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.