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

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 (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 transmission line, and hybrid techniques.

Ultra-Wideband Antenna and Design 145

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

useful properties for band-stop design within small antenna footprints.

**5.2. Parasitic stub** 

structures are presented in Fig.28.

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

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

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.
