**3.1. UWB planar monopoles**

The planar monopole antenna was firstly reported in 1976 by G. Dubost and S. Zisler [10]. It can be realized by replacing a conventional wire monopole with a planar monopole, where the planar monopole is located above a ground plane and commonly fed using a coaxial probe. Up to now, many planar monopole antennas have been introduced due to their wideband performance. Several representative structures are shown in Fig.9, and these antennas achieve the impedance bandwidth ratio from 2:1 to more than 10:1. *i.e.*, Agrawall *et al.* [11] carried out a bandwidth comparison of several planar monopoles with various geometries, such as circular, elliptical, rectangular, and trapezoidal monopoles. The results show that the circular and elliptical monopoles exhibit much wider bandwidth performance than those of others, and both can obtain the impedance bandwidth ratio

exceeding of 10:1 (Circular monopole: 1.17~12 GHz, Elliptical monopole: 1.21~13 GHz). Evans *et al*. [12] proposed a trapezoidal planar monopole antenna above the ground plane, also achieved a bandwidth ratio exceeding of 11:1. Besides the regular structures, Suh *et al*. [13] proposed an interesting structure, the planar inverted cone antenna (PICA), which can provide an impedance bandwidth ratio of more than 10:1 and the pattern bandwidth ratio of about 4:1. To improve the pattern bandwidth ratio, two circular holes are added in the PICA, as shown in Fig.9(e). This alteration improves the radiation pattern performance dramatically without impairing the impedance performance, where the radiation pattern of the two-circular-hole PICA antenna provides a good omnidirectional performance over a bandwidth ratio up to 7:1 and has a very low cross polarization, 20 dB or less. Later, Bai *et al*. [14] presented a modified PICA, where a leaf-shaped metal plate with three circular holes is vertically mounted on the ground plate and is covered by a dielectric plate instead of the conventional metal plate. It achieves the impedance bandwidth ratio better than 20:1, covering the frequency range from 1.3 to 29.7GHz, as shown in Fig.9(f).

Ultra-Wideband Antenna and Design 133

**Figure 10.** Various wideband techniques for planar square monopole antennas [15-19].

The aforementioned planar monopole antennas achieve an ultra-wideband performance based on various techniques, but they all need a perpendicular ground plane, resulting in increasing of the antenna volume and inconvenience for integration with monolithic microwave integrated circuits (MMICs). For the portal wireless device applications, the printed UWB monopole antennas are more popular due to their easier integration than the

The printed UWB monopole antenna commonly consists of a monopole patch and a ground plane. Both of them are printed on the same or opposite side of a substrate, and a microstrip or CPW feedline is used to excite the monopole patch. Since Choi *et al*. [20, 21] introduced this type of antenna with the wideband characteristics in 2004, various printed monopole antennas were studied in the following several years, mainly on the geometries of the

**3.2. UWB printed monopoles** 

planar UWB monopole antennas.

monopole and the ground plane.

Among various planar monopole antennas, the square planar monopole is the simplest in geometry, and its radiation pattern is usually less degraded within the impedance bandwidth. These favourable features attract many studies, mainly on the bandwidth enhancement since the square planar monopole only owns an impedance bandwidth ratio of 2:1. From the antenna geometry, the feed gap, the feed point location and the shape of the monopole's bottom, all may affect the impedance matching. Thus, several techniques such as notching, bevelling, double feed, trident-shaped feed, and etc., were proposed to expand the bandwidth of the square monopole antenna, as shown in Fig.10 *i.e.*, Su *et al*. [15] proposed a method of cutting a pair of notches at the two lower corners of the square planar monopole. With suitable dimensions of the notches chosen, the impedance bandwidth can be greatly enhanced to be about 3 times that of a corresponding simple square planar monopole antenna (2~12.7 GHz compared to 2~4.5 GHz). Antonino-Daviu *et al*. [16] proposed a method of double feed with aims to intense the vertical current distribution and suppress the horizontal distribution in the square planar monopole, which contributes to improvement in the impedance bandwidth and polarization properties. This method achieves an impedance bandwidth ratio of 9.5:1. However, in this double-feed design, an additional feeding network under the ground plane is required to excite the antenna at two separate feeding positions. To avoid using the external feeding network, Wong *et al*. [17] proposed a trident-shaped feeding strip structure. With the use of a trident-shaped feeding strip, the square planar antenna's impedance bandwidth can be enhanced to be larger than 3.5 times that of a simple feeding strip (1.4~11.4 GHz compared to 1.5 ~3.3 GHz). Moreover, hybrid techniques are also proposed, *i.e*., Thomas *et al*. [18] used a sleeved transmission line as a transformer together with the bevelling technique, the impedance bandwidth ratio reaches about 12:1 (0.5 ~ 6 GHz). Ammann *et al*. [19] adopted a method of combining the bevelling and the shorting techniques, the square planar monopole antenna's bandwidth ratio can be expanded to more than 13:1 (0.8 ~10.5 GHz).

