**5.1. Embedding slot**

Among various proposed techniques on the band-notched UWB antenna design. One common and simple way is to etch slots on the radiation patch or ground plane. Up to now, many shapes of embedding slots were studied, and some representatives are shown in Fig.26 *i.e.*, Kim *et al*. [63] proposed a CPW-fed planar UWB antenna with a hexagonal radiating element. By inserting a V-shaped thin slot with a length of λc/4 (λc is the wavelength of notched frequency) on the hexagonal radiating element, the narrow frequency band-notched is created, where the fractional bandwidth is approximately 8~10%. Chung *et al*. [64-66] introduced the printed UWB monopole antenna by inserting an inverted U-shaped, Π-shaped or rectangular slot. At the notched frequency, current is concentrated around the edges of the slot and is oppositely directed between the interior and the notched frequency. This leads to the desired high attenuation near the notched frequency. In [67], a band-notched printed monopole antenna is provided by using two modified U-shaped slots on the monopole. The U-shaped slot perturbs the resonant response and also acts as a half-wave resonant structure. At the notched frequency, the desired high attenuation near the notched frequency can be produced. Jiang *et al*. [68, 69] introduced a pair of inverted-L-shaped slots around the microstrip line on the ground plane; a frequency-notched response can also be achieved.

**Figure 26.** Notched-band designs with various slots on patch or ground [63-69].

#### Ultra-Wideband Antenna and Design 145

**Figure 27.** Notched-band designs with various periodic structure slots [70-73].

Since split-ring resonator (SRR), electric-LC (ELC) resonator, complementary split-ring resonator (CSRR) and complementary electric-LC (CELC) resonator are commonly used to design a material with negative permittivity and permeability, all these structures can also be applied in UWB antennas for the notched band design. Several representatives are shown in Fig.27. The SRR is generally composed of two concentric split ring strips. It has a favourable aspect in size since it can be designed as small as one-tenth of the resonance wavelength. In [70], a dual reverse split trapezoid slots, instead of the conventional strip-type SRRs, was proposed and implemented for a bandstop application. In [71], a slot-type CSRR is etched inside the tuning stub of the printed elliptical slot antenna, and implemented for a band-stop application. It was found that an alterable notched band could take place by adjusting the radiuses of the CSRR. In [72], the CELC resonator is etched inside the circular patch of the monopole antenna to achieve the notched frequency band. The CELC could provide a predominantly magnetic response. At the notched frequency, the current flows into the CELC region so that the desired high attenuation near the notched frequency would be produced. In [73], a fractal-binary tree slot embedding technique for the band-notched characteristics design was introduced. By etching a dual band-notched resonance slot using a four-iteration fractal binary tree, two additional filters are applied to the radiating element of the antenna. The fractal, which effect increases the possible length of isolated current paths on the radiating element, has clear and useful properties for band-stop design within small antenna footprints.

#### **5.2. Parasitic stub**

144 Ultra Wideband – Current Status and Future Trends

transmission line, and hybrid techniques.

**5.1. Embedding slot** 

(WiMAX) service from 3.3 to 3.6 GHz also shares spectrum with the UWB. This may cause interference between the UWB system and other exist communication systems. To address this problem, one way is to use filters to notch out the interfering bands. However, the use of an additional filter will result in increasing the complexity of the UWB system and also the insertion loss, weight and size for the UWB trans-receivers. Therefore, various UWB antennas with notched functions have been researched to overcome this electromagnetic interference. This section concludes the existing band-notched techniques, which can be classified into the following categories: embedding slot, parasitic stub, bandstop

Among various proposed techniques on the band-notched UWB antenna design. One common and simple way is to etch slots on the radiation patch or ground plane. Up to now, many shapes of embedding slots were studied, and some representatives are shown in Fig.26 *i.e.*, Kim *et al*. [63] proposed a CPW-fed planar UWB antenna with a hexagonal radiating element. By inserting a V-shaped thin slot with a length of λc/4 (λc is the wavelength of notched frequency) on the hexagonal radiating element, the narrow frequency band-notched is created, where the fractional bandwidth is approximately 8~10%. Chung *et al*. [64-66] introduced the printed UWB monopole antenna by inserting an inverted U-shaped, Π-shaped or rectangular slot. At the notched frequency, current is concentrated around the edges of the slot and is oppositely directed between the interior and the notched frequency. This leads to the desired high attenuation near the notched frequency. In [67], a band-notched printed monopole antenna is provided by using two modified U-shaped slots on the monopole. The U-shaped slot perturbs the resonant response and also acts as a half-wave resonant structure. At the notched frequency, the desired high attenuation near the notched frequency can be produced. Jiang *et al*. [68, 69] introduced a pair of inverted-L-shaped slots around the microstrip line on the ground plane; a frequency-notched response can also be achieved.

