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Design of a Small and Low-Profile 2 2 Hemispherical
Helical Antenna Array for Mobile Satellite
Communications
H. T. Hui, Edward K. N. Yung, C. L. Law, Y. S. Koh, and W. L. Koh
2
Abstract—A small and low-profile 2 2 hemispherical helical antenna
array was designed for INMARSAT satellite reception. With only four elements fed by a simple network, the array can produce a gain of 15 dB,
a 3-dB axial ratio bandwidth of nearly 14%, a cross-polarization level of
smaller than 20 dB over the half-power beamwidth, and a radiation pattern with a sidelobe level of 20 dB. These characteristics are better than
those obtained by previously proposed INMARSAT-M antennas.
0
0
Index Terms—Antenna array, circular polarization, hemispherical helical antenna, satellite communications.
I. INTRODUCTION
Mobile satellite communications rely very much on the design
of good vehicle antennas [1]. In order to combat the problem of
polarization change of the electric field by the atmosphere, circular
polarized antennas are generally used. In the INMARSAT-M system,
the mobile vehicle antenna is required to be small in size and have a
relative high gain of 13–16 dB. Several circular polarized antennas
or antenna arrays have been suggested for use in this system [2],
[3]. However, they are either too large in size and require additional
design for feeding network [2] or their gain is too low [3]. In
view of this, a completely new antenna array is designed to provide
Manuscript received July 10, 2002; revised March 9, 2003.
The authors are with the School of Electrical and Electronic Engineering,
Nanyang Technological University, 639798 Singapore, Singapore (e-mail:
ehthui@ntu.edu.sg).
Digital Object Identifier 10.1109/TAP.2003.822410
an improved performance for INMARSAT-M mobile vehicles. In
[4], a low-profile hemispherical helical antenna that can produce
circular polarization over a wide angular range was proposed by Hui
et al. Using this antenna as array elements, we designed a small
hemispherical helical antenna array for mounting on INMARSAT-M
mobile vehicles. With only four elements fed by a simple network,
this array can produce a gain of 15 dB, a 3-dB axial ratio bandwidth
of nearly 14%, a cross-polarization level of 015 dB over an angular
range of 80 , a cross-polarization level of smaller than 20 dB over the
half-power beamwidth, and a 020 dB sidelobe-level radiation pattern.
These characteristics are better than those obtained by previously
proposed INMARSAT-M antennas or antenna arrays [2], [3]. In this
paper, we demonstrate a rigorous design of this array. The design is
to optimize the circular polarization characteristics (the axial ratio)
with respect to the interelement spacings and the relative angular
displacements of the elements.
II. DESIGN PROCEDURE
The array is to be constructed using the central-feed hemispherical
helical antenna as shown in Fig. 1(a). We denote the radius of the
hemispherical helix by a and the distance of the lowest point of
the helix from the ground plane by h. The angle subtended by the
straight wire leading from the coaxial point to the helix with the
negative y axis is denoted by . The helix consists of N turns with
an equal vertical distance between adjacent turns. The equation of
the hemispherical helix can be found in [4]. It was shown in [4] that
a five-turn hemispherical helical antenna can produce pure circular
polarization radiation over a wide angular range. A low axial ratio of
0.5 dB can be obtained at C= = 1:23, where C is the circumference
of hemispherical helix and is the operating wavelength. This antenna
has an extremely low profile of about 0.2 in height and a relatively
high gain of about 9 dB. These characteristics make this antenna
especially suitable for small-size and high-gain antenna or antenna
array design for satellite communications. We design the array by
using four such antennas deployed in a 2 2 2 configuration as shown
in Fig. 1(b). The interelement spacings (between the centres of the
elements) are denoted, respectively, by dx in the x direction and
dy in the y direction. The relative angular displacements of the
antenna elements are denoted, respectively, by 1 , 2 , 3 , and 4 .
The design of the array is carried out by the moment method [5]
aided by experimental measurements. The purpose of the design is
to determine optimum interelement spacings and relative angular displacements in order to achieve a low axial ratio (AR) and a relatively
high gain in the main-beam direction. Previous studies have shown that
mutual coupling effect between antenna elements significantly affects
the characteristics of an array [3], [6]. This is especially true for the
axial ratio [3], which is sensitive to the array configuration, interelement spacings, and the orientations of the antenna elements. So we first
investigate the variations of the axial ratio and the coupling effect with
the interelement spacings and the relative angular displacements.
A. Determination of the Interelement Spacings
The variations of the array axial ratio in the normal direction (perpendicular to the ground plane) and the decoupling factor (DCF) [6] with
the distance between the centres of two hemispherical helical antenna
elements are shown in Fig. 2 with the relative angular displacements
of the elements all set to 0 . The DCF is used to indicate the mutual
coupling effect between the two antenna elements. The dimensions of
the antenna elements are N = 5, C= = 1:23, and h= = 0:05.
0018-926X/04$20.00 © 2004 IEEE
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 1, JANUARY 2004
347
Fig. 3. Variations of the axial ratio and the DCF with the relative angular
displacement of two antenna elements separated by a distance of 0.7 in the
x and y directions. The antenna elements are the same as those in Fig. 2.
