GB1311620A - Slot antenna - Google Patents

Abstract

1311620 Aerials AMPEX CORP 1 Oct 1971 [12 Oct 1970] 45850/71 Heading H4A An aerial comprises a slot having a length equal to an odd number, greater than one, of half-wavelengths at the operating frequency. A transmission line is connected to the slot at a point along its length, and means are provided for causing all radiation relative to said aerial to be associated only with sections of said slot of like field polarity without significantly affecting the phase velocity of signals as they pass between the said point and all other points along said slot. As described, a single continuous and narrow slot 12, Fig. 1, is formed in a planar sheet 10. A coaxial feed cable 18 is connected at the midpoint of the slot, which has a length of five halfwavelengths at the operating frequency. The central and end half-wavelength sections 12a, 12c, 12e of the slot are of one polarity, whilst the intermediate half-wavelength sections 12b, 12d are of the opposite polarity. U-shaped sheet metal half-wave chokes are provided along the sections 12b, 12d which effectively prevent radiation therefrom in the forward direction 22, while not interfering with power transmission to the sections 12a, 12b. A further half-wave choke 28 over the whole length of the slot prevents any radiation in the backward direction. In a second embodiment (Fig. 6, not shown) co-linearstraight half-wavelength sections of a continuous slot alternate with folded or U-shaped halfwavelength sections. The straight sections are of one polarity and radiate constructively, whilst there is little or no radiation from the folded sections, which are of the opposite polarity. In a third embodiment (Fig. 5, not shown), a transverse slot is interposed between each adjacent pair of half-wavelength sections of a single straight continuous slot. Said transverse slots are symmetrical about the axis of the main slot, and each has an overall length of a half-wavelength. At each intersection, opposite comers are connected by diagonal wires or bars. In this way, all half-wavelength sections of the main slot are made of the same polarity. A fourth embodiment features a radiating structure which is generally similar to that shown in Fig. 1. A continuous slot with a length of five half-wavelengths is defined by the aperture of a choke 52, Figs. 3, 4, and two forward facing chokes 56a, 56d are provided over the halfwavelength sections adjacent the central halfwavelength section 48. The structure is mounted upon a flanged vertical pipe or mast 40, the choke 52 being displaced to form an obtuse angle with the chokes 56 for this purpose, and a bracket 64 being provided. A coaxial fed cable 68 is run inside the mast 40, and is connected to the centre of the slot section 48. Radiation from the aerial is horizontally polarized, and in order to obtain an omni-directional radiation pattern, or one of another required shape, a pair of directors 70, 72 is associated with each radiating section of the slot. These directors are symmetrically arranged at the rear of the mast 40, and each comprises a pair of plate, mesh, or skeleton members 74 mounted on a rod 78 which extends from a bracket on the mast. A fifth embodiment, Fig. 7 (not shown), features a radiating structure similar to that described above with reference to, Fig. 5 (not shown), which is mounted on a vertical mast. The sheet containing the main and transverse slots is folded towards the mast so that it has a generally V-shape, and a coaxial feed cable is connected to the centre of the lowest half-wavelength section of the main slot. Directors are provided, as in the previous embodiment, except that there are common elements, extending along the structure, instead of separate elements for each radiating halfwavelength section of the slot.