GPS antenna design

C. DUMONT, engineer student, ENSEEIHT, 1999-2000

The aim of this project is double: a bibliography on Antennas for GPS applications(spiral, planar, etc.) and the design of a planar antenna with the following characteristics :

resonance frequency : 1.5754 GHz;right handed circular polarization;VSWR<2;50 Ω adaptation;Gain > 10 dBi.

To realize the patch antenna, we use Depron as dielectric. So, with this dielectric andthe resonance frequency given above, the theoretical patch dimension is 90mm.

To excite the metallic patch, three methods exist: by probe fed, by microstrip line andby coupling through the ground plane. The latter method is chosen because it allows an easypatch association.

To obtain the circular polarization, a Wilkinson power divider is designed. The π/2phase difference between the two Wilkinson output ports is obtained by a length of lilne.

To calculate the dimension of the iris on the ground plane, we only have the formula:ew6lmin +=

with w the width of the exciting line and e the dielectric thickness.Many simulations of a patch excited by a single microstrip line (i.e. in linear polarization) ismade in order to obtain correct dimensions.

The simulated structure is represented on figure 3.

Figure 3: Simulated structure.

The first simulations don't give good results, so the resonance frequency had beenshifted to 1.5754 GHz by modifying the patch dimensions. Now, his length is 87 mm. Weobtain the following results:

Figure 4: S11 parameter of the antenna.

Figure 5: VSWR of the antenna.

On the two graphs, we see a resonance frequency for 1.5754 GHz. The figure 6 shows theellipticity rate at f = 1.5457 GHz. We can note that this rate is a little higher than predicted.

Figure 6: Ellipticity rate of the antenna.

In conclusion, this first study of a patch antenna for GPS application gives correctresults, even if they can be better. A second study, which had modified the excitation, showedbetter results. Actually, a practical realization is underway.