**4. Directional UWB antenna and design**

Comparing to the omni-directional UWB antenna, the directional UWB antenna with a much higher gain is also required to meet various applications. In this section, several types of directional UWB antenna design such as the UWB printed wide-slot antennas, the UWB DRAs, and the DRAs with radiation reconfiguration, etc., are detail introduced.

#### **4.1. UWB printed wide-slot antenna**

The printed wide-slot antenna is another type of the most suitable candidates for UWB applications. This type of antenna is commonly consists of a wide-slot and a tuning stub connected with a microstrip or CPW feedline. Up to now, many wide-slot antennas, including different wide-slots or tuning stubs, have been extensively studied on the antenna operation bandwidth. Among various shapes of slot, the rectangular wide-slot is the simplest structure. Based on the rectangular wide-slot, several shapes of the tuning stub are studied, where the representative geometries are shown in Fig.16 and the bandwidth ratios vary from 1.8:1 to 3.6:1. *i.e.*, Jang *et al*. [40, 41] presented two rectangular wide-slot antennas fed by the microstrip line with a cross-shaped stub and a Π-shaped stub, respectively. The antenna bandwidth greatly depends on the length of the horizontal and vertical feed lines as well as on the offset position of the feedline. The two slot antennas achieve the impedance bandwidth ratios of 2.8:1 (1.7~ 4.9 GHz) and 3.5:1 (1.7~ 6.0 GHz), respectively. Later, Yao *et al*. [42] proposed a fanshaped microstrip stub together with a strip, which contributes to a little wider with the bandwidth ratio of 3.6:1 (0.5~ 5.7 GHz). A rectangular slot with a rectangular tuning stub was also studied [43], but with the bandwidth ratio of 1.8:1.

136 Ultra Wideband – Current Status and Future Trends

Fig.14.

miniaturized counterpart.

**Figure 14.** Radiation improvement techniques [37, 38].

**Figure 15.** Miniaturization techniques [39].

In fact, geometries of the monopole and the ground plane not only affect the antenna impedance bandwidth but the antenna radiation pattern and phase center over a wide bandwidth, which is an important phenomenon, especially for pulsed devices that need minimum signal distortion. Thus, the relation between antenna structure and its radiation are also studied. *i.e.*, Fortino *et al*. [37] introduced a CPW-fed printed triangular monopole antenna with a specific ground plane, such as the double folded structure, where the ground plane is optimized to obtain more constant radiation patterns versus frequency. Wu *et al*. [38] also pointed out the ground plane may affect the antenna's H-plane radiation pattern as its width could be comparable with the wavelength at the higher operating frequency. Thus, a compact ground plane with a trident-shaped feed structure is proposed to significantly improve the H-plane radiation pattern in a very wide impedance bandwidth, as shown in

Moreover, the wireless portable devices become smaller and smaller, which contributes the printed UWB antenna design on miniaturization. One of creative miniaturization techniques is proposed by Sun *et al*. [39], where a 40% size reduction is realized by simply exploiting its structural symmetry, as shown in Fig.15. It is found that the miniaturized bevelled planar monopole antenna exhibits a much wider impedance bandwidth, higher cross-polar radiation, and slightly lower gain at higher frequencies as compared with its un-

**Figure 17.** Various shapes of wide-slots [44-48].

**Figure 16.** Various shapes of tuning stubs [40-43].

It is also noted that the slot shape plays more important on affecting the antenna bandwidth compared to the tuning stub shape. Fig.17 gives several shapes of wide-slots, such as the tapered slot, the circular slot, the hexagonal slot, and etc. These antenna can provide the impedance than ratios from 3.1:1 to 15.4:1, which are much wider bandwidth those of rectangular wide-slot antennas. *i.e.*, Azim *et al*. [44] presented a tapered-shape slot excited by a rectangular tuning stub, which achieves an impedance bandwidth ratio of 3.1:1 (3~11.2 GHz) with a stable gain and radiation over the bandwidth. Denidni *et al*. [45] proposed a circular slot fed by a circular patch through a CPW feedline. This configuration offers a much larger bandwidth ratio of 6.1:1 (2.3~13.9 GHz). Furthermore, Angelopoulos *et al*. [46] investigated the elliptical slot with an elliptical tuning stub achieving an impedance bandwidth ratio of 15.4:1 (1.3~20 GHz), which is the widest bandwidth of the printed slot antennas in open literature.

