 have cut-out teeth over a quarter of their periphery (Fig. 8), the teeth on the two wheels being of different pitch, antennµ 1 to 4 will be energized cyclically for non- overlapping periods and the modulation of antennµ 1 and 3 and 2 and 4 will be different so providing the required beam characterization. If the wheels have teeth over a half of their periphery, antennµ 1 and 2 and antennµ 3 and 4 will be energized simultaneously (Fig. 9, not shown). In such a case interference patterns may be set up by the sources operating in unison. It is thought however that in the cases of antennµ 1 and 2 these may not be serious because the wave-guide mouths which act as sources are close together. Fig. 10 illustrates a modification in which each chamber has its own co-operating wheel and antennµ 1 and 2 are energized for half the cycle and antennµ 3 and 4 for non-overlapping portions of the other half of the cycle. Specification 550,660 is referred to.