Low-Profile Rolled Planar Antenna with Wide Bandwidth

Abstract

A novel size-reduction technique for a planar monopole type antenna is presented. By rolling the planar element of this antenna, a low-profile radiator with broad impedance bandwidth is obtained without excessively perturbing its radiation properties. The proposed antenna is capable of covering frequencies in the range 3.1 GHz to 15 GHz. The antenna yields a high gain with high radiation efficiency across the full bandwidth. Radiation patterns show good omnidirectional features, so it is a remarkable candidate for in-door or vehicular applications where large frequency range is required

Full text

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Dublin Institute of Technology
ARROW@DIT
Conference papers
School of Electronic and Communications
Engineering
2007-01-01
Low-profile rolled planar antenna with wide
bandwidth
Giuseppe Ruvio
Dublin Institute of Technology
Max Ammann
Dublin Institute of Technology, max.ammann@dit.ie
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Recommended Citation
Ruvio, G. and Ammann, Max: Low-profile rolled planar antenna with wide bandwidth. IWAT '07: International Workshop on
Antenna Technology: Small and Smart Antennas Metamaterials and Applications, 21-23 March,Cambridge, England, 2007,pp.77-80.

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Low-Profile Rolled Planar Antenna with Wide Bandwidth
G. Ruvio and M. J. Ammann
Centrefor Telecommunications Value-chain Research
School ofElectronic & Communications Engineering
Dublin Institute ofTechnology
Kevin Street, Dublin 8, Ireland
Email. giuseppe.ruvio@,tstudent. dit. ie
ABSTRACT: A novel size-reduction technique for a planar monopole type antenna is presented. By rolling the planar
element of this antenna, a low-profile radiator with broad impedance bandwidth is obtained without excessively
perturbing its radiation properties. The proposed antenna is capable of covering frequencies in the range 3.1 GHz to 15
GHz. The antenna yields a high gain with high radiation efficiency across the full bandwidth. Radiation patterns show
good omnidirectional features, so it is a remarkable candidate for in-door or vehicular applications where large
frequency range is required.
INTRODUCTION
In recent years, the design ofproducts capable of operating across several communication protocols has attracted great
interest in both the antenna industry and scientific community. One ofthe most important issues taken into account as a
constraint for such an important target has been a sustainable compromise between compactness, decent radiation
features and broad impedance bandwidth. In addition to that, a stable omnidirectional radiation pattern across the
working bandwidth has been considered as an advantageous characteristic for in-door and vehicular applications.
Monopolar antennas are very successful candidates to accomplish those requirements due to their large impedance
bandwidth and omnidirectional patterns [1, 2]. However, because oftheir bulky structure, several techniques have been
recently introduced in order to reduce the overall dimension and manufacturing efforts while radiation properties are not
heavily perturbed. Rolled planar monopoles have been already introduced in literature [3, 4]. This radiated element has
been demonstrated to possess very interesting broadband impedance features but its long shape drastically reduces
applications where more compactness is required. The antenna presented in this paper is a rolled monopolar antenna
horizontally placed on the ground plane. This solution reduces the overall height of the antenna to 0.17. at 3 GHz. At
the same time, different bandwidth enhancement techniques have been integrated.
impedance characteristics of a rolled monopole as seen in [3], but locating this radiating element horizontally with
respect to the ground plane, so that a lower profile is achieved and that bevelling techniques can be introduced. In fact
bevelling the bottom edge ofplanar monopolar antennas has been demonstrated to shift upward significantly the upper
edge frequency [5, 6]. A strong capacitive coupling occurs in this region and this is heavily dependent on both the bevel
angle and the separation between the bottom edge of the antenna and the ground plane. A strong current distribution is
concentrated in the gap between the ground plane and rolled monopole. This heavily influences the radiation properties
of the antenna across the band of operation. The excitation of more modes and a consequent improvement of the
impedance bandwidth response are obtained by the use ofan offset feeding point. This phenomenon is well documented
in literature [7]. As the antenna is folded in a roll, it assumes the shape of a cavity. This particular profile is able to
separate the radiated field from the field distribution occurring within the spirals ofthis antenna. In fact, the impedance
match of this particular planar monopole is aided by the current distribution on the roll, but its contribution to the
radiated field is shielded by the bigger spiral, which shrouds the internal spirals. Numerical simulations have been
obtained by Finite Integral Time Domain Method and compared to experimental results of the constructed prototype
with acceptable agreement.
The basic idea is to exploit the
ANTENNA DESIGN
The antenna geometry and coordinate system is shown in Fig. 1. The planar antenna is made of0.2mm thick brass sheet
rolled according to the constraints:
r2=rl-4mm
r3=rl-5.5mm
r4=rl-7mm r5=rl-8.5mm
1-4244-1088-6/07/$25.00 02007 IEEE.
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Fig.1 Geometry, coordinate system and cross-section ofthe antenna presented.fg=1.3mm, x=6.3mm,
g=lOOmm, rl=lOmm, wl=30mm, w2=9mm.
where rl is equal to 10mm. The total length ofthe rolled section is equal to 29.521 mm. The total height ofthe antenna
above the groundplane is given by h = fg + x + rl . The relation between the different radii of the spires of the
