The feasibility of using a low gain antenna (LGA) as a mobile terminal antenna for a helicopter is described in this paper. The objectives are to select the lowest cost antenna system which can be easily mounted on a helicopter and capable of communicating with a satellite, and to determine the best antenna position on the helicopter to mitigate the signal blockage due to rotor blades and the multipath effect from the helicopter's body. The omnidirectional LGA is selected because it is simple, reliable, and low cost. The helix antenna is selected among the many LGA's because it is the most economical one and has the widest elevation beamwidth. Both 2-arm and 4-arm helices are studied experimentally to determine the antenna's performance and the scattering effects from the helicopter's body. It is found that the LGA should be located near the tail section and at least eight inches above the helicopter

Divsalar, D.

Farazian, K.

Golshan, N.

Hinedi, S.

Wu, T. K.

English

NASA Technical Reports Server

N94-22835
L-band Mobile Terminal Antennas for Helicopters
T.K. Wu, K. Farazian, N. Golshan, D. Divsalar, S. Hinedi
L
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Dr.
Pasadena, CA 91109
TEL (818)354-1261
FAX (818)393-6875
ABSTRACT
The feasibility of using a low gain
antenna (LGA) as a mobile terminal antenna
for helicopter is described in this paper. The
objectives are (1) to select the lowest cost
antenna system which can be easily mounted
on a helicopter and capable of communicating
with a satellite, and (2) to determine the best
antenna position on the helicopter to mitigate
the signal blockage due to rotor blades and
the multipath effect from the helicopter's
body. The omni-directional low gain antenna
(LGA) is selected because it is simple,
reliable and low cost. The helix antenna is
selected among the many LGAs, because it is
the most economical one and has the widest
elevation beamwidth. Both 2-arm and 4-arm
helices are studied experimentally to
determine the antenna's performance and the
scattering effects from the helicopter's body.
It is found that the LGA should be located
near the tail section and at least 8" above the
helicopter.
INTRODUCTION
Helicopter satellite communication (H-
SATCOM) is of current concern, since it has
myriad applications, such as, emergency and
rescue missions, off-shore drilling, fire
fighting, rapid access, passenger
transportation, etc. For example, the
Norwegian air traffic controllers (ATCs) are
monitoring helicopter trips across the North
Sea to oil platforms using position data sent
automatically from the helicopters to ATCs
via the International Maritime Satellite
(Inmarsat) [1].
Jet Propulsion Laboratory (JPL),
under a contract with the Federal Aviation
Administration (FAA), is conducting a study
of implementing a very low-cost, small-size,
light-weight, real-time communication system
specifically for H-SATCOM. In this paper,
the feasibility of using LGAs for H-SATCOM
is studied. Here the helicopter's banking angle
is assumed to be _+.60 ° and the satellite is the
Inmarsat or American Mobile Satellite
Corporation (AMSC) or the Low Earth Orbit
(LEO) IRIDIUM satellite. The requirements
for the helicopter antennas are (1) complying
with industry standards, e.g., ARINC 741 and
Inmarsat LGA's specs for aeronautical mobile
terminals [2], (2) providing a 0 dBic gain in
360 ° azimuthal and from zenith to 40 ° below
horizon, (3) the transmit and receive
frequencies being 1.62-1.67 and 1.53-1.56
GHz, respectively, the transmit power being
19.2 watts, (4) the VSWR being 1.5:1, (5)
small-size, (6) light-weight, and (7) low-cost.
There are two unique technical
challenges in the determination of the best
helicopter antenna location. First is the
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https://ntrs.nasa.gov/search.jsp?R=19940018362 2020-06-16T15:56:14+00:00Z
periodic signal fading caused by the
helicopter's rotor blades. Second problem is
the multipath caused by the scattering from
the complicated shape of the helicopter body.
Thus the antenna study objectives are (1) to
select the low-cost, light-weight, and small-
