Simulated Performance of Conical Antennas Using Matlab-Based Finite-Difference Time Domain (FDTD) Code

The need of ultrawideband (UWB) antennas with omni-directional coverage is increasing for both military and commercial applications (Wiesbeck et. al., 2009; Minin, 2010). The UWB radio technology promises high resolution radar applications, sensor networks with a large number of sensors for industrial or home surveillance as well as high data-rate communication over short range for personal area networks. With a need for antennas with the characteristics of broad bandwidth and small electrical size, conical antenna structures have been a focus of research because of its broad bandwidth and omni-directional radiation pattern (Maloney & Smith, 1993; Sandler & King, 1994; Yu & Li, 2008; Palud et. al., 2008). The bi-conical antenna exhibits a very stable omni-directional radiation pattern in the plane normal to the dipole axis together with an excellent transient response. However, the feeding with a usual coaxial cable requires a balun, which transforms the asymmetric mode of the feed line into a symmetric mode at the feed point. For the coaxial balun the ultra wide bandwidth demands very high precision in the manufacturing process in order to get a good and stable matching especially for the high frequencies. The mono-cone antenna as asymmetric structure does not need any balun for an asymmetric feed line but it needs an infinite ground plane, which in reality can only be approximated. The theory of wide-angle conical antennas has been developed sufficiently to permit calculation of the transfer functions relating source voltage to radiated field and incident field to load voltage over the range of frequencies required in the study of transients. Such calculations were demonstrated in (Harrison & Williams, 1965). Due to their three-dimensional configurations, conical antennas are bulky and difficult to fabricate, integrate, and reconfigure. Moreover, since conventional conical antennas comprise of free-standing metal, they are typically heavy in order to achieve sufficient mechanical stability. Several configurations have been proposed to improve conical antennas’ mechanical performance (Ma et. al., 2009; Zhou et. al., 2009, Kliros et. al., 2010a). Resistive loading for conical antennas, which is investigated in (Maloney 1993), does not constitute the optimal solution as it reduces the antennas’ efficiency. Recently, investigations have been carried out on configurations that employ a dielectric or magnetic material to cover the conical antenna (Gentili et. al., 2004, Lu, 2007). Dielectric and magnetic coating of