Special Feature

Multibeam Phased Array Feed System
Using Beam Group Concept
Yoshinori Suzuki†, Akira Meguro, Kiyoshi Kobayashi,
and Masazumi Ueba
Abstract
The concept of beam groups should enable us to obtain a lightweight antenna feed system that offers
high antenna gain and high flexibility in radio-frequency (RF) power distribution among the beams ofthe next-generation multibeam communication satellite. Setting up groups of beams and exciting all the
beams in a group using the same amplitude weight improves the antenna gain over the entire coveragearea compared with the conventional array-fed reflector antenna and achieves flexible RF power distribution among the beams. The beams shared by different beam groups are combined with appropriate
phase and division ratios so as to further improve the RF power distribution flexibility. An antenna feeder that uses these techniques is only half the weight of conventional antenna feed systems.

1. Introduction
The design of an S-band next-generation broadband scalable mobile satellite communication system
for the ubiquitous network [1] will require various
innovations to reduce the weight and power consumption of the onboard equipment. The required
capacity of 1 Gbit/s can be achieved by using a multibeam system that offers high-antenna-gain beams
and extremely high levels of frequency reuse. Moreover, the radio frequency (RF) power resource allocated to each beam must be varied to match thebeam’s traffic [2]. The typical approach to achieving
these goals is to use a reflector antenna with a multibeam feeder. Two major technologies are important
for the onboard antenna system: an ultralightweightlarge antenna reflector [3] with an aperture diameterof about 20 m and a lightweight 100-beam-classantenna feed system.
This article describes a novel multibeam antenna
feed system that uses the concept of beam groups. In
each group, all the beams are excited using the same
amplitude weight. This improves the antenna gain
† NTT Access Network Service Systems Laboratories
Yokosuka-shi, 239-0847 Japan
Email: suzuki.yoshinori@lab.ntt.co.jp

Vol. 5 No. 1 Jan. 2007

over the entire coverage area compared with the conventional array-fed reflector antenna and achieves
flexible RF power distribution among the beams. To
further improve the RF power distribution flexibility
between beams, we devised a beam-forming methodthat combines beams from different beam groups. We
used these methods to design an onboard multibeam
reflector antenna for a broadband scalable mobile
satellite communication system. We confirmed theirvalidity by calculating the expected antenna performance. These new methods enable the weight of theonboard multibeam antenna feed system to be
reduced by about 50% compared with the conventional methods.
2. Requirements for onboard antenna systems
Our aim is to make the next-generation broadband
scalable mobile satellite communication system bybuilding an S-band 1-ton-class geostationary satellitewith a communication capacity of up to 1 Gbit/s [1].
The weight budgets for onboard equipment are
shown in Fig. 1. To meet these budgets, we must
halve the weight of the antenna feed system.
Achieving a 1-Gbit/s capacity in the 30-MHz bandwidth allocated to mobile satellite services in the Sband requires frequency reuse in excess of 20 times.
61

Special Feature

Par

900

abo

Array
feeder

800

efle

cto

r

Reflector

600
(1)
Focal
plane

500
400

Antennafeed systemθθFig. 2. Configurations of multibeam reflector antennas.

300
200

(2)

Frequency
converters
Multiport
amplifer

Beam-forming network

100

Feed
amplifer

Target3. Conventional multibeam reflector antennas
and feed systems
A typical onboard multibeam antenna configuration is a reflector antenna with a multibeam feed system. Such configurations fall into two classes according to the location of the feeder (Fig. 2) [4]. This section describes the configurations and the problems of
conventional feed systems.
3.1 Focal-plane array feed typeA system with the feeder at location (1) in Fig. 2 isa reflector antenna system because the energy transmitted from a feed element in the focal plane forms aplane wave whose direction corresponds to the feed
position. In this feed type, hereinafter called the
focal-feed type, the beam-forming network (BFN)
62

Beam 3

Beam 2

Fig. 1. Weight budgets for onboard equipment.

