Patent GB-1012738-A (GB)
Title: Helical aerials
IPC: H01Q19/15
Filed: 1962-10-04
Abstract: 1,012,738. Aerials. PATELHOLD PATENTVERWERTUNGS - ELEKTRO - HOLDING A.G. Oct. 4, 1962 [Oct. 6, 1961], No. 37518/62. Heading H4A. Relates to a wide band helical aerial 1, Fig. 1, connected at one end to a feeder 2, 5, 4, with a director disc 3 near that end and connected to the outer 4 of the feeder, in which the mean diameter of the helix 1 is between #/4 and #/3, the diameter of the director disc 3 being less than #/2 so that the aerial radiates in the direction from the open end of the helix to its fed end, i.e. the " backward " direction, as indicated by the arrow in contrast to known arrangements employing a disc of larger diameter acting as a reflector in which case radiation takes place in the reverse direction, i.e. the " forward " direction. With an aerial according to the invention an optimum directivity characteristic can be achieved with a small number of turns, e.g. three to five. The director disc may comprise a wire mesh or a perforated or non-perforate conductive sheet. The mean turn spacing of the helix should be between #/3 and #/15, e.g. #/5, and when a wire conductor is employed its diameter may be between #/20 and #/100; alternatively a strip conductor may be printed on an insulating core or on the inside of an insulating cylindrical housing or the helix may be pressed or cast into an insulating core. The inner conductor 2 of the feeder is connected by a radial piece 6 and this may be inclined at an angle α or it, and part of the first turn of the helix, may be in a plane parallel to the disc 3 with the remainder of the first turn forming a gradual transition to the helix for impedance matching as in Fig. 2 (not shown). In other modifications the director disc forms the end of the outer conductor of a coaxial feeder, Fig. 3 (not shown), a surface wavetrap is positioned between the external surface of the feeder and the director disc, Fig. 4 (not shown), the feeder tapers towards the disc, Fig. 5 (not shown), and the feeder is positioned inside the helix, Fig. 6 (not shown). The aerial may be supported at the focus of a parabolic reflector by means of its rigid feeder extending through the vertex of the reflector, Fig. 7 (not shown). In Fig. 9 a narrow beam is produced by a parabolic reflector 16 fed by a conventional forward wave helix 12 having a reflector disc 14 and a coaxial broader beam is produced directly by a backward wave helix 12<SP>1</SP> having a director disc 13. In a modification, Fig. 10, the reflector 16 is energized by a backward wave helix 12 and the broad beam is produced by a backward wave helix 12<SP>1</SP>, the director disc 13 being common to both helices. In both embodiments the helices are so wound that the two beams are circularly polarized in the same sense thereby decoupling the two aerials and in Fig. 9 the reflector disc 14 also serves as a decoupling screen between the two aerials. In Fig. 11 (not shown) two laterally spaced backward wave helices are energized in antiphase from a balanced feeder, the connections to the helices being offset by 180 degrees. An arrangement in which the aerial 12, Fig. 8, is fed at both ends from the inner conductor 15 of a coaxial feeder and there is provided near one end a director disc 13 to provide a backward wave in the direction shown together with a reflector 14 near the other end to provide a forward wave in the same direction is stated to result in a linearly polarized wave.