Index Terms—Directional patterns, finite ground plane, mi-crostrip line feed, mimi-crostrip slot antenna, omni-directional patterns, reflecting sheet, ultrawideband UWB.. Antenna Geomet
Trang 1(2.65–11.30 GHz) impedance bandwidth which exceeds the UWB
requirement of 110% (3.10–10.60 GHz) A ground plane of 50 mm
80 mm size is used which is similar to wireless cards for several
portable wireless communication devices The proposed antenna
covers only the top 20 mm or 25% of the ground plane length,
which leaves enough space for the RF circuitry Three variations
of the antenna design using the straight and rotated feed lines on
two different substrates are considered Effect of the conducting
reflecting sheet on back of the antenna is investigated, which can
provide directional radiation patterns but with reduced matching
criteria Finally, experimental verification of the fabricated
an-tenna for its impedance bandwidth is carried out, which shows
agreement with the simulated data.
Index Terms—Directional patterns, finite ground plane,
mi-crostrip line feed, mimi-crostrip slot antenna, omni-directional
patterns, reflecting sheet, ultrawideband (UWB).
I INTRODUCTION
T HE federal communications commission (FCC) has
allo-cated the frequency spectrum from 3.1 GHz to 10.6 GHz
as the ultrawideband (UWB) in the year 2002 Since then the
UWB technology has progressed a lot and is still emerging It
has created increased interest in the UWB antennas, as well
The UWB wireless communication antennas are special due to
very short and low-power impulse signals, which are transmitted
efficiently with less distortion Planar forms of the UWB
an-tennas can also be integrated between the radio frequency (RF)
front end circuitry and the radiating structure One way of
im-plementing planar forms of the antenna is using the microstrip
technology, which is widely used in wireless applications
Mi-crostrip antennas are popular because of its low profile, small
size, lightweight, low cost, high efficiency and economical
fab-rication features [1], [2] One form of the microstrip antennas is
the microstrip slot antenna, which radiates omni-directional
ra-diation patterns Microstrip slot antennas fed by a microstrip line
have shown wideband and ultrawideband performances [3], [4]
Manuscript received December 21, 2007; revised August 18, 2008 Current
version published May 06, 2009 This work was supported by the University
Grant Program (UGP), San Diego State University, CA.
The authors are with the Department of Electrical and Computer Engineering,
San Diego State University, San Diego, CA 92182-1309 USA (e-mail: sunil.k.
rajgopal@gmail.com; ssharma@mail.sdsu.edu).
Digital Object Identifier 10.1109/TAP.2009.2016694
tenna type is one of the most popular candidates for the UWB antennas In [7], a square slot (arc on one side) with a square shape feed and a triangular slot with a triangular shape feed provided bandwidths of 120% and 110%, respectively In [8],
a U-shaped tuning stub was introduced to enhance coupling be-tween the elliptical/circular slots and feed line so as to broaden operating bandwidth of the antenna The UWB antennas were achieved in [9] where slot antennas with U-shaped tuning stub and reflector was realized using two different types of the feed mechanisms In [10], a circular slot fed by a coplanar waveguide (CPW) line through a polygonal patch provided a large band-width from 2.6–15 GHz Some other types of the microstrip slot antennas have also been reported in [11]–[17]
In this paper, we investigate a novel planar pentagon shape microstrip slot antenna with the UWB impedance and radia-tion pattern characteristics Secradia-tion II presents the proposed an-tenna designs and anan-tenna performance results Effect of the conducting reflecting sheet on the antenna performance is pre-sented in Section III, where the aim is to get directional radiation patterns Section IV presents measurement verification of the impedance bandwidth and group delay, in addition to, the UWB antenna characteristics verification using a simulation study Fi-nally, Section V presents the conclusions The simulation results were obtained by employing the Ansoft Corporations Designer v3.0 and High Frequency Structure Simulator (HFSS) v10.0 tools, which are method of moments (MOM), and finite element method (FEM) based commercial full wave analysis programs, respectively [18]
II ANTENNAGEOMETRY ANDSIMULATIONRESULTS
A Antenna Geometry
In this study, three different antenna designs are considered, i.e., Design A: straight feed line on Rogers’s RT/Duroid 5880 substrate ( , ), Design B: tilted feed line on RT/Duroid 5880 substrate ( , ), and Design C: tilted feed line on FR-4 substrate ( ,
) The simulation model of the proposed an-tennas and photograph of the fabricated prototype are shown
in Fig 1(a) and (b), respectively, which consists of a pentagon shape microstrip slot, and tilted microstrip transmission feed line with a pentagon stub Dimension of the pentagon slot are
0018-926X/$25.00 © 2009 IEEE
Trang 21354 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL 57, NO 5, MAY 2009
Fig 1 The proposed pentagon shape microstrip slot antenna fed in using a
50
for the antenna Designs A, B & C, and (b) photograph of the fabricated prototype
of the antenna Design C on FR-4 substrate.
