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A Miniaturized Hilbert PIFA for Dual-Band Mobile
Wireless Applications
Mohammed Z. Azad
Motorola, Inc.,
azad@motorola.com
Mohammod Ali
University of South Carolina - Columbia,
alimo@engr.sc.edu
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Published in
IEEE Antennas and Wireless Propagation Letters,
Volume 4, 2005, pages 59-62.
http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?reload=true&punumber=7727
© 2005 by IEEE
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 4, 2005
59
A Miniaturized Hilbert PIFA for Dual-Band Mobile
Wireless Applications
Mohammed Z. Azad and Mohammod Ali,
Senior Member, IEEE
Abstract—A
miniaturized planar inverted-F antenna (PIFA) is
proposed for dual-band mobile phone application in the 900- and
1900-MHz bands. By employing a Hilbert geometry, an overall size
reduction of 50% was achieved compared to a conventional rect-
angular PIFA. The proposed antenna can be easily printed on the
inside surface of the plastic housing of a mobile phone or other
wireless device. An experimental prototype of this miniature an-
tenna was fabricated on 0.125 mm thick Duroid 5880 substrate.
Measured results demonstrate dual-band characteristics with good
radiation patterns.
Index Terms—Antenna,
Hilbert, miniature, planar inverted-F
antenna (PIFA), wireless.
I. I
NTRODUCTION
W
ITH the widespread proliferation of telecommunication
technology, the need for small, internal, multiband
antennas has increased greatly [1]–[6]. Lately the planar in-
verted-F antenna (PIFA) has gained much interest due to its
lower profile, good bandwidth, and ease of fabrication [1]–[5].
However, a dual-band PIFA operating in the 900/1900 MHz
mobile telephone operating frequency bands can occupy sub-
stantial space in order to satisfy the bandwidth requirements
(880–960 MHz for GSM and 1850–1990 MHz for PCS). Uti-
lizing geometrical configurations, such as meander or double
meander resulted in smaller antennas operating as monopoles
in [6].
Lately significant efforts have been made to study and op-
timize antennas based on the Hilbert type curve [7]–[9]. The
usefulness of a Hilbert wire antenna in order to lower the an-
tenna resonant frequency has been studied in [7]. In [8] a para-
metric study was conducted on a matched Hilbert antenna to un-
derstand its bandwidth and cross-polarization level. A printed
Hilbert antenna was proposed in [9] for operation in the UHF
band.
In this paper we introduce a PIFA that consists of a Hilbert
geometry that can support dual-band operations at 900 and
1900 MHz. Utilizing the Hilbert curve results in a significantly
smaller antenna than its conventional counterpart. The pro-
posed antenna occupies a volume of only 4.3 cm (40 mm by
10.65 mm by 10 mm). In contrast, a conventional dual-band
PIFA occupies a volume of 8.8 cm (40 mm by 22.18 mm
Fig. 1. Dual-band PIFA—Antenna A.
by 10 mm). Thus, a 50% saving in antenna volume is readily
achieved with our proposed design.
II. A
NTENNA
D
ESIGN
The antenna geometry conforming to the Hilbert profile ef-
fectively increases the length of the current flow path making
it possible to develop a miniature antenna. Since to achieve
dual-band performance at least two branches of a radiating el-
ement are needed [2], [5], we focus on studying antennas that
contain two branches and are connected to each other near the
feed [see Fig. 1]. For all antenna geometries, a ground plane
110 mm
made of a thin metallic conductor with length,
40 mm was considered. The antenna height
and width,
was fixed at 10 mm. Study and design of all antennas were con-
ducted using Ansoft HFSS. First an antenna consisting of two
Hilbert elements as shown in Fig. 1 was considered. The smaller
element near the feedpoint is responsible for resonance in the
high frequency band while the larger element is responsible for
the resonance in the low frequency band. The models of each el-
ement were constructed following a higher order Hilbert curve
as described in [8]. The individual segments and the trace width
(0.71 mm) were selected such that the two elements can be ac-
commodated within the width of the ground plane (40 mm). No
Manuscript received September 7, 2004; revised December 1, 2004. This
work was supported in part by the National Science Foundation (NSF) Career
Award ECS-0237783.
The authors are with the Department of Electrical Engineering, University of
South Carolina, Columbia, SC 29208 USA (e-mail: alimo@engr.sc.edu).
Digital Object Identifier 10.1109/LAWP.2005.844128
1536-1225/$20.00 © 2005 IEEE
60
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 4, 2005
(a)
(b)
(c)
Fig. 2. (a) Antenna B. (b) Antenna C on 0.125-mm substrate.
(c) Antenna D.
TABLE I
A
NTENNA
P
ARAMETERS
III. R
ESULTS
Computed VSWR and impedance data for antennas A and
B are shown in Figs. 3 and 4, respectively. Measured VSWR
data are shown in Fig. 5 for antenna C. The Hilbert geometry
for antenna A was created and adjusted several times to achieve
resonance at around 900 and 1900 MHz.
