Design of a Small Size Dielectric Loaded Helical Antenna for ...

Design of a Small Size Dielectric Loaded Helical Antenna
for Satellite Communications
C. H. Niow, K. Mouthaan, J. C. Coetzee * , H. T. Hui
Department of Electrical and Computer Engineering, National University of Singapore
Block E4, Level 5, Room 45, 4 Engineering Drive 3, Singapore 117576
g0800569@nus.edu.sg
* School of Engineering Systems, Queensland University of Technology
2 George Street, Brisbane, Queensland, Australia 4000
Abstract — The trade-offs in terms of performance and size of
an axial-mode helical antenna for satellite communications are
investigated. The antenna size is reduced by using a backfire
bifilar helical antenna (BBHA) combined with a dielectric core.
The antenna is designed using Ansoft’s HFSS and the results are
verified experimentally. A size reduction of 70% is achieved.
The gain at 2.4 GHz is 2.6 dBic with an axial ratio bandwidth of
400 MHz. The half power bandwidth is 80 degrees and the
antenna return loss is below -10 dB between 2.2 GHz to 2.6 GHz.
A two-element dielectric-loaded BBHA array is simulated using
HFSS and results are compared with a single dielectric-loaded
BBHA.
Index Terms – helical antennas, dielectric loaded antennas,
satellite communication.
T
I. INTRODUCTION
he helical antenna invented by Kraus, can operate in two
different modes: the normal mode and the axial mode [1].
The axial mode is the most practical, as it can be applied in
many applications where high directivity and circular
polarization are required [2]. In order to achieve a good
circular polarization for the axial mode, the ratio of the
circumference C and the wavelength � must be in the range of
3/4 < C/� < 4/3 and the spacing between each turn must be
approximately �/4. The circular polarization is optimum when
C/� = 1. The pitch angle should ideally be between 12° and
14° [2]. Due to the large dimensions required to achieve
circular polarization, the helical antenna is usually restricted
to higher frequency applications. Various methods have been
proposed to reduce the antenna size, such as modifying the
helical structure [3]-[5] or introducing dielectric loading [6].
However, antenna size reduction is achieved at the expense of
a narrower bandwidth.
In this paper, we propose a suitable combination of a
modified helical antenna structure and dielectric loading to
reduce the size of the helical antenna while maintaining
reasonable antenna performance with good return loss. The
performance of a closely spaced two-element dielectric
loaded helical antenna array is also investigated using
numerical simulations.
The Backfire Bifilar Helical Antenna (BBHA) is chosen for
the modified helical antenna structure, because it does not
require a ground plane unlike conventional helical antennas
[5]. However, the helical structure is still relatively large.
Hence, there is a need to introduce a dielectric rod to reduce
the size further [6]. The introduction of the dielectric rod
affects the gain and bandwidth of the antenna. Therefore,
trade-offs have to be made to reduce the antenna size while
achieving reasonable antenna performance with good return
978-1-4244-2802-1/09/$25.00 ©2009 IEEE
48
loss. This paper presents empirical results to identify the most
suitable design parameters.
II. DESIGN OF REDUCED SIZE HELICAL ANTENNAS
To design the BBHA, various antennas are designed using
both Macor and Teflon for the dielectric core. The antennas
are analysed using Ansoft HFSS version 11 and validated
through measurements.
A. Design of a 3-turn BBHA
One helix is connected to the signal feed while the other
helix is connected to the ground, as shown in Fig. 1. We
added two 20 mm × 10 mm matching strips near the feed
point to improve the return loss. Copper wire of 1 mm
diameter is used for the two helixes. The diameter of each
helix is 32 mm while the spacing between each turn is 25
mm. The pitch angle is 13.97°.
signal
ground
Top view
helixes
dielectric
50 ohm
coaxial cable
matching strips
Side view
Fig. 1. Model of a 3-turn BBHA
Without these two matching strips, the return loss is less
than 5 dB. With the matching strips, the antenna exhibits a
return loss of about 10 dB over a very wide bandwidth, as
shown in Fig. 2. The antenna shows good directivity with a
half power bandwidth (HPBW) of 80°, as depicted in Fig. 3.