Fig 2 The reflection coefficient ( S , dB) versus frequency (GHz) plot for the
antenna Designs A, B, and C generated using the Ansoft Designer For
com-parison, the Ansoft HFSS generated reflection coefficient result of the antenna
Design B is also included.
shown in Fig 1(a) which only requires 20 mm or 25% length
on the ground plane leaving enough space for the RF circuitry
For all the designs, the pentagon shape slot and stub dimensions
are kept invariant, which were selected after parametric study
but not shown here for the sake of brevity The thickness “h” of
the substrate material is kept 1.58 mm for all the designs For
the tilted feed line Designs B & C, the feed line is rotated by
15 The antenna is fed using a 50 coaxial SMA connector
connected to 50 microstrip transmission feed line The
ground plane size is 50 mm 80 mm for all the designs which
is similar in size to several portable wireless cards The ground
plane size selection is also based on the study presented in [5],
[6] on the microstrip slot antennas
B Impedance and Radiation Characteristics
The reflection coefficient results for the three Designs A, B,
and C are shown below in Fig 2 obtained using the Ansoft
Designer simulations, which considers infinite substrate
mate-rial but a finite ground plane size of 50 mm 80 mm The
tilted feed line Design B was also simulated using the Ansoft
HFSS to observe effect on the antenna performance of the
fi-nite substrate size, in addition to other fifi-nite dimensions of the
Fig 3 Gain radiation patterns of the antenna Design B at frequencies (a) 4 GHz, (b) 7 GHz, and (c) 10 GHz within the UWB range.
antenna It also includes the SMA connector effects on the an-tenna performance which is close to the anan-tenna geometry The HFSS simulated reflection coefficient result is shown in Fig 2 along with the Designer simulated reflection coefficient data The impedance bandwidth is generally defined for range of the frequencies which satisfy the VSWR 2:1 or the reflection coef-ficient, criteria It is observed that, for the an-tenna Design A, bandwidth is 106% (2.6–8.4 GHz with respect
to the (w.r.t.) center frequency) For the antenna Design B, band-width is 124% covering a frequency range from 2.65–11.3 GHz The antenna Design C showed a bandwidth from 2.4–9 GHz,
Trang 3Fig 4 The antenna Design B results for the (a) gain (dBi) versus frequency
(GHz) at the broadside angle ( = 0 ), and (b) Peak gain (dBi) versus
fre-quency (GHz).
which is 116% Similarly, the Design B, also simulated using
the HFSS, showed a bandwidth of 127% (2.8–12.6 GHz) There
are visible multiple resonances within the bandwidth, which
when joins provide impedance bandwidth exceeding the UWB
requirement of 110% (3.1–10.6 GHz) It can be observed that,
the Designs B & C with rotated feed lines exhibit enhanced
bandwidth than Design A which uses straight feed line It is also
evident that, the antenna Design B provides the maximum
band-width among all Further, the antenna Design B, which was
sim-ulated using both the Designer and HFSS programs, predicted
almost similar bandwidths of 124% and 127%, respectively, and
thus they agree well
Fig 3(a)–(c) shows the radiation patterns of the antenna
De-sign B within the UWB range obtained using the HFSS
sim-ulations The co-polarization ( at plane and at
plane) and cross-polarization ( at plane
and at plane) components gain patterns are plotted
at frequencies 4 GHz [Fig 3(a)], 7 GHz [Fig 3(b)], and 10 GHz
[Fig 3(c)] It is evident that, near omni-directional radiation
pat-terns can be obtained, which deteriorate towards the higher
fre-quency end The radiation patterns variation within the
band-width is attributed to the irregular pentagon shapes of both the
slot and the stub, and its effective electrical dimension
varia-tion with the frequency This can generate undesired current
distributions at higher frequencies, which is responsible for the
pattern deterioration at higher frequency end It can also be
ob-served that, the cross-polarization components increase with the
Fig 5 (a) Geometry of the antenna Design B backed by a reflecting sheet at
a spacing of d from the antenna, and its effect on the (b) reflection coefficient ( S , dB).
