Dual-band microstrip antenna for a robust navigation receiver

Abstract

The design of robust navigation systems requires the receiver to be nearly immune against interference coming from several different sources, such as hand-sets and base stations for mobile communications, and military radars. One way to minimize the effects introduced by such interferers is to place pass-band filters with low insertion loss between the antenna and the low-noise amplifier (LNA). In order to increase robustness, the antenna itself can be designed to operate only in the desired bands, so that only low power levels in the out-of-band region are received. In order to receive the signals in the E5a-E5b and L1 bands of the European Galileo system, a dual-band antenna may be used. Several topologies presenting multi-band characteristics have been already proposed in the literature. In some of the cases, the antennas do not present strong rejection in the out-of-band region. Moreover, most of them present only one output, where the signals received in all desired bands are present. For navigation systems, circularly polarized (CP) antennas with high polarization purity should be employed in order to reduce the positioning errors caused by multi-path. One way to obtain CP microstrip antennas is by using geometry-perturbation techniques, which is effective only for very narrow-band applications. One of the drawbacks is that the CP purity is strongly dependent on the fabrication tolerances. Another way is by designing microstrip patches that operate with two orthogonal modes simultaneously and with its outputs connected to power splitter that introduces a 90° phase shift between its two outputs. For dual-band CP antennas, this splitter is normally optimized at a frequency between both desired bands. For this reason, the CP purity of the antenna is not optimum in the bands of interest, since the performance of the splitter degrades for frequencies other than the one for which it has been optimized. This paper presents a novel microstrip antenna capable of separating the E5A-E5b and the L1 bands of the European Galileo system in two different isolated ports. With this feature, one 90°-hybrid is needed in each port to receive right-handed circularly polarized (RHCP) waves in each of the aforementioned bands. For this reason, the hybrids can be optimized to operate in the center frequency of each band, hence resulting in an improvement of the axial ratio. Moreover, since the antenna separates already the signals received at each frequency band, the two bands can be processed separately already right after the antenna. Since no diplexer is needed to separate the signals received in the E5a-E5b and L1 bands, volume, weight and cost are reduced in comparison to conventional solutions. In the final version of the paper, the geometrical details of the proposed antenna will be depicted. Simulated and measured results will be shown and discussed. The simulations performed up to now show that the radiation pattern presents nearly the same shape in the two frequency bands of interest. Moreover, due to a proper selection of the employed dielectric materials, the radiation pattern obtained is broad, which is important for receiving the signals coming from satellites passing near the horizon. At the zenith (90° of elevation), the simulations showed that the antenna presents a gain larger than 4 dBi. For an elevation angle of 5°, the computed gain is larger than -5 dBi. Further details about this antenna will be discussed in the final paper and in the conference