**Figure 26.** Notched-band designs with various slots on patch or ground [63-69].

Similar to the embedding slot technique in the UWB antenna design, another commonly used technique is a parasitic strip or stub in the aperture area of the antenna or a nearby radiator that forms a resonant structure and leads to a sudden change in the impedance in the notched band. Many parasitic strips or stubs were studied and several representative structures are presented in Fig.28.

For the UWB printed wide-slot antenna design, Liu *et al*. [74] proposed a UWB rectangular slot antenna with a fractal tuning stub to realize the notched function. Chui *et al*. [75] proposed a branch with a length of a quarter of the wavelength adding on the tuning stub to obtain the band-notched property. Cai *et al*. [76] studied a pair of elliptic arc-shaped strips inserted into a suitable aperture region to disturb the field distribution that generates the resonance at the designed notched-band. The total length of this pair of strips is adjusted to about half-wavelength at the desired notched-band, resonance will occur at the strips.

For the printed UWB monopole design, Zhang *et al*. [77] introduced a segmented circular planar monopole antenna with a notched band. Through cutting apart a circular monopole patch with a pair of symmetrical slots, the patch is divided into three segments: the center patch and two side patches. Practically, the side patches function as two parasitic elements and work as bandstop filters. Then, the band-notched property is achieved. Wu *et al*. [78] introduced a square looped resonator and an end-coupled resonator to achieve the gain suppression in the notched band. The square-looped resonator consists of two square loops whose physical length approximates half a wavelength at the notched frequency. Meanwhile, the end-coupled resonator is composed of a strip line with a pair of quarter wavelength folded open stubs. Compared with the band-notched methods using thin slits and plastic strips, this resonator has a small size and a fast rolloff rate well as 10–25 dB gain suppression (generally, the gain suppression of thin slits and plastic strips are usually less than 10 dB).

Ultra-Wideband Antenna and Design 147

**Figure 29.** Various bandstop transmission lines [79-82].

Using one notched-band technique will face two problems. Firstly, it is relatively difficult to create multiple frequency notches with a sharp and narrow stop band. Secondly, multinotched bands do not have any means to control independently because of the same technique. Therefore, various notched-band techniques have been together used to realize the WiMAX and WLAN bands rejection. The representative hybrid techniques are shown in Fig.30 *i.e.*, Abdollahavand *et al*. [83] studied the hybrid technique by adding parasitic strip and bandstop transmission line. A compound band-notched structure is formed by embedding Гshaped stubs in the radiation patch and a modified G-slot defected ground structure in the feeding line, which can provide two filtering frequencies in a certain band and function as a second-order filter. Ye *et al*. [84] studied the hybrid technique of a parasitic strip and a parasitic slit, where the parasitic strip is embedded inside the polygon slot and an isolated slit employed in the beveled T-stub. The desired excellent band-notched UWB operation can be obtained by choosing the sizes of the parasitic strip and slit. Zhou *et al*. [85] presented the hybrid technique of adding parasitic and embedding slot to realize dual notched bands of WiMAX and WLAN. Firstly, the circular patch is cut to an annular ring and a pair of Y-shaped strips is connected to the annular ring, the notched band of WiMAX centered at 3.5 GHz is realized. Then, an inverted V-shaped slot is etched on the patch, a notched band of 5.2–5.98 GHz for WLAN band is achieved. Niu *et al*. [86] used the hybrid technique of CCRC resonator and embedding slot, where a CCRC is to realize the 5 GHz WLAN notched-band and an elliptic arc-shaped slot is to realize the WiMAX notched band. Kim *et al*. [87] suggested a triple-band notched hybrid technique, which is based on a geometric combination of a meander shaped stub and the two rectangular complementary split ring resonators (CSRRs)

on the feedline, and an inverted U-shaped slot on the center of the patch.

**Figure 30.** Hybrid notched-band techniques [83-87].

**5.4. Hybrid techniques** 

**Figure 28.** Notched-band designs with various stubs [74-78].

#### **5.3. Bandstop transmission line**

The above mentioned notched-band techniques, such as embedding slot or ELC resonator, parasitic stub, will result in affecting the antenna radiation, especially for increasing of the cross-polarization. A transmission line with a bandstop characteristic to feed the UWB antenna can be considered as an integration design of the printed UWB antenna and the filter, which may have little affection to the antenna radiation. Several designs of microstrip feedline with the notched-band function are proposed, as shown in Fig.29 *i.e.*, Zhang *et al*. [79] proposed a Ushaped slot embedded in a microstrip feedline, and a band-notched characteristic was realized. Later, Nouri *et al*. [80] used the defect ground technique to realize the microstrip filter, where a vertical metal strip connected to the rectangular ring is embedded in the shovel-shaped slot that is located under the feedline at the center of ground plane. The notched frequency can be controlled by adjusting the dimensions of the filter structure. Moreover, the electromagnetic band-gap (EBG) structure has a characteristic of preventing wave propagation in special directions or at certain frequencies. In [81], square EBG cells are placed close to the microstrip feedline to obtain the desired notched bands. In [82], the dual band-notched characteristic has been achieved by introducing two open-circuited stubs from two sides of the microstrip feedline. By adjusting the length of two open circuited stubs approximately to one quarter of wavelength, a destructive interference of the current distribution takes place causing the antenna non-radiating at that notched frequency.