Ultra-Wideband Antenna and Design 139

bandwidth based on design of various special slots or stubs. Unfortunately, it maybe wastes a lot of time for antenna designers to find a suitable slot antenna structure according to a required operation bandwidth. So the investigation on the relationship between the slot structure and the bandwidth becomes very useful. For this purpose, reference [53] presented an interesting deep study on printed binomial-curved slot antennas, where the slot and the tuning stub both formed by a binomial curve function, thus various bandwidths can be

The CPW-fed printed binomial-curved slot antenna is shown in Fig.19. It consists of a wide slot, a tuning stub, and a CPW feedline, all printed on a single-layer metallic substrate of thickness *h* and relative permittivity *ε*r, and with dimension of *L* × *W*. The slot's outline size is denoted as *l* × *w*, where the coordinate of point **A** on the up-right of the slot is fixed to

= =⋅ ≤ ≤ <sup>1</sup>( ) (2 / ) , 0 / 2 *<sup>N</sup> y fx l xw x w* (3)

where *N* is the order of the binomial curve function. The slot is excited by a CPW feedline, where the signal strip width is *wf*, and the gap spacing between the signal strip and the coplanar ground plane is *g*. To achieve an efficient excitation and a wide impedance matching, the signal strip is terminated to a tuning stub with the same shape as the slot but with a smaller size, which has an offset *d* away from the bottom edge of slot. The stub-toslot's outline size-ratio is denoted as *τ*. Therefore, the coordinate of point **B** on the upperright of the tuning stub is denoted as (*τw*/2, *τl*+*d*), and the binomial curve function for the

(*w*/2, *l*) and the edge is formed by a binomial curve function, expressed as follows:

obtained based on the structure with different binomial curves.

edge of the tuning stub can be rewritten as follows:

**Figure 19.** Binomial slot antennas.

= =+ ⋅

τ

 τ

≤ ≤

<sup>2</sup> ( ) (2 / ) , 0 / 2 *<sup>N</sup> y fx d l x w x w* (4)

τ

Different from above regular shapes of the slot or tuning stub, several special geometries of printed slot antennas, such as dual annular slot, semi-elliptic slot, and etc., were also introduced for UWB applications, as shown in Fig.18. These antennas achieve the impedance bandwidth ratios from 3.7:1 to 7:1. For instance, Ma *et al*. [49] introduced a tapered-slot-fed annular slot antenna. The tapered-slot feeding structure serves as an impedance transformer and guides the wave propagating from the slot line to the radiating slot without causing pernicious reflection. The radiating slot is curved to distribute part of the energy to the reverse side of the feeding aperture. This antenna achieves an impedance bandwidth ratio of 3.7:1 (2.95~11 GHz). Gopikrishna *et al*. [50] introduced a semi-elliptic slot antenna. The antenna features a CPW signal strip terminated with a semi-elliptic stub and a modified ground plane to achieve a wide bandwidth ratio of 7:1 (2.85~20 GHz). Pourahmadazar *et al*. [51] studied a special square slot antenna for circular polarization, which is composed of a square ground plane embedded with two unequal-size inverted-L strips around two opposite corners of the square slot. The antenna owns an impedance bandwidth ratio of 4.7:1 (2.67~13 GHz) and a circular polarization bandwidth ratio of 1.5:1 (4.9~6.9 GHz). Sim *et al*. [52] proposed a compact microstrip-fed narrow slot antenna design for UWB applications. By properly loading a notch to the open-ended T-shaped slot and extending a small section to the microstrip feedline, an impedance bandwidth ratio of 3.7:1 (3.1 ~11.45 GHz) is obtained.

**Figure 18.** Special geometries of wide-slot antennas [49-52].

Commonly, the operation bandwidth depends on the requirement of the wireless communication system which needs various bandwidths. From above mentioned slot antennas, it is known that the printed wide-slot antenna may achieve a wide range bandwidth based on design of various special slots or stubs. Unfortunately, it maybe wastes a lot of time for antenna designers to find a suitable slot antenna structure according to a required operation bandwidth. So the investigation on the relationship between the slot structure and the bandwidth becomes very useful. For this purpose, reference [53] presented an interesting deep study on printed binomial-curved slot antennas, where the slot and the tuning stub both formed by a binomial curve function, thus various bandwidths can be obtained based on the structure with different binomial curves.

