GB651812A - Improvements in or relating to aerial systems - Google Patents

Abstract

651,812. Aerials; coupling arrangements. MINISTER OF SUPPLY. Sept. 3, 1948, No. 23227. [Classes 40 (v) and 40 (vii)] An annular slot aerial comprises a plane conduction surface formed with an annular slot backed by a conductive box enclosing a ' cavity of annular cross-section, coupling with the aerial being effected by one or more loops encircling the magnetic field within the cavity, each loop being formed partly by a conductor projecting into the cavity and partly by the walls of the cavity. As shown in section. in Fig. 1, the metal skin 3 of an aircraft has a circular hole enclosed at the rear by a metallic spinning 6 having a central column 5 supporting a disc 4 in the plane of the skin so forming an annular slot 1 backed by an annular cavity 7. Coupling with the cavity is effected by four loops spaced apart by 90 degrees and fed in phase by a network of coaxial cables, Fig. 2 (not shown). Each loop comprises an extension a-b of the inner conductor of a coaxial terminal fitting 9, a series condenser c formed between a mushroom head on the extension a-b and the adjacent surface of disc 4, a, part b-c of the disc 4, a part c-d of one side wall of the cavity and a part d-a of the bottom of the cavity. By employing four loops a uniform polar diagram in azimuth is obtained. The vertical distribution follows a cosine law when the slot diameter is made less than one wavelength and varies with the diameter for larger slots. With a single coupling loop the polar diagram becomes asymmetrical, and two loops disposed at radially-opposite points may be energized alternately to produce a split beam polar diagram. Alternative coupling loops comprise conductors extending radially from the central column and then downwardly and rearwardly to the back of the cavity. The central column may contain a variable matching condenser through which the loops are fed. In a detailed construction described, suitable for use over a broad band of frequencies, the slot diameter lies between one half and one wavelength throughout the band. In an embodiment described in the Provisional Specification two loops are employed, the condensers c omitted and the conductors extended to connect with the cavity. This embodiment has dimensions producing a narrow-band characteristic.