One possible beam allocation model uses 69 beams in
3-beam clusters. Each beam offers at least 52.9 dBW
as its edge of coverage (EOC) EIRP (equivalent/
effective isotropically radiated power). The antenna
gain is 43.5 dBi and the RF output is 9.4 dBW. Total
RF output power is 600 W (27.8 dBW).
The onboard antenna system must allocate RF
power to each beam according to the beam’s traffic
and it must be possible to concentrate the RF powerof each beam up to the power flux density limit, i.e.,
18% of the total RF system power.

Output hybrid network

Conventional
designInput hybrid network0

Beam 1Feeder link

Beam 3 Beam 2 Beam 1

Payload mass (kg)

700

lic r

Input/output
hybrid network

Divider/combiner
Fixed phase shifter
Fixed attenuator
Power amplifier
hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Hyb

Orthogonal hybrid

Fig. 3. Block diagram of a multibeam antenna feedsystem for a focal-feed-type antenna.

uses multiple elements around the focal position. A
block diagram of a multibeam antenna feed system
for a focal-feed-type antenna is shown in Fig. 3. This
example shows a 3-beam system with 8 elements,
where each beam is formed by 4 elements.
When we implement the power distribution function between beams, one option is a multiport amplifier (MPA) system [5]. This is because in the assignment of the required RF power, each beam input sigNTT Technical Review

Special Feature

nal level is proportional to the RF power and theexcited RF power of each feed element depends on
the RF power distribution. The MPA can make theoperating conditions of the high-power amplifiers(HPAs) independent of the RF power allocation ofeach beam input level. Unfortunately, the MPA needs
a lot of hybrid circuits before and after the HPAs, as
shown in Fig. 3. There are three problems with using
an MPA.
1) The circuit scale of the hybrid matrices increases drastically with the number of input/output
terminals.
2) The insertion losses of the output hybrid matrix
suppress the effective antenna gain.
3) Since the output hybrid matrix must handle
large RF power, it is much heavier than the
input matrix.
3.2 Off-focal-plane array feed typeIn the off-focal-plane-feed-type (hereinafter, offfocal-feed) reflector antenna, the array feeder islocated in front of the focal plane at (2) in Fig. 2. The
array feeder converts the wavefront at the array aperture to form the beam. Therefore, all elements arebasically used for beam forming and must be controlled via the appropriate amplitude and phaseweights. A block diagram of this type of antenna feedsystem is shown in Fig. 4. Although this type needs alarge number of power dividers/combiners, fixedphase shifters, and fixed attenuators, the required RF
power that needs to be handled before the HPA issmall. Therefore, they can be effectively integrated asmonolithic microwave integrated circuit devices [6].
In contrast to the focal feed type, this feed system

Element #m

g ne

min

for
am-

rk
two

Be

Com

Element #1

Beam 1

Beam n

n

n

j =1

j =1

pi = gi ∑ ti aij2 = gi ai2 ∑ t j = gi ai2

Com: power combiner(1)( ai1 = ai 2 = ⋅⋅⋅ = aik = ai ),where gi, tj, and aij indicate the i-th HPA gain, the RF
power distribution ratio of the j-th beam versus thetotal radiation output, and the amplitude weight of thej-th beam of the i-th feed element, respectively.
3.3 Issues for 100-beam-class feed systems thatuse conventional feed systems
Conventional feed systems are summarized inTable 1. The best approach to making a lightweight
100-beam-class feed system is to dispense with the
MPA. Therefore, with respect to the issue of weight,
the off-focal feed type has a big advantage over the
focal feed type. However, the key problem with theoff-focal-feed type is the low antenna gain. It is about
1–2 dB lower than that of the focal-feed-type antennabecause of spillover and imperfect conversion of the
wavefront. However, it does satisfy many of the otherperformance requirements, such as power distribution and lightweight. Therefore, we devised novel
off-focal-feed-type-based antenna feed systems withimproved antenna performances.
4. Multibeam antenna feed system using beamgroups
4.1 Concept of beam groups
In the off-focal-feed-type antenna, the antenna gain
decreases in proportion to its deviation from the gain
optimal weight. Using the same set of amplitude
weights allows the MPA to be dispensed with at theTable 1. Summary of conventional multibeam antenna
feed systems.
Configuration(array position)

Fig. 4. Block diagram of a multibeam antenna feedsystem for an off-focal-feed-type antenna.