The CPW-fed printed binomial-curved slot antenna is shown in Fig.19. It consists of a wide slot, a tuning stub, and a CPW feedline, all printed on a single-layer metallic substrate of thickness *h* and relative permittivity *ε*r, and with dimension of *L* × *W*. The slot's outline size is denoted as *l* × *w*, where the coordinate of point **A** on the up-right of the slot is fixed to (*w*/2, *l*) and the edge is formed by a binomial curve function, expressed as follows:

$$y = f\_1(\mathbf{x}) = l \cdot \left(2\mathbf{x} / w\right)^N, \qquad 0 \le \mathbf{x} \le w / 2 \tag{3}$$

where *N* is the order of the binomial curve function. The slot is excited by a CPW feedline, where the signal strip width is *wf*, and the gap spacing between the signal strip and the coplanar ground plane is *g*. To achieve an efficient excitation and a wide impedance matching, the signal strip is terminated to a tuning stub with the same shape as the slot but with a smaller size, which has an offset *d* away from the bottom edge of slot. The stub-toslot's outline size-ratio is denoted as *τ*. Therefore, the coordinate of point **B** on the upperright of the tuning stub is denoted as (*τw*/2, *τl*+*d*), and the binomial curve function for the edge of the tuning stub can be rewritten as follows:

$$y = f\_2(\mathbf{x}) = d + \pi l \cdot \left(2\mathbf{x} / \pi w\right)^N, \qquad 0 \le \mathbf{x} \le \pi w / 2 \tag{4}$$

**Figure 19.** Binomial slot antennas.

138 Ultra Wideband – Current Status and Future Trends

antennas in open literature.

(3.1 ~11.45 GHz) is obtained.

**Figure 18.** Special geometries of wide-slot antennas [49-52].

It is also noted that the slot shape plays more important on affecting the antenna bandwidth compared to the tuning stub shape. Fig.17 gives several shapes of wide-slots, such as the tapered slot, the circular slot, the hexagonal slot, and etc. These antenna can provide the impedance than ratios from 3.1:1 to 15.4:1, which are much wider bandwidth those of rectangular wide-slot antennas. *i.e.*, Azim *et al*. [44] presented a tapered-shape slot excited by a rectangular tuning stub, which achieves an impedance bandwidth ratio of 3.1:1 (3~11.2 GHz) with a stable gain and radiation over the bandwidth. Denidni *et al*. [45] proposed a circular slot fed by a circular patch through a CPW feedline. This configuration offers a much larger bandwidth ratio of 6.1:1 (2.3~13.9 GHz). Furthermore, Angelopoulos *et al*. [46] investigated the elliptical slot with an elliptical tuning stub achieving an impedance bandwidth ratio of 15.4:1 (1.3~20 GHz), which is the widest bandwidth of the printed slot

Different from above regular shapes of the slot or tuning stub, several special geometries of printed slot antennas, such as dual annular slot, semi-elliptic slot, and etc., were also introduced for UWB applications, as shown in Fig.18. These antennas achieve the impedance bandwidth ratios from 3.7:1 to 7:1. For instance, Ma *et al*. [49] introduced a tapered-slot-fed annular slot antenna. The tapered-slot feeding structure serves as an impedance transformer and guides the wave propagating from the slot line to the radiating slot without causing pernicious reflection. The radiating slot is curved to distribute part of the energy to the reverse side of the feeding aperture. This antenna achieves an impedance bandwidth ratio of 3.7:1 (2.95~11 GHz). Gopikrishna *et al*. [50] introduced a semi-elliptic slot antenna. The antenna features a CPW signal strip terminated with a semi-elliptic stub and a modified ground plane to achieve a wide bandwidth ratio of 7:1 (2.85~20 GHz). Pourahmadazar *et al*. [51] studied a special square slot antenna for circular polarization, which is composed of a square ground plane embedded with two unequal-size inverted-L strips around two opposite corners of the square slot. The antenna owns an impedance bandwidth ratio of 4.7:1 (2.67~13 GHz) and a circular polarization bandwidth ratio of 1.5:1 (4.9~6.9 GHz). Sim *et al*. [52] proposed a compact microstrip-fed narrow slot antenna design for UWB applications. By properly loading a notch to the open-ended T-shaped slot and extending a small section to the microstrip feedline, an impedance bandwidth ratio of 3.7:1

