Spar has evaluated and compared several antenna concepts for the North American Mobile Satellite. The paper describes some of the requirements and design considerations for the antennas and demonstrates the performance of antenna concepts that can meet them. Multiple beam reflector antennas are found to give best performance and much of the design effort has gone into the design of the primary feed radiators and beam forming networks to achieve efficient beams with good overlap and flexibility. Helices and cup dipole radiators have been breadboarded as feed element candidates and meausured results are presented. The studies and breadboard activities have made it possible to proceed with a flight program

Karlsson, Ingmar

Patenaude, Yves

Stipelman, Leora

English

NASA Technical Reports Server

N88-25742
ANTENNA SYSTEM FOR M-SAT MISSION
INGMAR KARLSSON, YVES PATENAUDE and LEORA STIPELMAN, Antenna Development
Engineering, Spar Aerospace Llmlted, Canada
SPAR AEROSPACE LIMITED
21025 Trans Canada Highway
Ste-Anne-de-Bellevue, Quebec H9X 3R2
ABSTRACT
Spar has evaluated and compared several antenna
concepts for the North American Mobile Satellite.
The paper describes some of the requirements and
design considerations for the antennas and demonstrates
the performance of antenna concepts that can meet them.
Multiple beam reflector antennas are found to give
best performance and much of the design effort has gone
into the design of the primary feed radiators and beam
forming networks to achieve efficient beams with good
overlap and flexibility. Helices and cup dipole radiators
have been breadboarded as feed element candidates and
measured results are presented.
The studies and breadboard activ±ties have made Spar
ready to proceed with a flight program.
INTRODUCTION
Since the late 1970's Spar Aerospace Llmlted has studied and
evaluated different concepts and payload designs for a Canadlan Moblle
Communication Satellite (M-SAT). These studles have been funded by the
Canadian Department of Communication and have been almed towards the
deslgn of a demonstration model. Growlng commercial interest in both the
USA and Canada has changed the program lnto an lndustry led commercial
program. From being a UHF band system the concept evolved lnto a
comblned L and UHF band system in 1986. The WARC conference in 1987
established the L-band as the frequency band for mobile satellite servlce
and M-SAT ls now an L-band only,system.
Spar has had a leading role in deflning system requirements and
proposing sultable designs. Reference 1 describes the basellne system
requirements. It ls assumed that two identical spacecrafts, one US and
one Canadian, each operated indePendently , would enable a commercial
system to be implemented by the early 1990's. Thls flrst generation
system would allow mutual backup in the event Of a spacecraft fallure.
Thls paper describes some of the antenna'configurations and hardware
that have been and are considered for the M-SAT space segment.
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https://ntrs.nasa.gov/search.jsp?R=19880016358 2020-03-20T06:51:50+00:00Z
ANTENNAREQUIREMENTS AND CONSIDERATIONS
The space segment of a mobile satellite communication system must
provide high power densities (EIRP) on the ground to allow mobiles with
small, low gain, antennas to communicate. The limited spectra allocated
for mobile services requires maximum possible frequency reuse. The user
distribution is uncertain and will vary during the satellite lifetime.
There is thus a need to have flexibility in spectrum and power
allocations over the coverage region.
High gain spot beam antennas are well suited to meet these
requirements. Electrically the challenge is to design a multiple beam
antenna system with good beam overlap, low sidelobes and high efficiency,
that also allows for flexibility. Mechanically the challenge is to
provide large antenna apertures meeting the mass and volume constraints
of the spacecraft. Mesh reflector suppliers have achieved good
performance with breadboarded reflectors and associated deployment
hardware.
Spar has systematically studied and evaluated suitable antenna
candidates, several of which are described in the next section. All
reflector concepts assume offset reflector with 5m projected aperture.
ANTENNA CONFIGURATIONS
g-Beam 15 Shared Feed Element Reflector Antenna Concept
Nine beams are generated by a 15 element feed cluster. The beams
share feed elements to get good overlap and antenna efficiency.
Typically 4 feeds with uniform excitation are used for each beam. The
feed cluster layout is shown together with the resulting coverage in
Figure I.
The minimum coverage dlrectivity is 33.0 dBi based on calculations
assuming cup dipole primary radiators. Frequencies can be reused every
third beam with 22 dB isolation.
