Figure 8.65 Symbolic patterns of the current distributions on the beveled square monopole antenna loaded with a thin inverted-U-slot for the first four characteristic modes ([39], copyrightC 2010 IEEE).

2.75 GHz, J2 is horizontal current parallel to the ground plane, associated with resonance at 7 GHz, and J3 is higher-order-mode vertical current with null near the base of the patch, associated with resonance at 12 GHz. With a U-slot on the patch, an additional current mode is produced, generating an additional resonance in the structure. Graphical representation of the current flows on the beveled square patch with a thin U-slot is illustrated in Figure 8.65, in which current flows of five modes, from J0 to J3 and an additional mode, referred to as a slot mode Js, are shown with arrows. These currents, except Js, flow in paths similar to those on the patch without the U-slot. Js is divided into two at the center of the patch, and each of them flows symmetrically circling around the U-slot as shown in Figure 8.65, and contributes to producing a sharp increase in antenna impedance, entailing a narrow rejection band. As a consequence, a steep band-reject performance is observed in the S11 characteristics as Figure 8.63(c) illustrated.

8.1.2.1.2.3.3.2 Circular/Elliptical slot UWB antennas

Circular/Elliptical CPW-fed slot UWB antennas are introduced in [40a], where a microstrip line-fed UWB antenna is also discussed. Antenna geometry along with dimensional parameters is illustrated in Figure 8.66. The antenna is comprised of a circular/elliptical stub that excites a similar-shaped slot aperture. Three models (elliptical/circular) fed by a CPW and one model (elliptical) fed by a microstrip line are examined to find that all models exhibit wide enough bandwidth for UWB operation with satisfactory radiation efficiency and radiation patterns. The antenna element is

Table 8.5 Dimensional parameters of four antennas I, II, III and IV (h = 1.575 mm and εr = 3) ([40a], copyright C 2006 IEEE)

L

Figure 8.66 Geometry of antenna and dimensional parameters ([40a], copyright C 2006 IEEE).

etched on the substrate of 1.575 mm thickness with εr = 3, and dimensions of the four types of antenna given in Table 8.5 are used.

From the experimental results, bandwidth observed for the Prototype I (elliptical model) is 17.35 GHz (from 2.65 to 20 GHz), or 153%. For the Prototype II (circular model) it is 17.05 GHz (2.95 GHz to 20 GHz), or 148%, and the Prototype III (circular model) demonstrates 175%, beginning from 1.3 GHz that is a lower frequency than that of other models, being at 2 GHz band, as a consequence of the enlarged ground plane. Regarding the radiation patterns, an almost omnidirectional profile is observed in lower frequencies, becoming somewhat directional in higher frequencies. The maximum gain obtained is about 4.5 dBi.

Design of a slightly modified circular/elliptical slot UWB antenna is described in [40b]. Geometries of the ellipticalor circular-shaped monopole antennas are depicted in Figure 8.67(a) and (c), respectively, and the complementary versions of these antennas are also shown in (b) and (d). The antenna structure is formed with planar conducting surfaces of either two ellipses or circles in a two-sided conductor-coated substrate. The primary radiating element and the microstrip feed are on one side of the substrate, while the ground plane is on the other side. The radiating slot is formed by the intersection

Figure 8.68 Geometry of printed elliptical slot antenna with U-stub fed by (a) microstrip line and

(b) CPW ([40c], copyright C 2006 IEEE).

of UWB antennas with suitable radiation characteristics, covering UWB bandwidth with well-behaved omnidirectional radiation patterns and more than 90% radiation efficiency.

Elliptical/circular slot antennas with U-shaped stub [40c] are studied both theoretically and experimentally and shown to have satisfying UWB characteristics with smaller size. Figure 8.68 illustrates antenna geometries of different feeding types by (a) microstrip