B. Determination of the Relative Angular Displacements
Fig. 1. Central-feed hemispherical helical antenna: (a) the element and (b) the
array.
In order for an antenna array to produce radiation with a good axial
ratio, the relative excitation phases of the elements have to be aligned
properly. In our design, the relative excitation phases of the elements
are controlled by changing the relative angular displacements [1 , 2 ,
3 , and 4 in Fig. 1(b)] of the elements. Fig. 3 shows the effect of the
relative angular displacements of two elements separated by a distance
of 0.7 along the x- and the y -axis on the axial ratio and the decoupling factor. The dimensions and the connection of the antennas are
the same as in Fig. 2. Fig. 3 shows that the axial ratio reaches two relative minima near = 0 and 180 for both cases. The DCF shows a
relative maximum at = 315 for elements separate along the x-axis
and a relative maximum at = 240 for elements separated along the
y -axis. But at these two relative angular displacements, the axial ratios
are quite large for both cases. As a compromise, we choose the relative
angular displacements to be 0 , i.e., 1 = 2 = 3 = 4 = 0 , at
which a moderate DCF of 33 dB is obtained.
III. ARRAY CHARACTERISTICS
Fig. 2. Variations of the axial ratio and the DCF with the interelement spacings
for two antenna elements with = 0 . The dimensions of the antenna elements
are N = 5, C= = 1:23, and h= = 0:05.
The results are obtained with element #2 being excited and element #1
being connected to a load that is a conjugate of the input impedance
of the elements. Two sets of data are plotted in the figure: one for
elements separated along the x-axis and the other for elements separated along the y -axis. In Fig. 2, both sets of data show that while the
DCF bears a roughly monotonically increasing function with the interelement spacing dx or dy , the axial ratio does not. The axial ratio
is a nonlinear function of dx or dy but we note that there is a relative
minimum near dx = 0:7 (at which AR= 2:4 dB) or dy = 0:7
(at which AR= 1:8 dB). Hence we take the interelement spacings
dx = dy = 0:7.
An array was constructed using the interelement spacings and the
relative angular displacements as determined above. The hemispherical helixes were wounded on the surfaces of four foam hemispheres,
each with a diameter of 7.5 cm. The calculated and measured input
impedances of a single central-feed hemispherical helical antenna are
shown in Fig. 4. At the frequency of 1.575 GHz (C= = 1:24), the
input impedance was measured to be 99 0 j 2
. A microstrip stub
matching circuit was designed to match each element to a 50-
line
and the elements were excited in-phase through a one-to-four power
divider. The matching circuits and the power divider were fabricated
on a separate board that was driven by a single feeding cable. The
matching-circuit and power-divider board was connected to the array
through four equal-length coaxial cables. The size of the array ground
plane is 36 2 36 cm2 . The calculated axial ratio and the power gain
in the main-beam direction are shown in Fig. 5. The 3-dB axial ratio
bandwidth is found to be 13.8%. The lowest axial ratio is found to be 1
dB at C= = 1:18. A very stable power gain of about 15 dB is found
within this bandwidth. The calculated and measured radiation patterns
of E at C= = 1:20 are shown in Fig. 6. The larger sidelobe level
of the measured radiation pattern is due to the finite size of the ground
plane. The copolarization and cross-polarization radiation patterns of
the array at C= = 1:20 are shown in Fig. 7. Within the elevation anglular range 040 40 , the cross-polarization suppression is at
348
Fig. 4.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 1, JANUARY 2004
Input impedance of a central-feed hemispherical helical antenna.
Fig. 7. Copolarization and cross-polarization radiation patterns of the array at
C= = 1:20.
cross-polarization suppression is 25 dB. Hence we see that very pure
circular polarization radiation can be produced within a broad angular
range. These characteristics are well suited for INMARSAT-M satellite reception [1] and better than those obtained by previously proposed
INMARSAT-M antenna arrays such as that in [3], which has a smaller
gain (13 dB) and a much higher profile (0.675 in height). Moreover,
the use of cylindrical helical antenna elements in [3] will definitely result in a small angular range of good circular polarization.
IV. CONCLUSION
A small and low-profile 2 2 2 hemispherical helical antenna
array has been designed for INMARSAT-M mobile vehicles. Using
only four elements fed by a simple network, the array can produce
a gain of 15 dB, a 3-dB axial ratio bandwidth of nearly 14%, a
cross-polarization level of smaller than 015 dB over an angular range
of 80 , a cross-polarization level of smaller than 020 dB over the
half-power beamwidth, and a radiation pattern with a 020 dB sidelobe
level. These characteristics outperform those of previously proposed
INMARSAT-M antennas or antenna arrays.
Fig. 5. Axial ratio and the power gain of the array.
REFERENCES
Fig. 6.
Calculated and measured radiation patterns of
C= = 1:20.
E
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for copolarization reception is below 20 dB. In the normal direction, the
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