Table 8.6 Optimized dimensions of printed elliptical/circular slot antenna ([40c], copyright C 2006 IEEE)

Table 8.7 Measured and simulated bandwidth of printed elliptical/circular slot antennas ([40c], copyright C 2006 IEEE)

line and (b) CPW. With optimized dimensions of four types of antennas tabulated in Table 8.6, measured and simulated bandwidth are given in Table 8.7, demonstrating much wider bandwidth compared with other antennas described in [40d–40f].

A novel modified UWB planar monopole antenna with variable frequency band-notch function is described in [41].

8.1.2.1.2.3.3.3 A rectangular monopole patch with a notch and a strip

A small printed rectangular patch with a notch and strip is described in [42a]. The antenna is featured in reduced ground plane effect by cutting a notch from the radiator and attaching a strip asymmetrically to the radiator, while keeping wide bandwidth covering the UWB band. Figure 8.69 depicts antenna geometry, showing (a) a printed rectangular monopole patch and (b) a rectangular monopole patch with a notch and a strip. By slotting the radiator and/or modifying the shape of the radiator as well as the ground plane, the size can be reduced to 30 × 30 or 25 × 25 (in mm) from 40 × 50 mm, a usual size for similar printed antennas [42b, 42c].

In addition, by adding a strip to the top side of the rectangular radiator, the length of the radiator can be reduced to 30 mm as illustrated in Figure 8.69(b). Further reduction of the

Figure 8.69 (a) Planar rectangular monopole with the finite ground plane and (b) the antenna with a strip ([42a], copyright C 2007 IEEE).

antenna size can be attained by cutting a notch on the radiator as shown in Figure 8.70(a), where dimensional parameters are also described. Presence of the notch on the radiator leads to concentration of the current distributions on the right portion of the radiator, where the notch is cut, while the currents at the left portion of the radiator as well as the ground plane are very weak. Figure 8.70 illustrates the current distributions on the radiator and the ground plane, comparing (b) with notch and (c) without notch at 3, 5, 6, and 10 GHz. Observing this current distribution, it can be said that the notch plays a significant role to determine the lower operating frequencies, and subsequent impedance matching at around 3 GHz will become more sensitive to the notch dimension than the shape and size of the ground plane. The reason for this is that the currents on the ground plane are much weaker than those on the radiator. Thus it is important to notice that the effects of the ground plane and RF cable on the antenna performance at lower frequencies can be suppressed greatly by the notch [42a]. As the operating frequency increases, current flow becomes stronger on the feeding strip, on the junction of the radiator and the feeding strip, and on the ground plane. Thus impedance matching is greatly affected by the gap g between the patch and ground plane. The lowest frequency f is determined by the path length L of the current flow around the notch on the right portion of the patch, which is the sum of the horizontal path from the feeding point,

the vertical path from the bottom of the radiator, and the length and the width of the

√

horizontal strip, giving f = c/λ (λ = 2L εr + 1/2).

Here f = 3.10 GHz. Simulated and measured return loss is provided in Figure 8.71. The radiation patterns are almost omnidirectional at lower frequencies, while more directional at higher frequencies. The radiation efficiency varies from 79% to 95% across the entire bandwidth 3.1–10.6 GHz.

50 mm

Figure 8.72 Pentagon-shape microstrip slot antenna fed by a microstrip line ([43], copyrightC 2009 IEEE).

8.1.2.1.2.3.4 Modified shaped UWB antennas

Planar antennas with modified shapes are also developed for the UWB applications. Two examples of them are (a) pentagonal shaped-slot antenna, and (b) sectorial loop antenna.

8.1.2.1.2.3.4.1 Pentagon-shape microstrip slot antenna

The antenna geometry is illustrated in Figure 8.72 [43]. Three models are considered; model A with a straight feed line, model B with tilted feed line and model C with tilted feed line on a different substrate from that of the model A and B. The substrate used for models A and B has εr = 2.20 and tan δ = 0.0004, whereas for model C, εr = 4.50 and tan δ = 0.02, The thickness of the substrate is 1.58 mm for all the models. The antenna can be designed to mount on the small substrate (ground plane) with the size of 50 mm × 80 mm, which is a similar size to the wireless card used in usual wireless equipment. The antenna will occupy only the top 20 mm or 25% of the ground plane length, leaving enough space available to mount RF devices and circuitry on it. Even with this small size, the impedance bandwidth obtained was maximum 124% (2.65– 11.30 GHz), exceeding the UWB bandwidth of 110% (3.10–10.60 GHz), as a result of combination of the pentagon-shaped slot, feed line, and pentagon stub. For models B and C, the feed line is rotated by 15◦. In terms of the bandwidth, model A exhibited 106% (2.6–8.4 GHz), model B provided the largest of all 124% and model C obtained 116%