increase in frequency, which is attributed to the pentagon shape stub and tilt of the feed line
The co- and cross-polarization components gain values at the broadside angle for both the and 90 cut planes, and the peak gain values with the frequency variation are also shown plotted in Fig 4(a) and (b), respectively An ex-amination of Fig 4(a) reveals that, the co-polarization gain com-ponents vary from 5.80 dBi at 2.80 GHz (start of the bandwidth)
to almost 0 dBi as frequency exceeds 8 GHz, and then it become 4.10 dBi at 12.60 GHz (end of the bandwidth) Similarly, Fig 4(b) shows that the peak gain varies between 3.00–6.25 dBi from around 3.50–13.0 GHz Therefore, the peak gain variation
is around 3.25 dBi for most of the frequencies falling within the UWB range, though at 3.10 GHz the peak gain increases to 7.50 dBi Thus the antenna radiates well throughout the range III EFFECT OFREFLECTING SHEET ONANTENNA
PERFORMANCE
The effect of a conducting reflecting sheet on back of the an-tenna Design B on the impedance matching and radiation pat-tern performance was also studied to see, if the reflecting sheet can be used to provide unidirectional radiation patterns such as
in the case of a microstrip patch antenna [1], [2] A square re-flecting sheet of dimension 50 mm 50 mm is placed at spacing
“d” from the antenna, as shown in Fig 5(a) The spacing “d” was
Trang 41356 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL 57, NO 5, MAY 2009
Fig 6 Gain radiation patterns of the antenna backed by a reflecting sheet at d = 5 mm or 10 mm within the UWB range frequencies (a) 4 GHz, (b) 5 GHz, (c) 6 GHz, (d) 7 GHz, (e) 9 GHz, and (f) 10 GHz.
varied from 5–25 mm at a step of 5 mm for the parametric study
All the design parameters of the antenna were kept the same
including thickness of the substrate material Fig 5(b) shows
the reflection coefficient variation versus frequency with the
re-flecting sheet spacing variation from to 25 mm From
Fig 5(b) it can be observed that, between to 15 mm,
the antenna shows matching level starting from about 5 dB to
better However, for and , the antenna is
matched better than 10 dB level from almost 2.50–11.50 GHz
For case, the impedance bandwidth is about 129%
(2.5–11.5 GHz)
For obtaining directional radiation patterns within the UWB
range, a combined spacing between and 10 mm can
be used Fig 6(a)–(f) show the gain radiation patterns with
or 10 mm spacing for frequencies in the UWB range, i.e.,
4, 5, 6, 7, 9, and 10 GHz, respectively generated using the HFSS
simulations For the lower (3–7 GHz) and upper (8–11 GHz)
spac-ings are found suitable, respectively An evaluation of the ra-diation patterns from Fig 6(a)–(f) reveals that, the patterns are fairly directional at the broadside angle (gain variation between 3–8 dBi) with front-to-back (F/B) ratios between 7–15 dB The patterns also show asymmetry and scan for some of the frequen-cies providing beam peak gain values between 4–8 dBi at angles other than the broadside angle Not presented here, but a single spacing can also be used to achieve directional patterns but with slightly reduced direction-ality This antenna can be further improved to achieve better antenna performance characteristics by implementing a recon-figurable spacing “d” Thus, a directive antenna within the UWB range can be obtained using the proposed slot antenna and a re-flective conducting sheet, which is also planar and compact in size It can be used for some wireless communication applica-tions if matching criteria of is acceptable Further, this can also be used to reduce the back lobe radiation in hand-held devices
Trang 5Fig 7 (a) S-parameters (dB) versus frequency (GHz), and (b) S phase
(de-grees) versus frequency (GHz) plots for the transmit-receive antenna system.
IV VERIFICATION OF THEANTENNA
The antenna was verified using the HFSS simulation for
the UWB communications using the technique outlined in
[19], [20], where a transmit/receive antenna combination was
considered Both transmit and receive antennas were similar
(Design B) and placed 100 mm apart facing each other as
suggested in [19] This transmit-receive antenna combination
can also be considered as a two-port network The
S-param-eters and phase versus frequency variations are shown in
Fig 7(a) and (b) The reflection coefficient results show similar
impedance matching behavior for most of the frequencies,
except that at the start and end of the bandwidth they do not
overlap The parameters provide all the important
system parameters in terms of the gain, impedance matching,
polarization matching, path loss and phase delay Therefore,
these parameters can be used to predict performance of the
UWB antenna system which is frequency dependent [19] From
Fig 7(a) it can be observed that, the transmission coefficients
of the antenna system cover the UWB frequency
range within near to the 10 dB variation Further as expected,
the phase is nonlinear within the UWB range [shown in
Fig 7(b)] The phase centers vary with frequency because the
Fig 8 (a) Comparison of the simulated and measured reflection coefficient ( S , dB) results for the fabricated prototype antenna shown in Fig 1(b), and (b) Measured group delay for the transmit-receive combination of the antennas when facing each other.
