Low-Profile, Multi-Element, Miniaturized Monopole Antenna

Abstract

A low-profile, electrically small antenna with omnidirectional vertically polarized radiation similar to a short monopole antenna is presented. The antenna features less than lambda/40 dimension in height and lambda/10 or smaller in lateral dimension. The antenna is matched to a 50 Omega coaxial line without the need for external matching. The geometry of the antenna is derived from a quarter-wave transmission line resonator fed at an appropriate location to maximize current through the short-circuited end. To improve radiation from the vertical short-circuited pin, the geometry is further modified through superposition of additional resonators placed in a parallel arrangement. The lateral dimension of the antenna is miniaturized by meandering and turning the microstrip lines into form of a multi-arm spiral. The meandering between the short-circuited end and the feed point also facilitates the impedance matching. Through this technique, spurious horizontally polarized radiation is also minimized and a radiation pattern similar to a short dipole is achieved. The antenna is designed, fabricated and measured. Parametric studies are performed to explore further size reduction and performance improvements. Based on the studies, a dual-band antenna with enhanced gain is realized. The measurements verify that the proposed fabricated antennas feature excellent impedance match, omnidirectional radiation in the horizontal plane and low levels of cross-polarization.

Full text

Page 1

72 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 1, JANUARY 2009
Low-Profile, Multi-Element, Miniaturized
Monopole Antenna
Wonbin Hong, Student Member, IEEE, and Kamal Sarabandi, Fellow, IEEE
Abstract—A low-profile, electrically small antenna with omnidi-
rectional verticallypolarized radiationsimilartoashort monopole
antenna is presented. The antenna features less than
sion in height and
?? or smaller in lateral dimension. The an-
tenna is matched to a 50 ? coaxial line without the need for ex-
ternal matching. The geometry of the antenna is derived from a
quarter-wave transmission line resonator fed at an appropriate
location to maximize current through the short-circuited end. To
improve radiation from the vertical short-circuited pin, the geom-
etry is further modified through superposition of additional res-
onators placed in a parallel arrangement. The lateral dimension
of the antenna is miniaturized by meandering and turning the mi-
crostrip lines into form of a multi-arm spiral. The meandering be-
tween the short-circuited end and the feed point also facilitates the
impedance matching. Through this technique, spurious horizon-
tally polarized radiation is also minimized and a radiation pattern
similartoashortdipoleisachieved.Theantennaisdesigned,fabri-
cated and measured. Parametric studies are performed to explore
further size reduction and performance improvements. Based on
the studies, a dual-band antenna with enhanced gain is realized.
The measurements verify that the proposed fabricated antennas
feature excellent impedance match, omnidirectional radiation in
the horizontal plane and low levels of cross-polarization.
?? dimen-
Index Terms—Dual-band antennas, electrically small antennas,
monopole antennas, omnidirectional antennas.
I. INTRODUCTION
T
discovery of electromagnetic wave radiation. From the early
stagestopresent,thedipoleantennaanditsvariationshavebeen
extensively used in the field of wireless communication for its
simple geometry and reliability. In addition when the antenna
is vertically mounted, the dipole antenna features an omnidirec-
tional radiation pattern, making it useful for terrestrial applica-
tions, non-line-of sight conditions, and situations where trans-
ceiver modules are often deployed randomly. For near ground
wave propagation applications where both the transmit and re-
ceive antennas are placed near the surface of the earth, it is
shownthatpropagationpathlossforverticallyorientedantennas
is by many magnitude lower than any other antenna orientation
configuration. Monopoles that emulate virtually identical per-
formance behaviors as that of a dipole antenna are often used
at lower frequencies. For air or ground vehicle applications it
HE half-wave dipole antenna is perhaps one of the most
fundamental and commonly used antennas ever since the
Manuscript received March 26, 2008; revised August 11, 2008. Current ver-
sion published March 04, 2009.
Authors are with the Radiation Laboratory, Department of Electrical Engi-
neering and Computer Science, The University of Michigan, Ann Arbor, MI
48109-2122 USA (e-mail: wonbin@umich.edu).