**Figure 10.** Various wideband techniques for planar square monopole antennas [15-19].

### **3.2. UWB printed monopoles**

132 Ultra Wideband – Current Status and Future Trends

shown in Fig.9(f).

(0.8 ~10.5 GHz).

exceeding of 10:1 (Circular monopole: 1.17~12 GHz, Elliptical monopole: 1.21~13 GHz). Evans *et al*. [12] proposed a trapezoidal planar monopole antenna above the ground plane, also achieved a bandwidth ratio exceeding of 11:1. Besides the regular structures, Suh *et al*. [13] proposed an interesting structure, the planar inverted cone antenna (PICA), which can provide an impedance bandwidth ratio of more than 10:1 and the pattern bandwidth ratio of about 4:1. To improve the pattern bandwidth ratio, two circular holes are added in the PICA, as shown in Fig.9(e). This alteration improves the radiation pattern performance dramatically without impairing the impedance performance, where the radiation pattern of the two-circular-hole PICA antenna provides a good omnidirectional performance over a bandwidth ratio up to 7:1 and has a very low cross polarization, 20 dB or less. Later, Bai *et al*. [14] presented a modified PICA, where a leaf-shaped metal plate with three circular holes is vertically mounted on the ground plate and is covered by a dielectric plate instead of the conventional metal plate. It achieves the impedance bandwidth ratio better than 20:1, covering the frequency range from 1.3 to 29.7GHz, as

Among various planar monopole antennas, the square planar monopole is the simplest in geometry, and its radiation pattern is usually less degraded within the impedance bandwidth. These favourable features attract many studies, mainly on the bandwidth enhancement since the square planar monopole only owns an impedance bandwidth ratio of 2:1. From the antenna geometry, the feed gap, the feed point location and the shape of the monopole's bottom, all may affect the impedance matching. Thus, several techniques such as notching, bevelling, double feed, trident-shaped feed, and etc., were proposed to expand the bandwidth of the square monopole antenna, as shown in Fig.10 *i.e.*, Su *et al*. [15] proposed a method of cutting a pair of notches at the two lower corners of the square planar monopole. With suitable dimensions of the notches chosen, the impedance bandwidth can be greatly enhanced to be about 3 times that of a corresponding simple square planar monopole antenna (2~12.7 GHz compared to 2~4.5 GHz). Antonino-Daviu *et al*. [16] proposed a method of double feed with aims to intense the vertical current distribution and suppress the horizontal distribution in the square planar monopole, which contributes to improvement in the impedance bandwidth and polarization properties. This method achieves an impedance bandwidth ratio of 9.5:1. However, in this double-feed design, an additional feeding network under the ground plane is required to excite the antenna at two separate feeding positions. To avoid using the external feeding network, Wong *et al*. [17] proposed a trident-shaped feeding strip structure. With the use of a trident-shaped feeding strip, the square planar antenna's impedance bandwidth can be enhanced to be larger than 3.5 times that of a simple feeding strip (1.4~11.4 GHz compared to 1.5 ~3.3 GHz). Moreover, hybrid techniques are also proposed, *i.e*., Thomas *et al*. [18] used a sleeved transmission line as a transformer together with the bevelling technique, the impedance bandwidth ratio reaches about 12:1 (0.5 ~ 6 GHz). Ammann *et al*. [19] adopted a method of combining the bevelling and the shorting techniques, the square planar monopole antenna's bandwidth ratio can be expanded to more than 13:1

