Making UWB antennas unidirectional: Phase coherence with an ultra-wide band frequency selective surface reflector

Research on ultra-wide-band (UWB) systems gained significant pace in the recent past; many printed configurations of monopole, and slot antennas have been demonstrated for these systems. These printed UWB antennas have an impedance bandwidth greater than 106% to cover the FCC UWB band from 3.1 GHz to 10.6 GHz. One notable common feature of these antennas is their bidirectional radiation patterns, indicating significant radiation to both upper and lower hemi-spheres. This is a direct result of the partial ground planes common to these printed UWB antennas. When such an antenna is integrated to the top of a device, all the power radiated into the lower hemisphere will be wasted inside the device. In most cases, this loss is close to 50%. Another intrinsic drawback of such antennas consists of the impossibility to install them parallel to and closely above a conducting surface, such as a hard-disk enclosure or a circuit board, because the parallel conducting surface destroys the antenna matching. This happens because a strong electric near field below these antennas exists and it is “short-circuited” by such parallel conductors. A planar UWB reflector for aforementioned antennas is highly beneficial for unidirectional radiation. Designing such a planar reflector with a bandwidth greater than 106% is a challenging task indeed. In narrowband systems, metallic screen reflectors are placed at a distance of λ/4 to provide in-phase reflection. Most of the research on AMCs has been limited to broadband or multi-band operations. The design, analysis and experimental validation of two different configurations have been chosen and illustrated, which manifests low transmission and good phase coherence over an ultra-wide frequency band.