Digital Object Identifier 10.1109/TAP.2008.2009731
is highly desirable to use low-profile antennas that can pro-
duce vertical polarization with omnidirectional radiation pat-
tern. Also applications such as network of unattended ground
sensors, low-profile antennas are highly desired. As wireless
communication devices continue to evolve, the large dimension
of the antenna is frequently problematic. Therefore it is imper-
ative to further investigate methods of realizing extremely short
monopole antennas with very small lateral dimensions. This
will allow integration of such antennas with the wireless device
package or platform.
A variety of space filling compression techniques have been
studied to reduce the vertical profile of monopole antennas
[1]–[3]. The dimension of a wire monopole antenna is greatly
reduced by folding the monopole antenna geometry through
several iterations. In [4], metallic paths patterned after Peano
and Hilbert curves are used to greatly reduce the vertical profile
of monopole antennas. The Peano-curve top-loaded monopole
featuresaverticalheightofapproximately
excellent monopole radiation pattern. In addition, a meandered
monopole antenna is further modified by extending a conductor
line for the end of a rectangular meander monopole to achieve a
dual-band operation [5]. In [6], [7], various fractal antennas are
further investigated and the resonant properties, bandwidth, and
impedance matching of compressed omnidirectional antennas
are reported.
The vertical height of a monopole antenna can also be re-
duced through antenna loading techniques. In [8], the antenna
is loaded with a dielectric cylinder and a dual-band behavior is
realized. Amonopole antenna is inductivelyloaded and itselec-
trical characteristic is studied [9], [10]. In [11]–[16], a low-pro-
file, omnidirectional antenna is reported by capacitively loading
a monopole antenna with modified disks. The heights of the
proposed antennas are in the range of
antennas feature excellent operational bandwidth. However, the
lateraldimensionoftheantennasarecomparabletowavelength.
Nonetheless, the impact of the Goubau antenna [16] is signifi-
cant in the antenna community due to the fact that it closely
approaches Chu’s fundamental limit. However, because of its
complex geometry, the Goubau antenna is difficult to be an-
alyzed using conventional methods. In [17], the Goubau an-
tenna is thoroughly analyzed using full-wave simulations. Re-
cently, artificial electromagnetic materials such as electromag-
netic band-gap (EBG) structures have been incorporated with
conventional wire antennas to feature a low-profile antenna that
behave equivalent to a vertical monopole antenna [18].
In this paper, a low-profile antenna with omnidirectional
vertically polarized radiation similar to that of a traditional
monopole antenna is proposed and presented using a different
approach. The proposed antenna is realized by modifying the
whilefeaturing
. In addition, these
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HONG AND SARABANDI: LOW-PROFILE, MULTI-ELEMENT, MINIATURIZED MONOPOLE ANTENNA 73
geometryofaquarter-wavemicrostriptyperesonatortoemulate
an electrically small antenna with a very small vertical profile.
The concept is based on superposition of multiple quarter-wave
segments that are meandered and spiraled around to virtually
negate the radiation from horizontal currents above the ground
plane. As a result, the antenna features a vertically polarized ra-
diation in the horizontal plane. The design and miniaturization
method for the antenna is presented in Section II. Simulation
and measurement results are also shown and discussed. The
antenna is further modified and parametric studies are done in
Section III. In Section IV, a dual-band multi-element monopole
antenna is designed and its measurement results are presented
and discussed.