Vol. 5 No. 1 Jan. 2007

can eliminate the input/output hybrid networks of the
MPAs. Since all beams can share the same set of
amplitude weights, the excited RF power of each feedelement, pi, does not depend on the RF power distribution, as shown by the following equation [7].

Focal feed

Off-focal feed

BFN

SimpleComplex

HPA system

MPA

Only HPA

Effective antenna gain*

Baseline

Less than 1–2 dB

WeightHeavy

Baseline* Includes insertion loss (2.0 dB) of output network.

63Special FeatureGroup 2
Overlapping
beam input

Variable
divider
Overlapping
beams

j -th
BFN

Com

Com

i -th
BFN

Com

Group 1x-th BFN: weight control for x-th beam group

Group 3Fig. 5. Example of beam groups and overlapping beams.

Fig. 6. Block diagram of our multibeam antenna feedsystem.

cost of an increase in amplitude weight error and adecrease in gain.
We focused on the properties of off-focal-feed-type
antennas and calculated several sets of amplitude
weights, each of which forms a beam group. In eachbeam group, all beams are excited by a common setof amplitude weights. Since each beam group contains beams that overlap ones in neighboring groups,
as shown in Fig. 5, the deviation of the amplitude
from the optimum value can be lowered.
How the RF power is excited is the key to eliminating the MPA. In the case of two beam groups, basedon our design concept, the excited RF power of eachelement is given by

3) Calculate a set of amplitude weights Ail that satisfies 1) and 2) above.
This can maximize the average gain among beams
while minimizing the antenna gain variance. Our
scheme achieves higher antenna gain over the entire
coverage area than is possible with a conventional
off-focal-feed reflector antenna excited with the same
set of amplitude weights.

n
n
k
⎞
⎛
pi = gi ∑ ti aij2 = gi ⎜ Ai21 ∑ t j + Ai22 ∑ t j ⎟⎝ j =1j =1
j = k +1 ⎠(2)

( ai1 = ⋅⋅⋅ = aik = Ai1 ),),( aik +1 = ⋅⋅⋅ = ain = Ai 2 ).
To keep pi constant, we introduce a constraint suchthat for each beam group, the subtotal RF powers
k

n

j =1

j = k +1

∑ t j and ∑ t j are fixed. Thus, the MPA is not necessary in this feed system given this constraint.
4.2 Method for determining amplitude weights
for beam groups
The amplitude weights for each beam group aredetermined by the following procedures.
1) Calculate the gain margin for the demands
assuming that each beam is excited using thegain-optimal weight.
2) Find permissible amplitude weight errors corresponding to each beam’s margin.
64

4.3 Beam-forming technique for overlappingareas
Our technique offers good antenna characteristics
while eliminating the MPA. However, it does restrict
the flexibility of RF power distribution and the maximum assigned power because the RF power allocatedto each beam group must be fixed. To solve these
problems, we set beams (hereinafter called overlapping beams) that belong to two or more beam groups,
as shown in Fig. 5. This configuration increases thepower distribution flexibility of the overlapping
beams while holding constant the subtotal power of
each beam group.
A block diagram of a feed system that supports
overlapping beams with the minimum number of
additional devices is shown in Fig. 6. Here, the i-thand j-th BFN control phase and amplitude of eachelement correspond to beam groups #i and #j, respectively. The signals in the overlapping beams areappropriately divided by a variable divider. The
divided signals are then input to the BFN ports that
correspond to the area from which beams are radiated. The overlapping beams are formed by the spatial
combination of the radiated beams. In this case, it isnecessary to adjust the phase of each beam appropriately.
NTT Technical Review