size antenna system for H-SATCOM, and (2)
to determine the best antenna position on the
helicopter to minimize the signal blockage
due to rotor blades and the multipath effect
from the helicopter's body. The study results
are summarized in the following sections.
ANTENNA SELECTION
To ensure that all the antenna options
are considered, the LGAs, the steerable
medium-gain antennas [3] and high gain
reflector antennas [4] are all included in this
exhaustive survey. Both JPL and Antenna
Industry publications in this specific
application were studied. The high gain (_> 20
dB) reflector antennas in L-band are usually
very large in size (at least 5' in diameter and
1.25' in height) and heavy in weight. In
addition, a bulky and expensive tracking
system is needed to steer this reflector
antenna beam to the satellite direction. Thus
it is not suitable for helicopter use.
In general, the medium-gain antennas
(including mechanically and electronically
steered arrays [3]) are more expensive and
less reliable than a low gain antenna due to
the fact that an additional tracking system is
required to steer the narrow antenna beam to
the satellite direction. However, the omni-
directional LGAs, as summarized in TABLE
1, are simple, reliable and low cost.
Furthermore, the low-gain antennas are
typically ten times smaller than the medium-
gain antennas. This makes the mounting of
the antenna on the helicopter reIafive_fy
easier. Therefore, the low gain antennas are
selected for the H-SATCOM.
Figure 1 shows a 4-arm helix (volute)
antenna [5], which gives a cardioid pattern as
depicted in Figure 2. Figure 3 shows a donut
shaped pattern of a 4-arm conical spiral
antenna [6]. Note that one can change the
shape, size or pitch angle of a crossed dipole
or helix antenna to optimize the gain in the
desired directions. The 2-arm helix and the
crossed drooping dipole antennas have the
widest bandwidth (covering both the transmit
and receive frequencies). The 4-arm helix
antenna is bandwidth limited and hence
requires two antennas for uplink and
downlink. But it is attractive since it only
costs about $20. Since the helix antenna has
the lowest cost, it is selected for the
helicopter use. It is also possible to use
multiple antennas or antennas in conjunction
with a gyro or compass to compensate for the
helicopter's maneuvers. However, due to cost
and complexity it is desirable to have one or
two antennas without a tracking system.
HELIX ANTENNA TEST RESULTS
Several off-the-shelf helix antennas
were tested in an out-door far-field range.
Figures 4 and 5 are the measured radiation
pattern of a 4-arm helix antenna without and
with a 23" by 23" ground plane, respectively.
Figure 6 shows the severe pattern distortion
as the helix antenna is placed 4" above the
ground plane. Figure 7 shows the measured 2-
arm helix antenna at 1.5754 GHz. From this
test data, we know that this helix antenna has
about 2.1 dBic peak gain, 5 dB axial ratio
above horizon, and 140 ° half-power
beamwidth. It seems that this antenna is
designed to have optimized circular
polarizations at 45 ° cone angle. The measured
helix antenna performances are summarized
in TABLE 2. Note that several minor
discrepancies are observed as compared to
TABLE 1. First, for the 4-arm helix antenna,
the peak gain is about 0.8 dB lower and the
half-power beamwidth (HPBW) is about 10 °
smaller. For the 2-arm helix antenna, the
HPBW is about 200 smaller and the axial
ratio is about one dB worse. These minor
discrepancies may be attributed to the
588
measurementtolerance and uncertainty.This
also implies that extra link margin shouldbe
considered for the H-SATCOM system
design. The helix antenna should also be
placedat least8" awayfrom the helicopter in
order to minimize the ground plane effects.
CONCLUSION and RECOMMENDATION
The helix antenna is selected for H-
SATCOM, since it is small-size, light-weight,
and low-cost. Several off-the-shelf helix
antennas were also tested. None of these
antennas will remotely meet the H-SATCOM
antenna requirements. But one can change
the shape, size, or pitch angle of the helix
antenna to meet the requirements. The 0 dBic
elevation beamwidth of a single helix antenna