II. THE MINIATURIZED MULTI-ELEMENT
MONOPOLE ANTENNA
Radiation powers from short monopole antennas are propor-
tional to the currents induced on them. In practice the level of
induced current is limited by the impedance mismatch between
the transmission line and the very small radiation resistance of
such radiating structures. External matching networks based on
lumped elements are lossy which render a rather poor radiation
efficiency. One approach to increase the induced electric cur-
rent on a short segment of a vertical wire above a ground plane
is to use the vertical wire as part of a resonant structure. The
smallest resonant structure can be formed from a quarter-wave-
length segment of a transmission line short-circuited at one end
and open-circuited at the other end (i.e., monopole). Consider
a microstrip line resonator formed by a strip above the ground
planeandshort-circuitedbyasmallverticalwire.Thisresonator
can be fed from the center as shown in Fig. 1(a). Impedance
matching of this quarter-wavelength resonator (monopole) is
facilitated by adjusting the feed position along the resonator
and choosing parameters
, the open-circuited segment and
, theshort-circuited segment.It is expectedthat thecurrent
flowing on the microstrip itself does not contribute to the total
radiated field due to cancellation of the far-field by its image in
the ground plane. The directionsof the currents atthe feed point
and on the short-circuited vertical wire are opposite of each
other; however, the magnitude of current at the short-circuited
wire can be significantly higher from which the net radiation
emanates.Theelectriccurrentflowingontheverticalelementof
the antenna is responsible for the vertically polarized radiation.
Ifthehorizontalelectriccurrentcanbeeffectivelyeliminatedwe
can achieve a completely vertically polarized radiation ignoring
the lateral electrical dimension for the time being. Cancellation
can be done by adding another monopole element while sharing
the same feed as seen in Fig. 1(b). The cancellation of the hori-
zontal electric current is achieved by introducing another set of
electric current that is in the opposite direction in the horizontal
plane of the antenna with the original electric current at elec-
tromagnetic resonance. In contrast, the vertical electric currents
flowing on the short-circuited pins of each elements is in phase
and as a result, behavesas the radiating elements of the antenna.
Thus, the two-element monopole antenna behaves as a small
vertically polarized antenna. To achieve omnidirectional radia-
tion patterns in the horizontal plane of the antenna while main-
taining low levels of cross-polarization levels,it is important for
Fig. 1. Design process of the miniaturized multi-element monopole antenna.
the antenna to be electrically small and symmetric. Additional
monopole elements are added in similar ways to negate the hor-
izontal currents. The increased number of short-circuited pins
provides increased number of radiators and improves the me-
chanical stability of the multi-element monopole antenna. Pa-
rameters
and must be modified to accommodate for
the changes in impedance caused by the additional monopole
elements. The topology of the four-element monopole antenna
is presented in Fig. 1(c).
The proposed topology of the multi-element monopole
antenna enables the height of a traditional monopole antenna to
be greatly reduced. However, as the height of the multi-element
monopole antenna reduces, the lateral dimension increases.
The miniaturization of the multi-element monopole antenna is
achieved by folding the open-circuited segments of the antenna
in a spiral-shaped geometry so the cancellation of horizontal
electric currents on the elements is maintained. To achieve
miniaturization, the short-circuited segments of the antenna
can also be meandered and placed slightly below the spiral
layer. A sketch of a miniaturized multi-element monopole
antenna is visualized in Fig. 1(d). The overall length of each
quarter-wavelength segment must be adjusted using a full-wave
approach to take the effects of near field mutual couplings into
account.
The miniaturized multi-element monopole antenna is de-
signed using Ansoft’s HFSS. The topology of the simulated
antenna is presented in Fig. 2. Parameters are optimized to
achieve operation around 460 MHz with lateral dimensions
less than
and the height to be around
cuited elements and the short-circuited elements are designed
on two separate vertical layers with 1 mm spacing for enhanced
space conservation and compactness. The open-circuited el-
ements are folded in a spiral-liked fashion and minimized to
mm in lateral dimension. The total
length of the open-circuited element
miniaturized open-circuited elements are fed by a single wire
feed.Themeanderedshort-circuitedelementsaredesignedtobe
placed 1 mm beneath the miniaturized open-circuited elements.
The elements in both layers are electrically connected by four
. The open-cir-
is 117 mm. The
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74 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 1, JANUARY 2009
Fig. 2. Topology of the miniaturized multi-element monopole antenna.
vertical pillars with a height of 1 mm. The short-circuited pins
are connected to each end of the four short-circuited elements
respectively. The total length from the vertical pillar to the
short-circuited pin
is 94.6 mm. Impedance matching
of the miniaturized multi-element monopole is achieved by
adjusting the length of the meandered short-circuited elements.