antenna radiation behavior is dependent upon the effective antenna dimension, which changes with frequency for a given physical antenna dimension
Two prototypes of the proposed antenna Design C were fab-ricated The photograph of one of them is already shown in Fig 1(b) Since the previously considered FR-4 substrate thick-ness of was not readily available in the Antenna and Microwave Laboratory (AML), being developed at the San Diego State University, therefore, the substrate thickness used for the fabrication was The antenna was again simulated using the HFSS for this substrate thickness, so that
it can be compared with the measured data The antenna re-flection coefficient was measured using a HP8510C Vector Net-work Analyzer The measured reflection coefficient along with the simulated data is shown plotted in Fig 8(a) The measured impedance bandwidth w.r.t is 117% which covers a frequency range from 2.6–10 GHz In comparison to this, the simulated bandwidth is 115% (2.5–9.3 GHz) Both sim-ulated and measured results show multiple resonances which are responsible for such a wide bandwidth performance The slight variation in frequency range can be attributed the fabrication er-rors Thus, it can be observed that, the simulated and measured
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results are in good agreement The antenna was also
experimen-tally verified for the group delay using the two antenna
arrange-ment [20], where the transmit and receive antennas are facing
each other while connected to the two ports of the Network
An-alyzer The 100 mm spacing between the antennas is equivalent
to (free space wavelength) at the 3 GHz and at 10 GHz
The group delay result is shown in Fig 8(b) The ripples may
be attributed to the scattering effect from the network cables It
can be observed that, the group delay between the antennas is
around 0.7 ns and varies by 0.125 ns within the bandwidth Thus
the antenna shows fairly constant group delay
V CONCLUSION
In this paper, a planar ultrawideband (UWB) pentagon
shape microstrip slot antenna is investigated for the impedance
matching and radiation pattern characteristics This antenna
occupies only 25% space on the 50 mm 80 mm size ground
plane along the length The antenna can find applications in
portable wireless communication devices Both straight feed
line and tilted feed line designs were investigated with two
different substrate materials of the same thickness It was
observed that, for the tilted feed line Design B an impedance
bandwidth of 124% (2.65–11.3 GHz) can be obtained, which
exceeds the required UWB range of 110% (3.1–10.6 GHz)
However, all three antenna Designs A, B, and C almost met
the UWB frequency range requirements, and provided nearly
omni-directional radiation patterns Further, by employing a
conducting reflecting sheet on the back of the antenna,
di-rectional radiation patterns can be obtained within the UWB
range but with the reduced matching criteria It can be used not
only to get directive antenna within the UWB range but also
to reduce the back lobe radiation The measured impedance
bandwidth of the fabricated antenna showed good agreement
with the simulated data The transmit/receive combination of
the proposed antenna showed acceptable UWB communication
performance in terms of the S-parameters and group delay
ACKNOWLEDGMENT
Authors would also like to thank C Meagher for helping in
the measurements, and the anonymous reviewer’s comments
that helped in improving the presentation of this paper
REFERENCES
[1] R Garg, P Bhartia, I Bahl, and A Ittipiboon, Microstrip Antenna
De-sign Handbook. Norwood, MA: Artech House, 2001.
[2] J L Volakis, Antenna Engineering Handbook, 4th ed. New York:
McGraw Hill, 2007.
[3] A A Eldek, A Z Elsherbeni, and C E Smith, “Microstrip-fed printed
lotus antenna for wideband wireless communication system,” IEEE
An-tennas Propag Mag., vol 46, no 6, pp 164–173, Dec 2004.
[4] A M Abbosh, M E Bialkowski, J Maziersha, and M V Jacob,
“A planar UWB antenna with signal rejection capability in the 4–6
GHz band,” IEEE Microw Wireless Compon Lett., vol 16, no 5, pp.
278–280, May 2006.
[5] S K Sharma, L Shafai, and N Jacob, “Investigation of wide-band
microstrip slot antenna,” IEEE Trans Antennas Propag., vol 52, no 3,
pp 865–872, Mar 2004.