Similar adjustments in the geometrical parameters were also
made for antennas B, C, and D. All antenna parameters are
listed in Table I. From the VSWR plots of Fig. 3 it is clear that
the bandwidths of antenna A are 9.4% and 4.4% within 2.5:1
VSWR. The bandwidth in the high frequency band for this con-
figuration is relatively narrow for the 1900-MHz band PCS ap-
plication. The bandwidths of antenna B are 10.4% and 7.3%, re-
spectively. Computed input impedance data for antennas A and
B are shown plotted in Fig. 4. Clearly, the impedances at 1850
and 1990 MHz for antenna A are much more spread than an-
tenna B. Thus, even though all antennas are fairly well matched
at their center frequencies antenna A is narrowband at the high
frequency.
Similarly, computed bandwidths for antennas A, B, and C
are shown in Fig. 5. Measured data for antenna C along with
an inset photograph of the fabricated antenna are also shown in
Fig. 5. Computed bandwidths for antenna C are 9.6% and 6.5%,
respectively while measured bandwidths are 8% and 5.6%, re-
spectively. The measured bandwidth is slightly narrower than
the computed, particularly due to the high inductance provided
by the center conductor of the coaxial feed line. In practical ap-
plications, metal strip type feed, as indicated in Figs. 1 and 2, is
generally used which should improve the bandwidth further.
attempt was made to reduce the trace width and further minia-
turize the antenna. We call this first model antenna A. All pa-
rameters for this antenna are listed in Table I.
Antenna A resonated at around 920 and 1920 MHz and had
good operating bandwidth in the low frequency band. In the
high frequency band the bandwidth was relatively narrow. This
prompted us to investigate a hybrid geometry consisting of a
Hilbert element for the low frequency band and a plate element
for the high frequency band. This resulted in the development
of antenna B [Fig. 2(a)]. Both antennas, A and B were studied
in the absence of any dielectric material.
To fabricate and measure the proposed antenna we considered
sub-
to use a thin (0.125 mm thickness) Duroid 5880
strate. A prototype antenna similar to antenna B was first mod-
eled and simulated on Duroid 5880. A slight modification in the
geometry of antenna B was required which resulted in antenna
C. Thus, antenna C represents the design on 0.125 mm thick
Duroid 5880 [Fig. 2(b)]. For comparison, a conventional PIFA
was also modeled (Antenna D, Fig. 2(c)]. The parameters of
40 mm,
18.65 mm,
the conventional antenna were:
20 mm,
4 mm,
16.47 mm,
3.53 mm,
0.71 mm,
20.76 mm, and
6 mm (distance be-
tween feed and shorting pin).
AZAD AND ALI: MINIATURIZED HILBERT PIFA FOR DUAL-BAND MOBILE WIRELESS APPLICATIONS
61
Fig. 3. Computed VSWR data for antennas A and B near (a) 900-MHz band and (b) 1900-MHz band.
Fig. 4. Computed input impedance data for antennas A and B near (a)
900-MHz band and (b) 1900-MHz band.
Fig. 6. Computed current distributions for Antenna C (a) at 910 MHz and
(b) at 1912 MHz.
Fig. 5.
and C.
Computed and measured VSWR characteristics of antennas A, B,
Computed current distributions of antenna C at 910 MHz and
1912 MHz are shown in Fig. 6. As apparent at 910 MHz the cur-
rent density is high in the Hilbert element which is responsible
for the low frequency resonance. At 1912 MHz the current den-
sity is high in both the polygonal and the Hilbert elements.
Computed radiation patterns for antennas C and D are shown
in Fig. 7. Patterns are computed at 910 and 1912 MHz for an-
tenna C and 920 and 1925 MHz for antenna D. In all cases,
shows uniform
the low resonant frequency pattern at
coverage in the azimuthal plane which is desirable in mobile
component being dominant. In
wireless application with the
the pattern resembles that of a
the elevation plane
dipole antenna with some cross-polarization (suppressed below
20 dB). Similarly, the azimuthal pattern in the high frequency
band is also fairly uniform with both components being close
to each other. In the elevation plane the pattern has a butterfly
shape. The peak gain at 910 MHz is 2.4 dBi and at 1912 MHz
it is 4.6 dBi. A longer antenna and ground plane at the high fre-
quency results in a more directive pattern and, hence, the high
gain. It is well known that current on the tip of monopole is zero,
. But the proposed PIFA
which results in zero fields along
providing better coverage in the
has nonzero fields along
upper hemisphere as compared to vertical monopole antenna.
IV. C
ONCLUSION
A miniaturized Hilbert type PIFA comprising of a Hilbert and
a plate-type element is proposed. This geometry occupies only
62
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 4, 2005
Fig. 7.
Computed radiation patterns for antenna C at (a) 910 MHz and (b) 1912 MHz; antenna D at (c) 920 MHz and (d) 1925 MHz.
about 50% of the volume (only 4.3 cm ) needed by a conven-
tional metal plate-type PIFA. Computed and measured band-
width of the antenna indicates good performance for dual-band
mobile phone application at around 900 and 1900 MHz.
R
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