is 140 °. Thus two helix antennas are needed
to provide a 260 ° coverage. Since the helix
antenna's radiation pattern is very dependent
on the nearby scattering objects, it is
appropriate to conduct a scale model test (or
full sized test) and a numerical study to
precisely determine the blockage effect of the
rotor blades and the helicopter body. The
rationale for doing this task is that via the
scale model testing we can efficiently
determine the best antenna position and
performance on the helicopter for SATCOM
and also validate the numerical modeling
software. Whenever a different helicopter or
antenna is superimposed, running the
computer model is the most efficient and
cost-effective way to provide the SATCOM
system designer the necessary and accurate
antenna performance data.
ACKNOWLEDGEMENT
The work is carried out by JPL,
California Institute of Technology, under
contract with FAA. The test results were all
obtained by C. Chavez. The far-field range
was prepared by W. Picket and R. Beckon.
The Micropulse 4-arm and Tecom's 2-arm
Helix Antennas are kindly loaned to us from
S. Ow and B. Bishop, respectively.
REFERENCES
[1] R. Riccitiello, "Satellites Modernize Air
Traffic Control, Communications," Space
News, Nov. 16-22, 1992.
[2] Inmarsat Aeronautical System Definition
Manual (SDM), July 1988, London, UK.
[3] K. Woo, et. al., "Performance of a Family
of Omni and Steered Antennas for Mobile
Satellite Applications," Proceedings of the
International Mobile Satellite Conference,
Ottawa, 1990, pp. 540-546.
[4] A.W. Rudge, et. al., The Handbook of
Antenna Design, Chapter 3, Peter Peregrinus
Ltd., London, UK. 1986.
[5] C. Kilgus, "Resonant Quadrifilar Helix,"
IEEE Trans., vol. AP-17, no. 3, May 1969, pp.
349-351.
[6] A. Atia, "Multiple-arm Conical Log-spiral
Antennas," Ph. D. Dissertation, UC Berkeley,
1969.
SHORTING
DISC --_ SPLIT SHEATH BALUN
WITH _4 TRANSFORMER
Fig. 1 4-arm helix (volute) antenna
configuration
589
TABLE 1. L-band mobile antennasummary
Antenna "l_'pe ° Size (cm) Gain
(dn)
HT Dia.
Mechanically
Steered Array
1.Yagi Array 3.8 53 _ I0
2.Tilt Array 15 51 > 10
Electronically
Steered Array
1.Ball 3.3 61 > 8
2.Teledyne 1.8 54 _ 8
Low-Gain Omni
1.Crossed Dipole 12 8 > 4
2.Helix (2-arm) 15.2 5.1 2
3.Helix (4-arm) 9 5 4.5
4.Conical Spiral
(2-arm) 14 6.9 3.8
5.Conical Spiral
(4-arm) 15.7 12.9 4.5
6.Cavity Backed
Slot 0.8 8.3 2
Band-
width
(%)
6.25
6.25
6.25
6.25
25
28
1.3
6.25
6.25
6.25
ttPBW
(*)
40
40
40
40
100
160
150
160
40
120
Axial
Ratio
(dB)
7
4
4
4
4.5
Beam (pattern)
Shape
Steered Beam in
AZ
Steered Beam in
Both AZ and EL
Cardioid/Donut
Cardioid/Donut
Cardioid/Donut
Cardioid
Donut
Cardioid
Co_t (S/unit) b
450
600
1600
1800
400
150
20
300
400
1451
a. All the antennas are right-hand circularly polarized.
b. The cost of each antenna unit is a ROM cost based on producing 10,000 units per year over a five-year period.
TABLE 2. Summary of helix antenna's performances
Antenna Frequency Axial Ratio Bandwidth Peak Gain HPBW
Type GHz dB GHz dB degree
4-arm Helix 1.57 4 0.06 3.7 140
2-arm Helix 1.62 5 0.24 2.1 140
59O
Fig.2 Typical cardioid pattern Fig. 3 Typical donut shaped pattern of
conical spiral antenna
,,,,,,
°1|._ ...... ,
: _,,t_,,_ In.
-Ila.M "135.R -gU.! -45.N NLm _4§.IIU _,_1|.N 413S.m *llll
SCAN ANGI_E (OEGS)
Fig.4 Measured 4-arm helix pattern
No ground plane
Fig.5 Measured 4-arm helix pattern
with ground plane
591
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_--n -28.1
Q
-38,8
J
rl
y-
II _a.8
-58.
-68
-188.88
I t I I I I ] I I I ! LLL_J_L -IJ.I I | I I I I I i I I I I I I l I | I I I [ I t I t 1 I m_LLLLLLLLLL_ .LLLLLL_:
-135.88 -58.8{} -45.88 +{}.88 +45.e8 +g8.88 +135.8{} +18{}.
SCRN RNGLE (DEGS)
Fig. 6 Measured 4-arm helix antenna pattern at 1.575 GHz
antenna right above the finite ground plane
+B.8
:.
"18"81 J
-28. e
v
-38.8
_1
Q_
y-
"-48.1
-_8. E
-68.8
-188._
'_-'t ..... '_L['''__L'''--_'"
f
-135.88 -88.1_
!
,i, ii1,,,,, !
i
dHic
i
I
-45.88 +8.8e +45._ +38._8 +135._ .188
SCRN 8NGLE (DEGS)
Fig. 7 Measured 2-arm helix antenna pattern
592


L-band mobile terminal antennas for helicopters

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