[6] S I Latif, L Shafai, and S K Sharma, “Bandwidth enhancement
and size reduction of microstrip slot antennas,” IEEE Trans Antennas
Propag., vol 53, no 3, pp 994–1003, Mar 2005.
[7] Y F Liu, K L Lau, Q Xue, and C H Chan, “Experimental studies of
printed wide-slot antenna for wide-band applications,” IEEE Antennas
Wireless Propag Lett., vol 3, pp 273–275, 2004.
[8] P Li, J Liang, and X Chen, “Study of printed elliptical/circular slot
an-tennas for ultrawideband applications,” IEEE Trans Anan-tennas Propag.,
vol 54, no 6, pp 1670–1675, June 2006.
[9] R Chair, A A Kishk, K F Lee, C E Smith, and D Kajfez, “Mi-crostrip line and CPW fed ultrawideband slot antennas with u-shaped
tuning stub and reflector,” Progr Electromagn Res., PIER, vol 56, pp.
163–182, 2006.
[10] S.-Y Lin, H.-R Huang, C.-W Guan, and S.-A Yeh, “A novel
com-pact slot antenna for ultra-wideband communication,” in Proc IEEE
Antennas Propag Society Int Symp., HI, 2007, pp 5123–5126.
[11] I.-T Tang, D.-B Lin, G.-H Liou, J.-H Horng, and C.-M Li, “A
compact slot UWB antenna with CPW-fed,” in Proc IEEE Antennas
Propag Society Int Symp., HI, 2007, pp 5143–5146.
[12] J.-Y Jan and C.-Y Hsiang, “Microstrip-line fed printed slot antenna
for ultra-wideband operation,” in Proc IEEE Antennas Propag Society
Int Symp., HI, 2007, pp 5139–5142.
[13] S H Choi, J K Park, S K Kim, and J Y Park, “A new ultra-wideband
antenna for UWB applications,” Microw Opt Technol Lett., vol 40,
no 5, pp 399–401, Mar 5, 2004.
[14] A Vasylchenko, W D Raedt, and G A E Vandenbosch, “Electric and magnetic type compact UWB antenna using u-shaped slot with
band-rejection characteristic,” in Proc IEEE Antennas Propag Society
Int Symp., HI, 2007, pp 4617–4620.
[15] J.-M Ju, G.-T Jeong, J.-H Yoon, C.-S Kim, H.-S Kim, and K.-S Kwak, “Design of multiple u-shaped slot microstrip patch antenna in
5 GHz band WLAN,” IEICE Trans Commun., vol E88-B, no 2, pp.
821–825, Feb 2005.
[16] T Dissanayke and K P Esselle, “UWB performance of compact
l-shaped wide slot antennas,” IEEE Trans Antennas Propag., vol 56,
no 4, pp 1183–1187, Apr 2008.
[17] A M Abbosh and M E Bialkowski, “Design of planar ultrawideband
planar monopole slot antennas of circular and elliptical shapes,” IEEE
Trans Antennas Propag., vol 56, no 1, pp 17–23, Jan 2008.
[18] Ansoft Corporations, Designer and High Frequency Structure Simu-lator (HFSS) [Online] Available: www.ansoft.com
[19] Z N Chen and T S P See, “Antennas for UWB applications,” in
Antennas for Portable Devices, Z N Chen, Ed. Hoboken, NJ: Wiley, 2007.
[20] Z N Chen, X H Wu, H F Li, N Yang, and M Y W Chia,
“Consid-erations for source pulses and antennas in UWB radio systems,” IEEE
Trans Antennas Propag., vol 52, no 7, pp 1739–1748, Jul 2004.
Sunil Kumar Rajgopal was born in Tuticorin, Tamil
Nadu, India, in 1985 He received the B.Eng degree
in electronics and telecommunication from Thakur College of Engineering and Technology, Mumbai, India, in 2006, and the M.Sc degree in electrical engineering from San Diego State University, San Diego, California, in 2008.
His main research interests are in small, planar and broadband antennas including ultrawideband antennas for handheld wireless applications.
Trang 7Winnipeg, Manitoba, Canada, from May 2001 to August 2006 Simultaneously,
he was also a Research Associate at University of Manitoba from June 2001 to
August 2006 In August 2006, he joined San Diego State University (SDSU),
San Diego, CA, as an Assistant Professor in the Department of Electrical and
and Propagation Symposia He was Chair of the Student Paper Contest of the IEEE Antennas and Propagation Society International Symposium 2008 held
in San Diego.