The aforementioned planar monopole antennas achieve an ultra-wideband performance based on various techniques, but they all need a perpendicular ground plane, resulting in increasing of the antenna volume and inconvenience for integration with monolithic microwave integrated circuits (MMICs). For the portal wireless device applications, the printed UWB monopole antennas are more popular due to their easier integration than the planar UWB monopole antennas.

The printed UWB monopole antenna commonly consists of a monopole patch and a ground plane. Both of them are printed on the same or opposite side of a substrate, and a microstrip or CPW feedline is used to excite the monopole patch. Since Choi *et al*. [20, 21] introduced this type of antenna with the wideband characteristics in 2004, various printed monopole antennas were studied in the following several years, mainly on the geometries of the monopole and the ground plane.

Ultra-Wideband Antenna and Design 135

For geometry of the ground plane, several representatives are also shown in Fig.12, and obtain the impedance bandwidth ratios from 3.8:1 to more than 10:1. *i.e.*, Huang *et al*. [28, 29] introduced an impedance matching technique by cutting a notch at the ground plane, and the impedance bandwidth can be enhanced by suitable size and position of notch chosen. Azim *et al*. [30] proposed to improve the impedance bandwidth by cutting triangular shaped slots on the top edge of the ground plane. The printed square monopole antenna with symmetrical saw-tooth ground plane obtains the impedance bandwidth ratio of 5.5:1 (2.9~16GHz). Considering high concentration of currents in the corners of the patch or ground, Melo *et al*. [31] studied a rounded monopole patch with a rounded truncated ground plane. It provides an impedance bandwidth ratio of larger than 4.7:1

**Figure 13.** Various printed monopole antennas with trapeziform ground [32-36].

enhance the bandwidth further [33-36].

One of interesting UWB printed monopole antenna designs is a trapeziform ground plane with a rectangular patch monopole aroused from the discone antenna, where the rectangular patch is used to replace the disc, the trapeziform ground plane is used to replace the cone, and the CPW is used to replace the coaxial feed, as shown in Fig.13 [32]. It is found that the printed rectangular antenna with a trapeziform ground plane achieves an impedance bandwidth ratio of 5.1:1, which is similar to that of a discone antenna. To enhance the bandwidth further, the input impedance is investigated by comparing bandwidths for various characteristics impedance of CPW feedline. The impedance bandwidth ratio expands to 12:1 when the characteristic impedance of CPW feedline is about 100Ω, which means the impedance bandwidth is enhanced by a factor of about 2.3. In order to match 50Ω SMA or N-type connectors, a linearly tapered central strip line is used as an impedance transformer, and an impedance bandwidth ratio of 10.7:1 (0.76~11 GHz) is obtained. Moreover, various printed monopoles and feed structures are also studied to

(2.55 ~12 GHz).

For geometry of the monopole patch, Fig.11 presents several representative structures. These antennas achieve the impedance bandwidth ratios from 2.3:1 to 3.8:1. Among various geometries of the monopole patches, the printed circular monopole antenna is one of the simplest [22], which achieves the impedance bandwidth ratio of 3.8:1 (2.69~10.16 GHz) with satisfactory omnidirectional radiation properties. Other monopoles such as octagon monopole [23], spline-shaped monopole [24], U-shaped monopole [25], knight's helm shape monopole [26] and two steps circular monopole [27], as shown in Fig.11, were also proposed and studied,. *i*.*e*., Ooi *et al*. [23] introduced the two-layer octagon monopole antenna based on the low-temperature co-fired ceramic (LTCC) technique, also obtaining an impedance bandwidth ratio of 3.8:1 (3.76~14.42 GHz). Lizzi *et al*. [24] proposed the spline-shaped monopole UWB antenna able to support multiple mobile wireless standards, covering DCS, PCS, UMTS, and ISM bands, with the bandwidth ratio of 2.3:1 (1.7~2.5 GHz).

