Axial ratio improvement of an Archimedean spiral antenna over a radial AMC reflector
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Axial ratio improvement of an Archimedean spiral antenna over a radial AMC reflector M. Grelier · C. Djoma · M. Jousset · S. Mallégol · A.C. Lepage · X. Begaud
Received: 29 February 2012 / Accepted: 10 October 2012 / Published online: 10 November 2012 © Springer-Verlag Berlin Heidelberg 2012
Abstract This paper presents a method to improve the circular polarization of an Archimedean spiral antenna placed over a radial Artificial Magnetic Conductor (AMC). Results have been compared with the same radiating element over a more classical AMC reflector. A prototype of an Archimedean two-wire spiral antenna has been built to operate from 0.5 GHz to 6 GHz. Measurement results with this radial AMC give a relative bandwidth of 79 %, in which the broadside RHCP gain is improved. In this bandwidth the axial ratio of Archimedean spiral antenna placed over a radial AMC is less than 2 dB whereas it is higher than 3 dB with a classical cartesian shape of AMC reflector.
1 Introduction The needs of the current wireless applications, both civilian and military, require antennas that are at the same time, low cost, low thickness and broadband. Planar spiral antennas are widely used to fulfill the aforementioned specifications. An Archimedean spiral antenna radiates a circularly polarized bi-directional beam. In most applications, this bidirectional beam must be transformed into a unidirectional one. A classical solution consists of backing a cavity filled by an electromagnetic absorber behind the radiating element. However, the antenna is bulky and loses one half of the radiated power. M. Grelier · C. Djoma () · A.C. Lepage · X. Begaud Institut Mines-Telecom, Telecom ParisTech—LTCI CNRS UMR 5141, 46 rue Barrault, 75634 Paris Cedex 13, France e-mail: [email protected] M. Grelier · C. Djoma · M. Jousset · S. Mallégol Thales Systèmes Aéroportés, 10 avenue de la 1ère DFL, 29238 Brest Cedex 3, France
Another approach consists of taking advantage of the backward radiated electric field by reflecting it in-phase with the forward radiated electric field thanks to an Artificial Magnetic Conductor (AMC) [1]. In [2], a reflector based on AMC called Quasi Artificial Magnetic Conductor (QAMC) has been presented. The term “quasi” has been chosen because the QAMC is composed of a few elementary cells. In this paper we present the improvement of the Cartesian QAMC by a reflector suitable to the geometry and the radiation of the Archimedean spiral antenna. The spiral antenna is presented in Sect. 2. The method to design the QAMC is introduced in Sect. 3. In Sect. 4 the spiral antenna is placed over the two different QAMC and compared in order to validate the new design. Then measurements are presented and discussed.
2 Archimedean spiral antenna The two-wire Archimedean spiral antenna introduced by Kaiser [3], is widely used in airborne systems due to its wideband intrinsic characteristics. This kind of antenna has active areas which depend on the frequency. Thus, these areas are defined by D = λ/π , where D is the diameter of
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