Commonly, the operation bandwidth depends on the requirement of the wireless communication system which needs various bandwidths. From above mentioned slot antennas, it is known that the printed wide-slot antenna may achieve a wide range

Ultra-Wideband Antenna and Design 141

into the UWB antenna. One of effective methods to expand the DRA's bandwidth is the hybrid technique, which combine the DRA and the monopole antenna. Both antennas provide the similar radiation patterns but with different operation frequency bands. Several representative UWB hybrid DRAs are shown in Fig.22 *i.e.*, Lapierre *et al*. [54] firstly proposed a hybrid antenna design by combining the properties of a quarter wavelength monopole antenna with an annular DRA. This design can be used to retrofit existing monopole antennas: by introducing an appropriate DRA, the original narrow-band monopole can be transformed to achieve an UWB performance, with bandwidth ratio of 2.6:1 (6.5~16.8 GHz). To enhance the antenna bandwidth further, Ruan *et al*. [55] proposed a double annular ring DR with different permittivity, which achieves an impedance bandwidth ratio of 3.7:1 (1.8~6.9 GHz). Later, Jazi *et al*. [56] proposed a skirt monopole antenna used to excite an inverted conical-ring-shape dielectric resonator. In this design, three different methods of impedance matching, dielectric, and ground plane shaping procedures have been applied to increase the antenna bandwidth. The results show that by shaping the dielectric structure and impedance matching method, the input impedance and location of the higher part of the frequency bandwidth can be controlled by exciting higher order mode (TM012+δ) of the same family with dominant resonant mode inside the DR (TM01δ). The lower part of the input impedance bandwidth can be adjusted using the ground plane shaping and matching method at the feed. This antenna achieves the bandwidth ratio

(a) L-shaped (b) U-shaped (c) Z-shaped

of 3.8:1 (1.8~6.9 GHz).

**Figure 22.** UWB hybrid DRAs [54-56].

**Figure 23.** Photos of various UWB DRAs [58-60].

**Figure 20.** Various binomial slot antennas.

Several shapes for different *N* are given in Fig.20. As *N* equals to 1, both the slot and the tuning stub are the triangular shape. As *N* increases, the bottom widths of the slot and the tuning stub expand gradually, and their shapes look like bowls. As *N* approaches infinity, both the slot and the tuning stub are transformed to the rectangular shape. From Fig.21, it is found that the operation bandwidth range varies from 10~20 % to 20~40 %, 25~60 %, 60~90 %, 70~110% and 85~110%, as the order *N* increases from 1 to 2, 3, 6, 12 and ∞, respectively, meaning that the larger order *N* is selected, the wider bandwidth may be obtained. It is also known that a proffered wideband slot antenna can be easily designed based on suitable selection of parameters, such as *w*/*l*, *N*, according to the operation bandwidth in applications, which is convenient for the wideband antenna design.

**Figure 21.** Various bandwidths for orders *N*.

#### **4.2. UWB dielectric resonator antenna**

Dielectric resonator antenna is a new type of directional UWB antenna. It owns a much smaller size and higher efficiency than the UWB printed monopole and wide-slot antennas. Recently, many studies have been proposed to expand the DRA's bandwidth and promote it into the UWB antenna. One of effective methods to expand the DRA's bandwidth is the hybrid technique, which combine the DRA and the monopole antenna. Both antennas provide the similar radiation patterns but with different operation frequency bands. Several representative UWB hybrid DRAs are shown in Fig.22 *i.e.*, Lapierre *et al*. [54] firstly proposed a hybrid antenna design by combining the properties of a quarter wavelength monopole antenna with an annular DRA. This design can be used to retrofit existing monopole antennas: by introducing an appropriate DRA, the original narrow-band monopole can be transformed to achieve an UWB performance, with bandwidth ratio of 2.6:1 (6.5~16.8 GHz). To enhance the antenna bandwidth further, Ruan *et al*. [55] proposed a double annular ring DR with different permittivity, which achieves an impedance bandwidth ratio of 3.7:1 (1.8~6.9 GHz). Later, Jazi *et al*. [56] proposed a skirt monopole antenna used to excite an inverted conical-ring-shape dielectric resonator. In this design, three different methods of impedance matching, dielectric, and ground plane shaping procedures have been applied to increase the antenna bandwidth. The results show that by shaping the dielectric structure and impedance matching method, the input impedance and location of the higher part of the frequency bandwidth can be controlled by exciting higher order mode (TM012+δ) of the same family with dominant resonant mode inside the DR (TM01δ). The lower part of the input impedance bandwidth can be adjusted using the ground plane shaping and matching method at the feed. This antenna achieves the bandwidth ratio of 3.8:1 (1.8~6.9 GHz).