The antenna is then fabricated on a 0.3 mm thick copper
sheet. The open-circuited and short-circuited elements on each
vertical layers are milled separately and soldered together. A
1 mm thick dielectric spacer is placed between the two layers
to minimize fabrication and simulation discrepancies. The
short-circuited pins are milled to be 20 mm in height
then soldered to each ends of the meandered short-circuited el-
ements respectively. The short-circuited pins are then soldered
to a
final height of the fabricated antenna is 21.3 mm.
Fig. 3 shows the simulated and measured input reflection
coefficient of the proposed antenna. The fabricated antenna
features 3% 2:1 VSWR bandwidth. The slight discrepancy
between the simulated and measured responses can be at-
tributed to alignment errors during the fabrication process. The
radiation patterns of the miniaturized multi- element monopole
antenna is measured in the anechoic chamber of the University
of Michigan at the resonant frequency of 460 MHz. Fig. 4
shows the E- and H- Plane, co- and cross-polarized patterns of
the antenna. The measured antenna features omnidirectional
radiation pattern in the H-Plane, similar to a monopole. In
addition, a null is observed at
larization levels in both planes are mainly caused by the close
proximity of the feed network cable and the small ground
plane size. This can be confirmed with simulated radiations
patterns of the featured antenna on an infinite ground plane as
presented in Fig. 5. The gain of the miniaturized multi-element
monopole antenna is measured in the anechoic chamber using
a dipole antenna with a known gain. The gain is measured to
and
mm copper ground plane. The
in the E-Plane. Cross-po-
Fig. 3. Measured and simulated ?
ment monopole antenna.
response of the miniaturized multi-ele-
be
such electrically small ground plane dimension
the effect of the edge currents on the ground plane becomes
significant to the radiation pattern, directivity, and gain of the
proposed antenna. Strong levels of edge currents result in the
rise of cross-polarization levels, increased ohmic loss leading
to degradation of directivity and gain. In addition, the low gain
suggest that relatively high levels of currents on the open-cir-
cuited and short-circuited elements may be causing ohmic loss.
Therefore it is important to further investigate the topology of
the antenna and the effects of the ground plane.
dBi. The directivity is calculated to be 1.0 dBi. For
III. THE MODIFIED MULTI-ELEMENT MONOPOLE ANTENNA
A. Antenna Design
Now that the functionality of the proposed miniaturized
multi-element monopole antenna has been established, addi-
tional modifications are made to the antenna design to further
improve its performance. To reduce fabrication errors and
improve rigidity, the antenna is newly designed on a dielec-
tric substrate. As discussed in the previous section, the input
impedance matching of the antenna is achieved by adjusting
the length of the meandered element of the antenna. However,
such modification is rather complex, requiring the topology
of the antenna to be redesigned and fabricated. Therefore,
impedance matching is difficult to achieve through simple
tuning of the antenna topology. It has been shown that the
length of a slot antenna can significantly be reduced by in-
serting short-circuited narrow slot-lines along the radiating
segment of a slot antenna [19]. Basically, the short-circuited
narrow slot-lines behave as series inductive elements. Due to
the insertion of series inductive elements, the electric current
on the ground plane then transverses a longer path. As result
the resonant frequency decreases. The meandered short-cir-
cuited elements of the miniaturized multi-element monopole
antenna are replaced with a similar geometry. The meandered
short-circuited elements are first straightened. Then a pair of
0.5 mm wide open-circuited microstrip lines are inserted along
the straightened elements. The insertion of narrow open-cir-
cuited microstrip lines introduces shunt capacitance stubs
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HONG AND SARABANDI: LOW-PROFILE, MULTI-ELEMENT, MINIATURIZED MONOPOLE ANTENNA 75
Fig. 4. Measured and simulated radiation pattern of the miniaturized multi-
element monopole antenna. (a) E-Plane. (b) H-Plane.
which have a similar size reduction effect. Thus, the physical
lengths of the straightened short-circuited elements are re-
duced. Impedance matching is obtained by simply adjusting
the location (
mm) and lengths (
inserted narrow microstrip lines. The right angle edges of the
spiral-shaped open-circuited elements are replaced with curved
edges. The final lengths of the open-circuited elements
mm) of the
Fig. 5. Simulated radiation pattern of the miniaturized multi-element
monopole antenna on an infinite ground plane. (a) E-Plane. (b) H-Plane.