**Figure 29.** Various bandstop transmission lines [79-82].

#### **5.4. Hybrid techniques**

146 Ultra Wideband – Current Status and Future Trends

**Figure 28.** Notched-band designs with various stubs [74-78].

antenna non-radiating at that notched frequency.

**5.3. Bandstop transmission line** 

than 10 dB).

For the printed UWB monopole design, Zhang *et al*. [77] introduced a segmented circular planar monopole antenna with a notched band. Through cutting apart a circular monopole patch with a pair of symmetrical slots, the patch is divided into three segments: the center patch and two side patches. Practically, the side patches function as two parasitic elements and work as bandstop filters. Then, the band-notched property is achieved. Wu *et al*. [78] introduced a square looped resonator and an end-coupled resonator to achieve the gain suppression in the notched band. The square-looped resonator consists of two square loops whose physical length approximates half a wavelength at the notched frequency. Meanwhile, the end-coupled resonator is composed of a strip line with a pair of quarter wavelength folded open stubs. Compared with the band-notched methods using thin slits and plastic strips, this resonator has a small size and a fast rolloff rate well as 10–25 dB gain suppression (generally, the gain suppression of thin slits and plastic strips are usually less

The above mentioned notched-band techniques, such as embedding slot or ELC resonator, parasitic stub, will result in affecting the antenna radiation, especially for increasing of the cross-polarization. A transmission line with a bandstop characteristic to feed the UWB antenna can be considered as an integration design of the printed UWB antenna and the filter, which may have little affection to the antenna radiation. Several designs of microstrip feedline with the notched-band function are proposed, as shown in Fig.29 *i.e.*, Zhang *et al*. [79] proposed a Ushaped slot embedded in a microstrip feedline, and a band-notched characteristic was realized. Later, Nouri *et al*. [80] used the defect ground technique to realize the microstrip filter, where a vertical metal strip connected to the rectangular ring is embedded in the shovel-shaped slot that is located under the feedline at the center of ground plane. The notched frequency can be controlled by adjusting the dimensions of the filter structure. Moreover, the electromagnetic band-gap (EBG) structure has a characteristic of preventing wave propagation in special directions or at certain frequencies. In [81], square EBG cells are placed close to the microstrip feedline to obtain the desired notched bands. In [82], the dual band-notched characteristic has been achieved by introducing two open-circuited stubs from two sides of the microstrip feedline. By adjusting the length of two open circuited stubs approximately to one quarter of wavelength, a destructive interference of the current distribution takes place causing the Using one notched-band technique will face two problems. Firstly, it is relatively difficult to create multiple frequency notches with a sharp and narrow stop band. Secondly, multinotched bands do not have any means to control independently because of the same technique. Therefore, various notched-band techniques have been together used to realize the WiMAX and WLAN bands rejection. The representative hybrid techniques are shown in Fig.30 *i.e.*, Abdollahavand *et al*. [83] studied the hybrid technique by adding parasitic strip and bandstop transmission line. A compound band-notched structure is formed by embedding Гshaped stubs in the radiation patch and a modified G-slot defected ground structure in the feeding line, which can provide two filtering frequencies in a certain band and function as a second-order filter. Ye *et al*. [84] studied the hybrid technique of a parasitic strip and a parasitic slit, where the parasitic strip is embedded inside the polygon slot and an isolated slit employed in the beveled T-stub. The desired excellent band-notched UWB operation can be obtained by choosing the sizes of the parasitic strip and slit. Zhou *et al*. [85] presented the hybrid technique of adding parasitic and embedding slot to realize dual notched bands of WiMAX and WLAN. Firstly, the circular patch is cut to an annular ring and a pair of Y-shaped strips is connected to the annular ring, the notched band of WiMAX centered at 3.5 GHz is realized. Then, an inverted V-shaped slot is etched on the patch, a notched band of 5.2–5.98 GHz for WLAN band is achieved. Niu *et al*. [86] used the hybrid technique of CCRC resonator and embedding slot, where a CCRC is to realize the 5 GHz WLAN notched-band and an elliptic arc-shaped slot is to realize the WiMAX notched band. Kim *et al*. [87] suggested a triple-band notched hybrid technique, which is based on a geometric combination of a meander shaped stub and the two rectangular complementary split ring resonators (CSRRs) on the feedline, and an inverted U-shaped slot on the center of the patch.

**Figure 30.** Hybrid notched-band techniques [83-87].