**Figure 22.** UWB hybrid DRAs [54-56].

140 Ultra Wideband – Current Status and Future Trends

**Figure 20.** Various binomial slot antennas.

**Figure 21.** Various bandwidths for orders *N*.

**4.2. UWB dielectric resonator antenna** 

Several shapes for different *N* are given in Fig.20. As *N* equals to 1, both the slot and the tuning stub are the triangular shape. As *N* increases, the bottom widths of the slot and the tuning stub expand gradually, and their shapes look like bowls. As *N* approaches infinity, both the slot and the tuning stub are transformed to the rectangular shape. From Fig.21, it is found that the operation bandwidth range varies from 10~20 % to 20~40 %, 25~60 %, 60~90 %, 70~110% and 85~110%, as the order *N* increases from 1 to 2, 3, 6, 12 and ∞, respectively, meaning that the larger order *N* is selected, the wider bandwidth may be obtained. It is also known that a proffered wideband slot antenna can be easily designed based on suitable selection of parameters, such as *w*/*l*, *N*, according to the operation

bandwidth in applications, which is convenient for the wideband antenna design.

Dielectric resonator antenna is a new type of directional UWB antenna. It owns a much smaller size and higher efficiency than the UWB printed monopole and wide-slot antennas. Recently, many studies have been proposed to expand the DRA's bandwidth and promote it

(a) L-shaped (b) U-shaped (c) Z-shaped

**Figure 23.** Photos of various UWB DRAs [58-60].

Apart from the UWB hybrid DRA design, recently, Liang *et al*. [57-61] proposed a patch feed technique and DRAs with various alphabet structures, such as cross-T-shaped [58], Lshaped [57], U-shaped [59], and Z-shaped [60], as shown in Fig.23, where the DRA bandwidth ratios of 2.1:1~9.4:1 were obtained. Fig.24 presents the trapezoidal patch-fed Lshaped DRA design process. To explain the wideband operation of the patch-fed L-shaped DRA, three reference antennas are used as references. Fig.24 (a) is a rectangular DR on a ground plane; Fig.24 (b) is a rectangular DR on a single-sided copper-clad substrate, and Fig.24 (c) is an L-shaped DR on a single-sided copper-clad substrate. The three antennas are excited by the probe feed mechanism and their optimized numerical results in terms of bandwidth are compared with patch-fed L-shaped DRA, as shown in Fig.24. It is observed that the impedance bandwidth can be expanded by using an inserted intermediate substrate, an L-shaped DR, and an inverted-trapezoidal patch feed mechanism. Table 1 lists several proposed UWB DRAs, including the antenna geometry, the feed mechanism and the bandwidth.

