INTRODUCTION

On 14th February in the year of 2002 the Federal Communication Commission (FCC) in the United States had released a bandwidth of 7.5 GHz i.e. from 3.1GHz to 10.6 GHz for Ultra-wideband (UWB) wireless communication [1] which is currently a rapidly growing advancement as a high data rate wireless communication technology. This UWB is the defined radio system having a bandwidth of 10dB which is larger than 25 percent of its center frequency. Since 2002 the researchers have designed and also proposed so many antennas for UWB applications by taking care of the various requirements such as compactness, less fragile, light weight, low cost and portability in hand held devices in the UWB system. To achieve good impedance band-widths, Omni-directional far field beam patterns and radiation efficiencies the strongest contenders are elliptical and circular disc monopoles [2]. The designs for these antennas can be made printed and also can be allowed for simple integration and low-cost fabrication with the association of UWB electronics.
Like the conventional wireless communication system UWB antennas also face some challenges in broadband impedance matching, gain characteristics which need to be appropriate and also in getting stable radiation patterns. But over this UWB frequency band, there also exist some narrowband wireless local area network (WLAN) operating bands such as the 5.2 GHz (5150–5350 MHz) and 5.8 GHz (5725–5825 MHz) band. World Interoperability for Microwave Access (WiMAX) service from 3.3-3.6 GHz also occupies in the UWB band [1],[3], [7]. So, the interference may be caused with these bands with UWB operations. In some UWB antenna designs, filters can also be used to notch out the interfering bands. But the demerit is that it increases the weight and complexity of the UWB system. Hence to overcome this problem, various UWB antennas with notch functions have been developed to mitigate the interference between the narrowband wireless systems and UWB systems. Incorporation of various shaped slots of different sizes into the main radiator is the most preferable simple and common approach. Band notch characteristics are achieved by using different shapes of slots with coupling strips. By varying the width and position of the slot we can control the band width and Centre frequency of the notched band. Two band notches can be introduced in a planar monopole UWB antenna using two different types of slots [4-5]. Especially at higher frequencies to improve the impedance matching the patch radiators are slotted. The current distribution at the radiators gets changed by the radiator cut slots with the change in input impedance and current path. A notch band of 5.12 GHz to 5.99 GHz can be realized by inserting a slot of U-shape in the radiating patch of half elliptical ring. Similarly, by loading two approximate half-wavelength slots of U-shape the band notch functions can be realized. These functions change the distribution of current on the y-shaped patch [6-7]. For annular ring UWB antennas with microstrip feed, WLAN and DSRC (dedicated short-range communication) band notch property can be achieved simply by inserting a partial annular slot in the radiator of antenna. To construct the left- hand materials, Pendry was developed Split ring Resonator (SRR). In these materials electromagnetic waves behave in the reverse direction in comparison to conventional rule of right-handed materials [8]. The SRR gives interest in its resonant behavior specifically. SRRs are generally considered as the electronically small resonator having a very high Q. The SRRs are very useful where sharp notch is required in the construction of filters and also to pass a band of certain frequency range [9]. The SRRs provide two types of properties such as resonance and anti-resonance properties. Inherently due to these properties the flow of the electromagnetic field can be passed or stopped which are localized and polarized along the SRR array. This is due to the resonance permeability and anti-resonance permittivity of the SRR [10-13]. Comparing UWB antennas with narrow band antennas it is found that due to the optimization in its wide bandwidth UWB system has greater impact and instead of continuous signals these UWB antennas transmit pulsed signals [14]. For narrow band antennas the standard parameters such as return loss, gain, radiation efficiency, etc. have been defined in frequency domain. Therefore, it is not enough to analyze the UWB antennas only in the frequency domain since it is needed to control the pulse distortion which is an important parameter [15-16]. Hence also in time domain these antenna characteristics must be studied. As the antennas behave differently during transmission and reception due to the involvement of large fractional bandwidth pulses the time domain analysis is equally important in the UWB antenna design used for high speed pulse communication [17-19]. Equivalent circuit of the patch antenna had been also derived by using lumped elements. So, for the applications of UWB frequency band several antenna configurations have been studied [1]-[24].
Based on the different kinds of designs of UWB notch-antennas mentioned above in literature survey, a simple, compact microstrip line fed notch antenna is proposed. The proposed antenna has a circular radiating element. For the proper impedance matching, a square shaped slot is created symmetrically in the circular radiating element, an open-ended slot is created in the middle of the ground plane and an open-ended slit ring created in the thick protruding stub from the upper portion of the square shaped slot created in the circular radiating element. The thick protruding stub helps in creation of a preliminary notch at the notch frequency centered at 4.7 GHz, which is not appropriate center of the notch frequency for the WLAN band of operation. Hence for the appropriate creation of the center of the notch frequency at 5.5 GHz (5 GHz to 6 GHz, WLAN), a split ring resonator SRR is embedded in the left portion of the microstrip line. Because of this SRR, the center of the notch frequency is tuned properly to 5.5 GHz from 4.7 GHz. The proposed antenna provides the bandwidth from 3.097GHz to 13.326GHz which is the operating bandwidth for the UWB system and a notch band from 5 GHz to 6 GHz (WLAN) with a center notch frequency of 5.5 GHz. This proposed notch antenna provides the average gain of 7.1 dBi which is suitable for operation in UWB system. The consistent average group delay is around 0.2 ns in the UWB band of operation and around 2.8 ns in the notch band of operation. The proposed antenna provides stable radiation patterns in the UWB band of operation with the appropriate level of cross-polar radiation. The equivalent circuit is developed for the proposed UWB notch antenna. The time domain analysis is performed for this proposed antenna. The proposed UWB notch antenna exhibits tremendous capability of handling the UWB short pulses. The proposed antenna is fabricated and all the antenna parameters are effectively measured. The measured results are in highly accordance with the simulation results.
The structural configuration, chronological development, conceptual analysis of the proposed UWB antenna are described in section I. Section II depicts the equivalent circuit analysis of the proposed antenna. Section III includes the comparative analysis of simulated and measured results of the proposed antenna. Section IV examines the time domain behavior of the proposed antenna, which is followed by the conclusion in section V.