I1 + I2

Probe feed

(c)

Figure 8.73 (a) A sectorial loop antenna, (b) coupled sectorial loop antenna (CSLA), and (c) a half of CSLA over the ground plane ([47], copyright C 2005 IEEE).

(2.4–9 GHz). The tilted feed in models B and C can enhance the bandwidth compared with model A. The radiation pattern is nearly omnidirectional, but becomes directional in higher-frequency ranges. The gain variation was 3.25 dBi (3.00–6.25 dBi) over the frequency range between 3.5 GHz and 13.0 GHz.

In order to obtain a directional pattern, a conducting sheet as a reflector is applied on the back of model B. A 50-mm square sheet is placed with spacing d, varying from 5 mm to 25 mm and it was found for obtaining the directional patterns within the UWB range that d = 10 mm was suitable for the lower-frequency range (3–7GHz) while d = 5 mm was suitable for the higher-frequency range (8–11 GHz).

Other types of wideband or multiband slot antenna are a tapered ring slot antenna [44], antenna using square ring resonator [45], and a V-shaped antenna [46].

8.1.2.1.2.3.4.2 Sectorial loop antenna (SLA)

A compact loop antenna, being referred to as a sectorial loop antenna (SLA), composed of an arch and two sectors, is depicted in Figure 8.73, showing (a) topology of the SLA,

(b) topology of the coupled SLA, and (c) a half of an SLA placed on the ground plane [47]. The antenna can provide wide enough bandwidth to cover the UWB operation band by combining two identical SLAs in parallel and controlling the mutual coupling between them as shown in Figure 8.73(b). When two identical SLAs are connected in parallel, the antenna system is equivalently expressed in two-port network terms by using relationships between the impedances Zij and currents Iij (i = 1 for one antenna and j = 2 for another antenna) as follows: