18Novel Modified UWB Planar Monopole Antenna With Variable Frequency Band-Notch Function
112IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS,VOL.7,2008 Novel Modi?ed UWB Planar Monopole Antenna With Variable Frequency Band-Notch Function
Reza Zaker,Changiz Ghobadi,and Javad Nourinia
Abstract—A novel modi?ed microstrip-fed ultrawideband (UWB)planar monopole antenna with variable frequency band-notch characteristic is presented.By inserting two slots in the both sides of microstrip feedline on the ground plane, much wider impedance bandwidth can be produced.A modi?ed H-shaped conductor-backed plane with variable dimensions is used in order to generate the frequency band-stop performance and control its characteristics such as band-notch frequency and its bandwidth.The designed antenna has a small size of 22222mm2and operates over the frequency band between3.1 and14GHz for VSWR<2while showing the band rejection performance in the frequency band of5.1to5.9GHz.
Index Terms—Frequency band-notch function,microstrip-fed monopole antenna,ultrawideband(UWB)antenna.
I.I NTRODUCTION
R ECENTLY,ultrawideband(UWB)technology has been widely used in various radars and has attracted much at-tention for communication systems.The design of an antenna is one of the most exciting challenging tasks in these systems. For UWB systems,released by the FCC in2002[1],planar monopole antennas are good candidates.These antennas owing to its attractive merits such as wide impedance bandwidth and omnidirectional radiation pattern have been widely used for a long time.For impedance bandwidth enhancement,techniques such as using a pair of notches at the lower corner of patch[2], inserting a slot in the tapered radiating element[3],using the notch structure in the ground plane[2],[3]and adding a narrow slit in one side of the monopole[4]have been reported.On the other hand,the frequency range for UWB systems between3.1 to10.6GHz will cause interference to the existing wireless com-munication systems,for example the wireless local area network (WLAN)for IEEE802.11a operating in5.15–5.35GHz and 5.725–5.825GHz bands,so the UWB antenna with a band-stop performance is required.Recently to generate the frequency band-notch function,modi?ed planar monopoles with inverted U-slot[4],U-slot[5]and small strip bar[6]are used.
In this letter,a new band-notched printed monopole antenna is presented.The notched band,covering the5-GHz WLAN band, is provided by using a modi?ed H-shaped conductor-backed plane instead of changing the patch or feedline shapes of the antenna.Also by inserting two slots in the both sides of mi-crostrip feedline on the ground plane,much wider impedance
Manuscript received January16,2008;accepted February1,2008.This work was supported by Iran Telecommunication Research Center(ITRC).
The authors are with the Department of Electrical Engineering,Urmia Uni-versity,Urmia,Iran(e-mail:st_r.zaker@urmia.ac.ir;ch.ghobadi@urmia.ac.ir; j.nourinia@urmia.ac.ir).
Digital Object Identi?er
10.1109/LAWP.2008.919621
Fig.1.Geometry of the proposed planar monopole antenna(unit:mm).
bandwidth can be produced,especially at the higher band.Ex-
perimental and simulated results of the constructed prototype
are presented.
II.A NTENNA D ESIGN
Fig.1shows the con?guration of the proposed wideband
monopole antenna which consists of a simple rectangular patch,
a truncated ground plane with two slots and H-shaped con-
ductor-backed plane.The proposed antenna is constructed on
FR4substrate with thickness of1.0mm and relative dielectric
constant of4.4.The
width of the microstrip feedline is?xed at1.86mm to achieve
50-characteristic impedance.On the front surface of the substrate,a rectangular patch with size
of
is printed.The rectangular patch has a distance of2mm to the ground plane with a length of6mm printed on
the back surface of the substrate.
Regarding to defected ground structures(DGS),the creating
slots in the ground plane provide an additional current path.
Moreover this structure changes the inductance and capacitance
of the input impedance which in turn leads to change the band-
width.The DGS applied to a microstrip line causes a reso-
nant character of the structure transmission with a resonant fre-
quency controllable by changing the shape and size of the slot
[7].Therefore by inserting two slots at the ground plane and
carefully adjusting its
parameters,much enhanced impedance bandwidth may be achieved.As shown in Fig.1,
these slots placed at a distance of1mm
(about)from the ground’s center line.
As illustrated in Fig.1,the H-shaped conductor-backed plane
is placed under the radiating patch and is also symmetrical with
respect to the longitudinal direction.The conductor-backed 1536-1225/$25.00?2008IEEE
ZAKER et al.:NOVEL MODIFIED UWB PLANAR MONOPOLE ANTENNA
113
Fig.2.Simulated VSWR characteristic for various length L .(W =2mm).
plane perturbs the resonant response and also acts as a para-sitic half-wave resonant structure electrically coupled to the rectangular monopole [8].At the notch frequency,the current ?ows are more dominant around the parasitic element,and they are oppositely directed between the parasitic element and the radiation patch [9].As a result,the desired high attenuation near the notch frequency can be produced.
The variable band-notch characteristics can be achieved by
carefully choosing the parameters
(
,,
and )for the H-shaped conductor-backed plane.In this structure,the
width
,is the critical parameter to control the ?lter bandwidth and the coupling value between radiating patch and parasitic ele-ment.On the other hand,the center frequency of the notched
band is insensitive to the change
of
.The resonant frequency of the notched band is determined
by
and .In this design,the optimized
length is set to band-stop resonate at approxi-
mately
,
where
and corre-sponds to band-notch frequency (5.5GHz)and
also
is ?xed at 10mm (the width of the optimized radiating patch).
III.R ESULTS AND D ISCUSSIONS
The simulated results are obtained using the Ansoft high-fre-quency structure simulator (HFSS)[10].The parameters
(
and ),based on the parametric analysis of the proposed an-tenna,are optimized to achieve the maximum impedance band-width and good impedance matching.The simulated VSWR
curves with different values
of
are plotted in Fig.
2.From the simulation results in Fig.2,it is observed
that the impedance bandwidth increases as the
length
in-creases.The simulated VSWR curves with the optimal
length
for various slot
widths are plotted in Fig.3.
As the slot
width increases,the impedance bandwidth in-creases.On the other hand,the lower frequency of the band
is insensitive to the changes
of
and .We observe from Figs.2and 3that a new resonant frequency at 11GHz is added.
We conclude that the
parameters
and ,are the critical factors to determine the upper operating frequency,impedance bandwidth and impedance matching.
The simulated VSWR curves with different values
of
are plotted in Fig.4.As the
height of the conductor-backed plane increases from 9.0to 14.5mm,the center frequency of notched band is varied from 8.9to 5.3GHz.Also the
height is the other parameter to determine band-notch frequency
[9].
Fig. 3.Simulated VSWR characteristic for various length W .(L =
7:5mm)
.
Fig.4.Simulated VSWR characteristic for various length
L .(
D =
5:5mm)
.
Fig.5.Simulated VSWR characteristic for various length
W .
From these results,we can conclude that the notch frequency is
controllable by changing the two heights
of
and .Fig.5illustrates the simulated VSWR characteristics with
various
widths
.As the
width of the conductor-backed plane increases from 9.2to 14mm,the ?lter bandwidth is varied from 0.6to 2.2GHz.The optimal parameters of the conductor-backed plane are as
follows:
,
,
and .
Fig.6shows the measured VSWR performance of the pro-posed monopole antenna with and without H-shaped conductor-backed plane.For comparison,the VSWR of the simple rectan-gular monopole antenna is also shown in Fig.6.As illustrated in Fig.6,simple monopole antenna has poor VSWR character-istics at the frequency band over 8.9GHz.After inserting the two slots on the ground plane,the impedance bandwidth ranges
from 3.1GHz to more than 14GHz
.It is also
114IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS,VOL.7,
2008
Fig.6.Measured VSWR characteristic with and without H-shaped conductor-backed
plane.
Fig.7.Measured x 0z plane and y 0z plane radiation pattern for proposed antenna at (a)4GHz;(b)8GHz;and (c)11GHz.
observed that the sharp frequency band-notch characteristic is obtained very close to the desired frequency of 5.5GHz when a modi?ed conductor-backed plane is added to the
antenna.
Fig.8.Measured antenna gain.
The measured radiation patterns in
the
and plane are plotted in Fig.7(a)to (c),respectively.From an overall view of these radiation patterns,the antenna behaves quite similarly to the typical printed monopoles.Fig.8shows the measured maximum gain of the proposed antenna with and without con-ductor-backed plane.A sharp decrease of maximum gain in the notched frequency band at 5.5GHz is shown.For other frequen-cies outside the notched frequency band,the antenna gain with the ?lter is similar to those without it.
IV .C ONCLUSION
A novel UW
B planar monopole antenna with variable band-stop characteristics has been presented.Two inserted slots on the ground plane are used to increase the impedance bandwidth.Band-stop characteristics are achieved by adding the H-shaped conductor-backed plane.The proposed monopole antenna has
the frequency band of 3.1to over 13.9
GHz
with a rejection band around 5.1to 6GHz.
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