geometry can be kept fixed by appropriate transparent spacers. This radiating element is placed at the centre of a 100 x
100 mm ground plane and the input signal is launched to the antenna through a 50Q coaxial connector located at the
back of the ground plane. The base of the radiating element is appropriately shaped to enhance the match by the
insertion of a double asymmetrical bevel and an offset feed point. The optimum geometry for the double asymmetrical
bevel at the base ofthe radiator was obtained by Finite Integral Time Domain Method, so that W is given by the sum of
wl=30mm and w2=9mm. The distance x from the tip ofthe bevelled plate to the first planar half-cylinder ofradius rl is
6.3 mm.
IMPEDANCE AND RADIATION CHARACTERISTICS
The antenna presented covers a very broad 1OdB Return Loss bandwidth as shown in Fig. 2. Three notches are visible at
3.7-3.9, 5.8-6.8 and 6.9-7.1GHz in the measurement for which the return loss does not reach 1OdB; however, 7 dB is
achieved from 3.1 to 15GHz.
-10 0
V
U/)
-20
-30
-40
68
Frequency [GHz]
Fig. 2 Simulated and measured 10dB Return Loss Bandwidth.
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Measured Exy
----- Measured Exy-xpol
..........
Simulated Exy-xpol
Measured Exz
----- Measured Exz-xpol
Simulated Exz
----Simulated Exz-xpol
90
90
120
60
SimulatedExy
120
60
..........
-10
10
20
150
30
2
150
30
1B0
rt2|10
-30
30--
40 -1800
40 -1800
30
30-
-20
-20
210
3
210
330
-10
10lo
0
240
0
02430
270
270
Measured Eyz
----- Measured Eyz-xpol
Simulated Eyz
-Simulated Eyzxpol
90
0 - ..
120 60
-10-
2
15030
-30-
401800
-30
20
2
-10
240300
270
Fig. 3 Radiation patterns at 6GHz. xy-, xz- andyz-cut.
A good agreement is obtained with simulations just at the lower part ofthe spectrum investigated. In fact, the radiating
element is made ofa very thin metal sheet (thickness roughly equal to )/500 at the lower edge frequency) rolled in three
spires. Such a complex round geometry can be only generally approximated by the mesh grid. The resolution of this
approximation must be carefully weighted against the computational effort. Moreover, due to its shape, the antenna is a
very highly resonating structure. This makes even more difficult to reach the convergence of the algorithm within a
significant threshold of accuracy and reasonable computational time. However, the package we used is still able to give
an idea (although generally pessimistic) ofthe trend ofthe structure presented.
All the enhancement techniques introduced are demonstrated to be successful to stretch the impedance bandwidth of
this antenna especially at the upper part ofthe spectrum.
The radiation pattern is illustrated in Fig. 3, which is normalised to maximum gain. The maximum gain was found to
be between 7.5 dBi and 8.5 dBi in the broadside direction across the operative bandwidth. Three plane cuts of the
radiation pattern at 6GHz are shown for the four configurations investigated: sb,0 = 900, (xy-plane), 0, 5 = 0°, (xz-
plane) and 0,0= 900 (yz-plane) cuts. All the measured radiation patterns are illustrated against the simulated
component in 10dB/division scaled plots. Moreover, each radiation pattern cut is presented together with the simulated
co- and cross-polar component. In particular, the xz-cut shows a very different behaviour when compared to a planar
monopole. The monopolar null is mitigated for this structure. This suggests that the operating mode of the antenna is
mainly transmission-line or patch mode [8]. This is due to the horizontal elements in radiator structure with respect to
the ground plane. In fact, through simulations of the far field at low frequencies, a strong distribution of the electric
field points in the z-axis; this is typical of a patch radiation. Also the current distribution in Fig 4 supports this thesis
because the contribution to the far field of the strong current distribution in the inner spirals is limited, so that the
dominant role is played by the horizontal currents on the upper part of the radiator. The offset feeding technique is
employed to generate an unbalanced current on the antenna structure as shown in Fig 4. However, its does not
significantly degrade the radiation pattern in the xy-plane, where the antenna presents good omnidirectional features. On
the other hand, the xz-cut pattern is shown to be asymmetric as the strong x-component ofthe far field is augmented by
an unbalanced current in that direction on the radiator and also reflected on the ground plane. Indeed, a maximum in the
pattern is observed between 0= 00 and 0=300 from Fig. 3.
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3GHz
6GHz
~~~~~~~~~~~~~~~~~~A/mn
38 6
27.E
19. 5
M
~~~~~~~~~~~~~~~~13. 4
9GHz 12GHz
8.94
+%4~~~~~~~~~~~~~~~5.63-
3.17
.~~~~~~~~~~~~~~~~~~~~~l .35
Fig. 4 Current distribution at the cross-section of the antenna.
CONCLUSIONS
A novel rolled horizontal planar monopole antenna has been presented. This radiating element presents broadband
features together with interesting omnidirectional pattern. The metallic spirals that form this antenna create a
constructive current distribution that improves the impedance match and reduces the overall size and profile, without
heavily affecting the radiation pattern. A full investigation of the antenna has been made and a description of the
broadbanding techniques has been presented. Moreover, a study ofthe current distribution on the structure investigated
has been carried on at various frequencies within the working bandwidth. Both experimental and numerical analyses are
in reasonable agreement.
ACKNOWLEDGEMENTS
This work was supported by Science Foundation Ireland.
REFERENCES
[1]
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technique," Microw. Opt. Techn. Letters, July 2001, vol. 30, no. 4, pp. 229-232.
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80
3GHz6GHz
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