Fig. 6. Topology of the modified multi-element monopole antenna. The in-
serted open-circuited microstrip is shown in the right.
and short-circuited elements
respectively. The modified geometry is shown in Fig. 6.
are 144 mm and 20 mm
B. Fabrication and Measurement
The modified antenna is simulated and then fabricated using
mm Rogers5880 with thickness
of 1 mm, dielectric constant of
. The two layers of the antenna geometry are
etched on the top and bottom side of the dielectric substrate.
The two layers are then connected through via holes. The short-
a
, and loss tangent of
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76 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 1, JANUARY 2009
Fig. 7. Measured ?
tenna as a function of ground plane dimension.
response of the modified multi-element monopole an-
circuited pins are connected to each end of the four short-cir-
cuited elements. The integrated antenna structure is then sol-
dered on to a
The antenna is fed from the center using a coaxial feed. The
antenna height
is set to 20 mm. The return loss of the fab-
ricated modified multi-element monopole antenna is presented
in Fig. 7. The antenna displays good impedance matching and
3.5% 2:1 VSWR bandwidth. The far-field co-polarized
and radiation patterns of the antennas are presented in
Figs. 8(a) and 9(a) respectively. The measured pattern indicates
the radiation pattern of the antenna remains similar to that of
the original miniaturized multi-element monopole antenna after
the modification. As expected due to the small ground plane
dimension, the modified antenna features relatively high levels
of cross-polarization levels. The gain of the modified multi-ele-
ment monopole antenna is measured to be
the improved geometry enhances the radiation efficiency of the
antenna compared to the original miniaturized multi-element
monopole antenna.
mm ground plane.
dBi, indicating
C. Parametric Studies
The dimension of the ground plane is increased to examine
how the parameter affects the performance of the modified
multi-element monopole antenna. The antenna is subsequently
fabricated and measured on four different ground planes with
increasing dimension. No additional modifications to the
topology of the antenna is performed as the dimension of the
ground plane changes. The measured input reflection responses
of the antennas are presented in Fig. 7. It is observed that the
antenna remains to be well matched as the size of the ground
plane varies. The shift in resonant frequency can mostly be
attributed to the radiating ground plane currents which slightly
modify the radiation of the antenna. The measured 2:1 VSWR
bandwidths, gains, and the calculated directivities of the an-
tennas associated with different ground plane sizes are listed
in Table I. The measured and simulated radiation patterns in
Figs. 8 and 9 show as the ground plane dimension increases,
cross-polarization levels decrease accordingly. As expected
the gain increases as a result. It can be seen from Table I that
Fig. 8. Measured and simulated E-Plane radiation pattern of the modified
multi-element monopole antenna as a function of ground plane dimension.
(a) ? ? ?
? ?? ?? ? ?? mm. (b) ? ? ?
(c) Infinite ground plane.
? ??? ?? ? ??? mm.
the gain of the modified multi-element monopole antenna is
enhanced by more than 5 dB as the dimension of the ground
plane increases to 200 mm
It is important to study the effect of the height of the modi-
fiedmulti-elementmonopoleantennaonitsgainandbandwidth.
The height of the antenna from the metal ground plane is in-
cremented from
mm to
the height of the short-circuited pins. The modified antennas are
then fabricated on a
plane. Fig. 10 shows the measured input reflection response of
the antennas with different heights. The measured 2:1 VSWR
bandwidthsandgainsarelistedinTableII.Themeasuredresults
indicates that the height of the antenna can be further reduced to
less than
without suffering severe gain degradation which
mm.
mm by adjusting
mm ground
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