The results of both simulations and measurements in the far field zone are represented in Figures 6 and7. The half-power beamwidth is of 8.5°in the H-plane and 12°in the E-plane, and the crosspolarization component is lower than Ϫ25 dB in both principal planes.
The measured antenna gain for different numbers of dielectric slabs is presented in Figure 8. It shows clearly that the antenna gain increases with the number of dielectric slabs, varying from 6 dBi without any slabs, to 14 dBi with one slab, to 19 dBi with two slabs. Concerning the antenna bandwidth, defined as the Ϫ3-dB criterion when considering the antenna gain, it covers almost the whole observed frequency band (38 -40 GHz) for the cases of the slot antenna and of the structure using one dielectric slab; by using two slabs for the focusing system, the bandwidth is still of 0.5 GHz, which stays very interesting for millimeter-wave applications. A more significant increase of the number of dielectric slabs would conduct to a focusing system acting as a higher order spatial passband filter, where the resulting antenna would still be centred around 38.9 GHz, but would operate with a much lower bandwidth.
4. CONCLUSION
A waveguide-fed slot antenna has been optimized for an operation in the band of 38 -40 GHz, with a return loss coefficient lower than Ϫ16 dB. A focusing system has been added to the previous structure in order to increase the antenna performances, by developing a Fabry-Perot cavity resonator made of two slabs of dielectric material. This increases the antenna gain up to 19 dBi, with a central frequency of 38.9 GHz, having a good polarization purity and a working bandwidth of 0.5 GHz. The performances obtained by the association of two technologies, the waveguide and the Fabry-Perot-based cavity resonators, makes it possible to consider such antennas structures a good choice for point-to-point and point-to-multipoint telecommunication systems. REFERENCES 1. D.R. Jackson and N.G. Alexopoulos, Gain enhancement methods by printed circuit antennas, IEEE Trans Antennas Propagat 33 (1985), 976 -987. 2. H.Y. Yang and N.G. Alexopoulos, Gain enhancement methods for printed circuit antennas through multiple superstrates, IEEE Trans Antennas Propagat 35 (1987), 860 -863.