Performance Study on Modern Ultra Wideband Monopole Antennas

Abstract

The proposed use of ultra wideband (UWB) technology in communication services has motivated the research towards more design and performance studies of modern UWB antennas, (FCC, 2002). Interference of UWB antennas with the existing technologies, phase centre stability of modern UWB antennas, low cross-polarization UWB antennas, high gain directional UWB antennas with stable phase centre, symmetrical Eand HPlane patterns and single or double linear polarization patterns are some of the important research topics nowadays. Monopole disc antennas, with circular, elliptical and trapezoidal shapes, have simpler two-dimensional geometries and are easier to fabricate compared to the traditional UWB monopole antennas with three-dimensional geometries such as spheroidal, conical and teardrop antennas. These disc monopole antennas can be designed to cover existing and upcoming UWB communication applications, (Honda et al., 1992) & (Hammoud & Colomel, 1993). In this study, different square, circular and elliptical disc monopole antenna geometries are designed and analysed for both omnidirectional and directional applications. The feeding structure is optimized to have a maximum impedance bandwidth starting at 3 GHz. One of the general principles of small-element antenna design is the outline design equivalence one. It states that an outline of a planar antenna element performs approximately like the original planar one, (Mohamed & Shafai, 2007) and (Schantz, 2005). In this chapter, a study is made on the gradual removal of metal from the interior of all studied antennas while keeping the impedance and radiation characteristics unchanged. The minimum metal width that could be achieved is about 50% from the total radiator area. The effect of implementing a notch close to the feeding structure on impedance bandwidth is also studied. It is found that implementing notches close to the trident-feeding strip structure did not increase the impedance bandwidth of circular and elliptical antennas. Further, implementing a notch decreased the impedance bandwidth of the square monopole antenna. On the other hand, removing metal from the interior structure of the square, circular and elliptical monopoles gives the same impedance bandwidth for square and circular monopole but increased the bandwidth of the elliptical one. The circular and elliptical monopole antennas with metal removed showed better omnidirectional behaviour at higher frequencies. The performance of a UWB quasi-circular monopole, with rectangular and trapezoidal ground planes was studied, (Wu et al., 2010). It was shown that antenna with trapezoidal