**Figure 12.** Various ground structures [28-31].

For geometry of the ground plane, several representatives are also shown in Fig.12, and obtain the impedance bandwidth ratios from 3.8:1 to more than 10:1. *i.e.*, Huang *et al*. [28, 29] introduced an impedance matching technique by cutting a notch at the ground plane, and the impedance bandwidth can be enhanced by suitable size and position of notch chosen. Azim *et al*. [30] proposed to improve the impedance bandwidth by cutting triangular shaped slots on the top edge of the ground plane. The printed square monopole antenna with symmetrical saw-tooth ground plane obtains the impedance bandwidth ratio of 5.5:1 (2.9~16GHz). Considering high concentration of currents in the corners of the patch or ground, Melo *et al*. [31] studied a rounded monopole patch with a rounded truncated ground plane. It provides an impedance bandwidth ratio of larger than 4.7:1 (2.55 ~12 GHz).

134 Ultra Wideband – Current Status and Future Trends

**Figure 11.** Various monopole structures [21-27].

**Figure 12.** Various ground structures [28-31].

(1.7~2.5 GHz).

For geometry of the monopole patch, Fig.11 presents several representative structures. These antennas achieve the impedance bandwidth ratios from 2.3:1 to 3.8:1. Among various geometries of the monopole patches, the printed circular monopole antenna is one of the simplest [22], which achieves the impedance bandwidth ratio of 3.8:1 (2.69~10.16 GHz) with satisfactory omnidirectional radiation properties. Other monopoles such as octagon monopole [23], spline-shaped monopole [24], U-shaped monopole [25], knight's helm shape monopole [26] and two steps circular monopole [27], as shown in Fig.11, were also proposed and studied,. *i*.*e*., Ooi *et al*. [23] introduced the two-layer octagon monopole antenna based on the low-temperature co-fired ceramic (LTCC) technique, also obtaining an impedance bandwidth ratio of 3.8:1 (3.76~14.42 GHz). Lizzi *et al*. [24] proposed the spline-shaped monopole UWB antenna able to support multiple mobile wireless standards, covering DCS, PCS, UMTS, and ISM bands, with the bandwidth ratio of 2.3:1

**Figure 13.** Various printed monopole antennas with trapeziform ground [32-36].

One of interesting UWB printed monopole antenna designs is a trapeziform ground plane with a rectangular patch monopole aroused from the discone antenna, where the rectangular patch is used to replace the disc, the trapeziform ground plane is used to replace the cone, and the CPW is used to replace the coaxial feed, as shown in Fig.13 [32]. It is found that the printed rectangular antenna with a trapeziform ground plane achieves an impedance bandwidth ratio of 5.1:1, which is similar to that of a discone antenna. To enhance the bandwidth further, the input impedance is investigated by comparing bandwidths for various characteristics impedance of CPW feedline. The impedance bandwidth ratio expands to 12:1 when the characteristic impedance of CPW feedline is about 100Ω, which means the impedance bandwidth is enhanced by a factor of about 2.3. In order to match 50Ω SMA or N-type connectors, a linearly tapered central strip line is used as an impedance transformer, and an impedance bandwidth ratio of 10.7:1 (0.76~11 GHz) is obtained. Moreover, various printed monopoles and feed structures are also studied to enhance the bandwidth further [33-36].

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 Fig.14.

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 unminiaturized counterpart.

Ultra-Wideband Antenna and Design 137

**4. Directional UWB antenna and design** 

also studied [43], but with the bandwidth ratio of 1.8:1.

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

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

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

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

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

DRAs, and the DRAs with radiation reconfiguration, etc., are detail introduced.

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

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