Ultra Low Multi-Path Antennas
an ESA Project (G00/B31.EEA-200)
An ESA project (2003 - 2006): GAPS is the development of a dedicated planar antenna for GALILEO, the future European satellite-based positioning system, which is currently in its testing phase. The main application of the antenna is high accuracy positioning, such as required for geodetic applications. In the past, choke ring structures were used to reduce the effect of multi-path. These devices are usually bulky and rather heavy (around 5 kg). Yet, with the advent of new technologies in the microwave region, like
Electronic
Band
Gap (
EBG) materials, it becomes possible to design more compact and light-weight antenna front-ends. Here, the design of a planar antenna for GALILEO receivers based on EBG-technology is presented.
The application considered here is high accuracy geodetic positioning systems. In this case, the reduction of multi-path becomes a key issue, as multi-path contributions have a direct influence upon the accuracy of the received GALILEO signal. A Right Hand Circular Polarised (RHCP) satellite signal is reflected by the surface of the Earth. Its polarisations is thus changed into LHCP (Left Hand Circular Polarised). This reflected signal arrives at the backside of the antenna, causing multi-path interference.
A solution to this problem is reducing the back radiation of the antenna by means of shielding. However, if a simple metal plate is used, it strongly influences the radiation pattern and phase centre stability of the antenna. For this reason, choke rings are generally applied.
To avoid the use of choke rings, it is also possible to employ alternative technologies such as EBG structures, in order to reduce the back radiation. The principle of an EBG substrate is: A metal plate is use for shielding the antenna element from the indirect satellite signals. This plate is covered with EBG cells on the antenna side, to suppress the propagation of surface waves. Thus, the influence on the radiation properties of the antenna of the metal ground plane and its edges is largely reduced.
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