The measured gain at 2.4 GHz is 3.27 dBic while the highest
measured gain is 3.99 dBic at 2.6 GHz. The antenna exhibits
a good axial ratio (AR) of less than 3 dB below 2.8 GHz, as
shown in Fig 4. The antenna has a wide gain bandwidth of
around 800 MHz, as shown in Fig. 5.
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Fig. 2. Computed and measured return loss for 3-turn BBHA
Fig. 3. Computed and measured normalized radiation pattern for
3-turn BBHA at 2.4 GHz
Fig. 4. Computed and measured AR for 3-turn BBHA
Fig. 5. Computed and measured normalized gain for 3-turn
BBHA
B. 3-turn BBHA loaded with Teflon
A Teflon rod (�r = 2.1) is introduced inside the helix, which
reduces the antenna size by 50% in volume. The size of the
copper strips is optimized using HFSS to 20 mm × 10 mm.
The diameter of the antenna is reduced to 26 mm while the
spacing between each turn reduces to 20 mm. The pitch angle
is 13.76°. Fig. 6 shows that the antenna still exhibits a return
loss of at least 10 dB from 2.2 GHz onwards. It exhibits good
directivity with an HPBW of 70° and a wide gain bandwidth
of about 800 MHz, as shown in Figs. 7 and 8 respectively.
49
The gain at 2.4 GHz is 3.27 dBic. The highest gain is 3.98
dBic at 2.6 GHz. The computed gain bandwidth of an
unloaded antenna of the same size shifts to the higher
frequencies as compared to the antenna loaded with the
dielectric. The AR bandwidth is about 400 MHz, as shown in
Fig. 9. The computed AR bandwidth of an unloaded antenna
of the same size shifts to higher frequencies.
Fig. 6. Computed and measured return loss for 3-turn BBHA
loaded with Teflon
Fig. 7. Computed and measured normalized radiation pattern for
3-Turn BBHA loaded with Teflon at 2.4 GHz
Fig. 8. Computed and measured normalized gain for 3-turn
BBHA loaded with Teflon
Fig. 9. Computed and measured AR for 3-turn BBHA loaded with
Teflon
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C. 3-turn BBHA loaded with Macor
The dielectric Macor has a permittivity of 5.8 at 2.4 GHz.
With its introduction, the dimensions of the antenna change
to a diameter of 20 mm and a spacing between each turn of
20 mm. The size of the copper strips is optimized to 20 mm ×
5 mm. The pitch angle is 17.7°. The antenna volume is only
30 % of the original antenna.
The antenna has a return loss of at least 10 dB from 2.2
GHz onwards, as shown in Fig. 10. While the antenna retains
its directivity, the back lobe has increased substantially. The
HPBW of the antenna is 80° (as shown in Fig. 11) which is
10° wider than the antenna loaded with Teflon. The gain
bandwidth is reduced with the introduction of the higher
permittivity dielectric load. The measured gain at 2.4 GHz is
2.6 dBic. The highest gain is 3.7 dBic at 2.6 GHz. This gain
is lower than the gain of the antenna loaded with Teflon, as
shown in Fig 12. The gain bandwidth shifts to higher
frequencies for the antenna of the same size without the
dielectric. The AR bandwidth is about 400 MHz and it shifts
to higher frequencies when the antenna is not loaded with the
dielectric, as shown in Fig. 13.
Fig. 10. Computed and measured return loss for 3-turn BBHA
loaded with Macor
Fig. 11. Computed and measured normalized radiation pattern
for 3-turn BBHA loaded with Macor at 2.4 GHz
Fig. 12. Computed and measured normalized gain for 3-turn
BBHA loaded with Macor
50
Fig. 13. Computed and measured AR for 3-turn BBHA loaded with
Macor
D. Performance Comparison
The dimensions and performance of the three antennas are
compared in Table I, Figs. 14 and 15. It is found that there is
no change in the gain bandwidth when the antenna size is
reduced and loaded with Teflon. However, the gain
bandwidth becomes narrower when the antenna is loaded
with Macor, while the antenna size is reduced further. The
AR bandwidth becomes smaller as the permittivity increases
while the antenna size reduces. The gain at 2.4 GHz reduces
when the permittivity increases.
