A very low profile and high efficiency antenna array for WLAN at S-Band

A high efficiency planar array operating in the 802.1 lb/g WLAN band of 2.4 to 2.485 GHz is required to provide range extension for WLAN access points. In addition, a low profile planar array would also improve the aesthetic aspect at its site of deployment. To satisfy these requirements, we have developed one of the thinnest S-band array reported to-date with dimensions of 19.5 x 19.5 x 0.65 cm. The conventional microstrip printed array on a thin substrate has about 1 - 2% frequency bandwidth. To meet the bandwidth requirement of 3.5%, additional parasitic elements have to be added. This together with the dielectric cover and integral beamforming network would lead to a thick array exceeding the specification. A better approach is to locate the probe-fed element in a metallic cavity with a dielectric radome. The air cavity layer not only gives needed rigidity to the array panel but also lowers the antenna loss. With the appropriate cavity and probe sizes, the cavity-backed element is shown to meet both the bandwidth and low height requirements. To compute the performance of a periodic array of these elements, the modal technique is used. The field distributions within the cavity and the patch radiator are represented by the modes of a rectangular waveguide with an inner conductor. These modes are obtained by the transverse resonance technique. The various discontinuities in a periodic cell consisting of sections of circular coax guide, rectangular coax guide, patch radiator, radome and free space are characterized by generalized scattering matrices (GSMs). These GSMs are combined to give an accurate overall model of the array performance. This procedure is used to design a very low profile array with high efficiency. Test results from this array agree very well with the predictions.