Low level beam forming and distributed amplifiers are used to
facilitate the excitation. Power flexibility can be achieved by
combining varying numbers of power modules or by varying the collector
voltage of a fixed size amplifier. However, this flexibility results in
lower power efficiency and has implications on weight and complexity.
g-Beam 11 Shared Feed Element Reflector Antenna Concept
The region of interest can be covered by the same number of beams
with fewer feed elements with Just a slight reduction in coverage gain.
The feed cluster Iayout is shown in Figure 2 with the corresponding
coverage. TypicalIy _ f_ed elements are used for each beam. Two beams
are generated by the same _ feeds using reciprocal non unlform
excitation in the North-South direction i.e., for the US and Canada
beams. The beams share feed elements in the East-West direction in order
to achieve good beam overlap. The resulting coverage directivity is G2.5
dBi except for small areas in the South of the USA. Frequencies can be
reused every third beam with 20 dB isolation.
A low level beam forming system similar to the one described in the
previous section can also be used here.
This antenna concept is the main candidate for M-Sat.
438
g-Beam Independent Feed Element Reflector Antenna Concept
This concept utilizes one feed element per beam so that no beam
forming is required. The feed element must be physically small to
achieve the desired beam separation. Small feed elements result in low
efficiency because of high spillover losses. Spar has investigated the
use of cup dipole radiators and helix radiators for this concept.
Despite the high aperture efficiency of the cup dipole the minimum
coverage directivlty is only 30.5 dB1. The helix radiator has an
effective aperture that is larger than its physical area and can thus be
,n]°n_d--- closer together to achieve better ......_- overlGp. _v_±y_...... spaced
helices have strong interaction (mutual coupling) and their radiation
patterns are greatly affected. However, the performance can benefit from
mutual coupling effects as the active patterns can be shaped to have very
flat top and steep fall-offs. Figure 3 shows a layout of the helix
cluster and the achieved coverage contours Frequencies can be reused
every third beam with 18 dB isolation. The coverage is calculated using
measured primary patterns of helices located in their representative feed
cluster environment.
The coverage directivity is 32 dBi except for a small corner in the
south of the USA. This is 1.5 dB better than what can be achieved with
cup dipole radiators, but still 0.5 dB lower than for the main candidate
system. However, excitation errors of the shared feeds reduces the gain
difference to about 0.3 dB.
The independent feed per beam concept significantly reduces the
complexity and weight of the transponder and feed assembly. No beam
forming networks are required and the number of radiators, filters and
amplifiers is reduced. This concept is considered as a good candidate
for a low cost system.
Direct Radiating Array Antenna Concept
The advantage of a direct radiating array with scanning beams is the
increased flexibility. The beams can be scanned to increase the gain in
specific areas either due to changed traffic requirements or a change in
orbital location. Beams can also be repointed to compensate for failure
of an amplifier or of a complete beam.
One of the antenna candidates studied is a 5.1 x 4.5 m hexagonal
direct radiating array. The radiating elements used are high efficiency
short backfire radiators. A total of 127 elements are arranged in 23
groups, each powered by a separate amplifier. The groups are formed such
that the amplifiers are loaded with equal power. The power is
distributed within each group using a low loss TEM line network._ With
this design concept the target area is covered by 9 scannable beams each
with its variable low level beam forming network.
The grouping of the elements and the associated non ideal phase
excitation makes the outermost beams less efficient resulting in a
minimum coverage directivity of 32.3 dBi.
The main drawback of using a hexagonal direct radiating array is the
complex deployment scheme required and the weight of the antenna. The
mass and complexity also make it impossible to have separate transmit and
receive antennas which increases the risk of distortions caused by
passive intermodulation. A hexagonal direct radiating array is thus not
seen as an attractive option for M-SAT. However, a rectangular array as
described in reference 2 can be implemented and offers good flexibility.
439
Imaging Reflector Antennas Concept
Single and dual reflector imaging antennas with electronlc beam
scanning have also been investigated.
The single reflector imaging system Is realized using a reflector