TABLE I
DIMENSIONS OF EACHBBHA LOADED WITH DIFFERENT
DIELECTRIC
Dielectric �r
Diameter
Spacing
between
each
turn
Size
compared
to
unloaded
BBHA
Air 1 32 mm 25 mm 100 %
Teflon 2.1 26 mm 20 mm 50 %
Macor
5.67 –
6.03
20 mm 20 mm 30 %
Fig. 14. Measured AR for 3-turn BBHA loaded with dielectric of
different permittivity
Fig. 15. Measured gain for 3-turn BBHA loaded with dielectric of
different permittivity
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III. A REDUCED SIZED TWO-ELEMENT BBHA ARRAY
For satellite communications, usually high-gain antennas
are required. To this end, we have designed a two-element
array using the Macor-loaded BBHA, which is different from
the conventional dielectric loaded helical antenna array [7].
This is because the inter element spacing between the
BBHAs of 0.2� at the operating frequency of 2.4 GHz is
much smaller and there is no need for a ground plane. Hence,
the overall size of the Macor-loaded BBHA array is smaller
compared to the conventional dielectric loaded helical
antenna array. Fig. 16 shows the coordinate system employed
here for the two element BBHA.
�
y�
2�
1�
Fig. 16. Coordinate system of a two element BBHA.
The computed normalized radiation pattern of this array is
shown in Figs. 17 and 18 in different � planes. It is found
that when the two elements are placed very close together
with port excitations 180º out-of-phase with each other, the
gain increases significantly.
Fig. 17. Computed normalized radiation patterns for 1-element
BBHA and 2-element BBHA at � = 0º.
Fig. 18. Computed normalized radiation patterns for 1-element
BBHA and 2-element BBHA at � = 90º.
��
x
51
IV. CONCLUSION
This paper considers methods for the size reduction of the
helical antenna. There is little freedom to vary the pitch angle
and diameter of a helical antenna to reduce the size without
affecting the antenna performance. In addition, the ground
plane of a conventional helical antenna needs to be large
enough for good antenna performance.
The bifilar helical antenna (BBHA) is a suitable candidate
for size reduction, since it does not require a ground plane.
Introducing a suitable dielectric rod can further reduce the
size. It is found that the antenna size can be reduced by a
dielectric rod with higher permittivity, but at the expense of
the gain and the bandwidth.
In this paper size reductions in volume are achieved of a
BBHA with a Teflon dielectric and a Macor dielectric of 50%
and 70% respectively.
ACKNOWLEDGEMENT
This work was partly supported by the departmental vote of
the Department of Electrical and Computer Engineering,
National University of Singapore.
REFERENCES
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Volume 37, Issue 3, March 1949 Page(s):263 - 272W.
[2] Constantine A. Balanis, “Antenna Theory: Analysis and
Design”, Second Edition, Page(s): 505-512.
[3] R.M. Barts, W.L. Stutzman, “A Reduced Size Helical
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[4] S.H. Zainud-Deen, N.F.M. Soliman, K.F.A. Hussein, A.A.M.
Shaalan, “Electromagnetic Characteristics of Spiro-Helical
Antenna”, Radio Science Conference, 2004. NRSC 2004.
Proceedings of the Twenty-First National, Volume, Issue, 16-
18 March 2004 Page(s):B9 - 1-10.
[5] W.T. Patton, “The Backfire Bifilar Helical Antenna”, Univ.
Illinois Antenna Lab Tech. Rept. 61, Sept. 1962.
[6] H. T. Hui, Edward K. N. Yung, and K. W. Leung, "Numerical
and experimental studies of a helical antenna loaded by a
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1997.
[7] Y.A. Ho, H.T. Hui, E.K.N. Yung, “A 1×2 dielectric-loaded
helical antenna array”, Antennas and Propagation Society
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21-26 Jul 1996 Page(s):1962 - 1965 vol.3.
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