Bidirectional Circularly Polarized Microstrip Antenna for GPS Applications A.Z. Narbudowicz #1 , X.L. Bao #2 , M.J. Ammann #3 # Antenna & High Frequency Research Centre Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland 1 adam.narbudowicz@student.dit.ie 2 xbao@dit.ie 3 max.ammann@dit.ie Abstract— A bidirectional circularly polarised (CP) antenna for 1.575 GHz GPS L1 band is presented. The antenna consists of two back-to-back coupled patches and is fed directly by 50 Coplanar Waveguide (CPW). It is compact and relatively easy to manufacture compared to other antennas with similar properties. The bidirectional CP pattern is achieved by proper adjustment of patch dimensions as well as feed parameters. A study of the key parameter is provided. I. INTRODUCTION In recent years the use of the GPS system is becoming more popular. Despite this growth, there is a limited number of circularly polarised (CP) antennas capable of receiving signal independently of their orientation; in most cases good axial ratio (AR) is achieved either for just one direction, or, if bidirectional, one side radiates right-hand and other left-hand CP. An exception to this state is [1], where a bidirectional antenna for RFID systems is presented, radiating LHCP on both sides. The antenna is fed using a quarter-wave transformer, resulting in oversized dimensions and a large ground plane (90 x 60 mm for 4.39-4.47 GHz). Another interesting design is presented in [2]. The antenna consists of square ring radiator with integrated Wilkinson divider and phase shifter. It is fed with a 50Ω probe. Recently some work has been carried out on omnidirectional CP antennas. A method involving meta- materials is described in [3]. The horizontal component is generated by four quarter-wave stubs and the vertical component by a mushroom structured middle patch. A different approach employing two back-to-back microstip patches with truncated corners is reported [4]. Despite its simplicity, the performance of this antenna is described by authors as almost omnidirectional with an AR better than 4 dB. Another type of omnidirectional antenna which has been successfully applied to the GPS system is the linearly polarised antenna. A case study on commercially available state-of-art unidirectional GPS antennas was performed with the use of U- Blox EVK-5 GPS/Galileo receiver. Each antenna was studied in three cases: with the main beam pointing towards sky (case A), towards ground (case B) and towards horizon (antenna on its side, case C). For most investigated antennas the drop of carrier to noise ratio (CNR) between cases A and B is around 8-9 dB (see table 1). However the drop between cases A and C was only around 1-2 dB. This proves usefulness of bidirectional GPS antenna, since it prevents the occurrence of case B. Please note, that for case C (which is still possible for bidirectional antenna) the CNR drop can be easily kept below 3 dB. Antenna Carrier to noise ratio [dB] Case A Case B Case C 50 mm circular patch on ceramic/woven glass 39.53 30.43 38.47 25 mm square patch on ceramic 33.36 24.15 31.15 active patch antenna 41.81 33.85 39.06 Table 1. Carrier to noise ratio of different state-of-art unidirectional GPS antennas. In this paper we present a bidirectional circularly polarised GPS antenna. It is fed directly from 50Ω coplanar waveguide (CPW), which allows easy integration with microelectronic components. The antenna radiates right-handed circular polarisation (RHCP) towards its front and back directions (θ = 0 O and θ = 180 O ). This property allows tracing small objects (like bags or mobile phones), where the orientation of antenna is unknown or may vary with time. Since reflected RHCP GPS signals are left-handed circularly polarised (LHCP), any possible distortion due to reflection is strongly suppressed with good cross-polarisation properties. It is sometimes believed, that good GPS antennas should have a pattern rejecting signals from low angles below 5 O above horizon. The reason for this is that such signals are more sensitive to ground reflection and therefore might cause inaccuracies. This rejection is however unnecessary, since for GPS applications the angular coordinates of each satellite are generally known and such unwanted signals can be easily rejected in the signal processing. II. POLARISATION ISSUES FOR GPS SYSTEM 978-1-4244-7306-9/10/$26.00 ©2010 IEEE 2010 Loughborough Antennas & Propagation Conference 8-9 November 2010, Loughborough, UK 205   978-1-4244-7307-6/10/$26.00 ©2010 IEEE 