Ultra-Wideband Antenna and Design 143

**Bandwidth ratio** 

**VSWR ≤ 2** 

**Frequency range** 

patch 9.8 3.87~8.17 GHz 2.1:1

patch 9.8 3.56~7.57 GHz 2.1:1

9.8 2.5~10.3 GHz 4.1:1

9.8 3.9~12.2 GHz 3.1:1

**No Antenna Geometries Feed** 

4 L-shaped DR [57] Trapezoidal

5 Cross-T-shaped DR [58] Trapezoidal

<sup>1</sup>Annular ring

<sup>2</sup>Double annular-ring

<sup>3</sup>Conical-ring DR

7 Z-shaped DR [60]

9 Rectangular DR[62]

**Table 1.** Various bandwidths of UWB DRAs.

**Figure 25.** UWB DRAs with a radiation reconfiguration.

**5. Band-notched UWB antenna and design** 

**mechanisms** *ε***DR**

DR+monopole [54] probe 10 6.5~16.8 GHz 2.6:1

DR+monopole [55] probe 4&36 3~11.2 GHz 3.7:1

+ skirt monopole [56] probe 10 1.8~6.9 GHz 3.8:1

6 U-shaped DR [59] Triangle patch 9.8 3.1~7.6 GHz 2.4:1

Beveled rectangular patch

8 Circular DR [61] Crescent patch 35 1.6~15 GHz 9.4:1

Bevelrectangular patch

The Federal Communication Commission released the frequency band 3.1~10.6 GHz for the UWB system in 2002. But along with the UWB operating bandwidth, there exist some narrowband wireless services, which occupy some of the frequency bands in the UWB band. The most well-known among them is wireless local area network (WLAN) IEEE802.11a and HIPERLAN/2 WLAN operating in 5.15~5.35 GHz and 5.725~5.825 GHz bands. Apart from WLAN, in some European and Asian countries, world interoperability for microwave access

The above mentioned UWB antennas could provide the monopole-like radiation or mushroom-like radiation in an ultra-wideband. While some portal UWB wireless devices need both the monopole-like radiation and the mushroom-like radiation since their position are not fixed in communication. For this purpose, Liang *et al*. [62] proposed a UWB DRA with the configurable radiation pattern design, where a rectangular DR excited by dual bevel-rectangular patches. Two bevel-rectangular metal patches are attached on the opposite sides of the DR for excitation and both connect to the 50-ohm microstrip lines, as shown in Fig.25. The bevel-rectangular patch-fed DRA has been proposed to achieve an UWB operation. A reconfigurable radiation pattern performance in terms of the monopole-like radiation and the mushroom-like radiation is obtained through the in-phase feed and the out-phase feed of two input ports, respectively. For the in-phase feed, the antenna performs the monopole-like radiation pattern in the entire operation band. While for the out-phase feed, the antenna performs the mushroom-like radiation in the same operation band.


**Table 1.** Various bandwidths of UWB DRAs.

bandwidth.

**Figure 24.** Wideband DRA design.

Apart from the UWB hybrid DRA design, recently, Liang *et al*. [57-61] proposed a patch feed technique and DRAs with various alphabet structures, such as cross-T-shaped [58], Lshaped [57], U-shaped [59], and Z-shaped [60], as shown in Fig.23, where the DRA bandwidth ratios of 2.1:1~9.4:1 were obtained. Fig.24 presents the trapezoidal patch-fed Lshaped DRA design process. To explain the wideband operation of the patch-fed L-shaped DRA, three reference antennas are used as references. Fig.24 (a) is a rectangular DR on a ground plane; Fig.24 (b) is a rectangular DR on a single-sided copper-clad substrate, and Fig.24 (c) is an L-shaped DR on a single-sided copper-clad substrate. The three antennas are excited by the probe feed mechanism and their optimized numerical results in terms of bandwidth are compared with patch-fed L-shaped DRA, as shown in Fig.24. It is observed that the impedance bandwidth can be expanded by using an inserted intermediate substrate, an L-shaped DR, and an inverted-trapezoidal patch feed mechanism. Table 1 lists several proposed UWB DRAs, including the antenna geometry, the feed mechanism and the

The above mentioned UWB antennas could provide the monopole-like radiation or mushroom-like radiation in an ultra-wideband. While some portal UWB wireless devices need both the monopole-like radiation and the mushroom-like radiation since their position are not fixed in communication. For this purpose, Liang *et al*. [62] proposed a UWB DRA with the configurable radiation pattern design, where a rectangular DR excited by dual bevel-rectangular patches. Two bevel-rectangular metal patches are attached on the opposite sides of the DR for excitation and both connect to the 50-ohm microstrip lines, as shown in Fig.25. The bevel-rectangular patch-fed DRA has been proposed to achieve an UWB operation. A reconfigurable radiation pattern performance in terms of the monopole-like radiation and the mushroom-like radiation is obtained through the in-phase feed and the out-phase feed of two input ports, respectively. For the in-phase feed, the antenna performs the monopole-like radiation pattern in the entire operation band. While for the out-phase

feed, the antenna performs the mushroom-like radiation in the same operation band.

**Figure 25.** UWB DRAs with a radiation reconfiguration.