and a relatively small feed array. The feed array is located between the
reflector and the focal point. This way, the field distribution of the
feed array is approxlmately recreated over the reflector aperture and a
scan of the feed array results in a proportional scan of the antenna.
A dual reflector imaging system comprises a main reflector, a sub-
reflector, and o feed array. The sub-reflector is located in the near
field of the feed array and the reflectors are arranged confocall y such
that a magnified image of the array is formed over the main reflector
aperture (5). Compared to a direct radiating array the number of
radiators is smaller which leads to a simplification of the beam forming
network and a substantial reduction of the mass of the array itself. A
large array can hardly be folded while a large deployable reflector can
be stowed in a relatively small volume compatible with launcher
envelopes. It is also feasible to have separate transmit and receive
antennas.
Both systems were optimized using a 5 m diameter main reflector.
The dual reflector imaging system resulted in a minimum coverage
directivity of 51.6 dBi compared to 51.2 dBi for the single imaging
system. The sidelobe performance of the imaging systems is poor,
reducing the possibility of frequency reuse. Imaging antenna systems are
thus not considered a very attractive option.
RADIATOR DEVELOPMENT
Cup Dipole Radiator
A cross dipole with unequal arm lengths fed by a split-coax balun
and placed inside a clrculor or a square cup provides a simple and
efficient circularly polarlzed radiator for narrowband applications. The
cup dipole radiator is thus a very good candidate for the M-SAT primary
feed radiators. Table 1 presents measured data for three different cups
over the M-SAT transmit frequency band 1550 - 1559 MHz.
Helix Radiator
Helices have been investigated as feed elements, especially for the
independent beam configuration. In particular, the effects of mutual
coupling between helices in an array have been studied.
The helix test model uses an 8 turn copper plated steel spring
conductor which is supported by a high density polystyrene cross support.
The helix circumference Is 1.1X Four tapered turns are used to improve
the axial ratio performance. The helix is placed in a circular cup whose
diameCer and depth are experimentally determined to optlmlze mutual
coupllng effects. The measured axlal ratio on-axis ls better than 0.2 dB
for the isolated helix and better than 1 dB for a helix in a cluster.
Figure _ compares the radiation patterns of an Isolated helix, and the
same helix in a 6-element cluster. The active pattern has lower peak
gain but has a flat top which makes it efficient as a primary radiator
for a reflector. The test results together with the secondary pattern
calculations demonstroCe that helices can be used efficiently as
independent feed elements for multiple beam antennas.
440
CONCLUSION
Desirable features of antenna systems for Mobile Communication
Systems have been defined and a number of suitable antenna designs have
been described.
The performances of the candidate concepts have been systematically
evaluated by Spar and some of the major considerations are given in this
paper. The 9 beam shared feed horn reflector concept with 1_ feed
elements is presently considered to be the best approach. This system
offers efficient beams with good overlap, sidelobes that allow frequency
reuse, good power flexibility and separate transmit and receive antennas.
Design and evaluation of concepts are continuing as the system
requirements are becoming mare well defined.
Some hardware has been developed to support the conceptual designs.
Test results from cup dipole and helix radiators are presented here.
Both radiators show more than adequate performance for M-SAT
requirements.
Spar is also co-operating with mesh reflector suppliers and has
performed tests of a 5 m mesh reflector together with Aerospatiale.
Similar tests will be performed with MBB during 1988.
REFERENCES
[1] Hing, K.' et al. Considerations for spacecraft design for M-Sat.
paper to be presented at The Mobile Satellite Conference JPL
1988
[2] Rosen, H. Frequency Addressable beams for Satellite Communications.
Telcom 87 Geneva
[3] Dragone, C. Gang, M.J. Imaging reflector arrangements to form a
scanning beam using a small array. The Bell System Technical
Journal. Vol. 58, No. 2, February 1979.
Table 1. Cup Dipole Performance
EFFICIENCY AXIAL RATIO [dB] RETURN LOSS [dB]
1530 1545 1559 MHz 1530 1545 1559 MHz
Circular cup
0 = 1.3X
Circular cup
0 = 0.65X
Square cup
d = 0.65X
99 1.8 0.8 1.2 -27 -29 -51
97 0.4 0.3 0.3 -16 -20 -21
97 0.5 0.4 0.2 -17 -25 -24
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Antenna system for MSAT mission

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