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A ring arrangement of four wires near to the excitation point of a quadrifilar helix antenna can be used to achieve an impedance match. The matching technique is applicable to resonant, fractional turn antennas, and to shaped conical beam backfire mode antennas.
Introduction: Resonant, fractional turn quadrifilar helix antennas as described by Kilgus [1], are broad beam circularly polarised radiators that are used extensively with, for example, small satcom terminals on boats and vehicles [2] and for GPS applications [3]. The backfire mode quadrifilar helix antenna, also described by Kilgus [4], is a related type of radiator with a different geometry, which exhibits a shaped-conical circularly polarised beam. With both of these general types of quadrifilar helix structure, except for electrically small ones, antenna radiation resistance is of the order of 100-300 O. In practice, feeder systems for these antennas can be made in the form of a pair of axially located orthogonal baluns with impedance transformation [5], usually to a 50 O coaxial transmission line system. An alternative arrangement is with the feeder lines being part of the helical radiating structure, using coaxial cables or microstrip lines and with 'frequency independent' baluns at the excitation points [6]. With either of these design approaches, it can be difficult to achieve an acceptable impedance match. A technique for achieving impedance matching without the need for impedance transformation in the antenna feeder system has now been devised, and is described here.
Impedance matching arrangement: Fig. 1 shows the matching...