Special Feature

5. Antenna design for broadband scalable
mobile communication satelliteThis section describes the design of an onboard
antenna for a broadband scalable mobile communication satellite that uses our beam-group concept.
5.1 Antenna design
We designed an onboard multibeam reflector
antenna that uses our new feed system and meets the
expected system requirements. Antenna performanceis greatly dependent on the number of beam groups:
EOC gain increases in proportion to the number ofbeam groups. The flexibility of RF power distribution
and the maximum assigned power both fall as thenumber of beam groups increases.
The antenna configuration is shown in Fig. 7 andthe antenna parameters are shown in Table 2. To minimize the amount of onboard equipment, the numberof feed elements and the number of beams accommodated in beam group were restricted to powers of two.
Design results for 3 beam groups and 11 overlapping
beams are shown in Fig. 8. The subtotal RF power
values of beam groups A, B, and C were 40%, 40%,
and 20% of the total RF system power, respectively.
We confirmed that this scheme can radiate sufficient
RF power. The sum of the subtotal RF powers allocated to the beam groups can be assigned to the corresponding overlapping beams. For example, 80% of
the total RF system power can be assigned to eachoverlapping beam between groups A and B. Therefore, the overlapping beams will normally be allocated to the main island of Japan because that area is
expected to have the heaviest traffic.
5.2 Evaluation of antenna performances
To evaluate the performance of our antenna design,
we examined a 69-beam antenna with different strucTable 2. Antenna parameters.

ParabolicreflectorD

ArrayfeederXfρFrequency

2.50 GHz

Aperture diameter: D

19.0 m

Focal length: f

13.3 mDefocus length: ρ

1.2 m

Offset angle: ξ

43.5°

Feed element

Microstrip patch

Feed element spacing

120 mmNumber of elements

64

Number of beam groupsξ

3 (A: 32, B: 32, C: 16)

Number of overlapping beams

f

Focus

(a) Antenna configuration

11

Group AGroup B

Group CA and B

B and C

C and A

2E

Xf
Elevation (degrees)

1

Yf

D

0

–1

G

F

–2
–2
(b) Feed element layout

Fig. 7. Designed multibeam reflector antenna.

Vol. 5 No. 1 Jan. 2007

–1

0

1

2

Azimuth (degrees)Fig. 8. Example of beam groups and overlapping beams.

65Special Feature51: 43.5 dBi

Group B

: 33.5 dBi: 23.5 dBi–248
Elevation (degrees)

Antenna gain (dBi)Group C

45

42

39
–2.5

–1.5

–0.5

0.5

1.5

2.5

1

0

–1

–2
–2

Azimuth angle (degrees)

–1

0

1

2

Azimuth (degrees)Fig. 9. Calculated antenna patterns for our feeder.
Fig. 11. Example of calculated antenna beam patterns.
(Areas: E, F and G, contours: 43.5, 33.5, and23.5 dBi)

Antenna gain (dBi)

51
48
45
42
39
–2.5

–1.5

–0.5

0.5

1.5

2.5

Azimuth angle (degrees)(a) Focal feeder with MPA

Antenna gain (dBi)

51

48

45

42

39
–2.5

–1.5

–0.5

0.5

1.5

2.5

Azimuth angle (degrees)(b) Off-focal feeder with the same amplitude weights

Fig. 10. Calculated antenna patterns with conventional
feeders.

tures. To provide a benchmark, we evaluated a multibeam reflector antenna with conventional feed systems. We assumed the same reflector aperture diameter as for our antenna. In the focal array feed config66

uration, each beam was formed by seven elements
and the amplifier part was assumed to consist of three
23-beam MPAs to achieve the required RF power
concentration. MPA output loss was estimated to be
1.5 dB.
Some examples of calculated antenna patterns areshown in Figs. 9 and 10. They correspond to theazimuth cut patterns (elevation angle: –0.7°) of 9
beams in Fig. 8. In these figures, the dotted lines show
the minimum EOC gain of the 9 beams. Our new feedsystem offers the same minimum EOC gain as the
focal-feed system; it has larger gain than the offfocal-feed system with the same amplitude weights.
The minimum EOC gains of all 69 beams with our
new feed system and the conventional focal feed system are 43.92 and 44.27 dBi, respectively. To evaluate the sidelobe characteristics, we calculated contour
patterns of the south-west, south-east, and north-eastedges (Fig. 11). Contours for 43.5, 33.5, and 23.5 dBi(only the south-west edge) are plotted. These calculations confirm that the antenna requirements are satisfied and that our scheme yields almost the same EOC
gain as the conventional focal feeder.
We also calculated the antenna patterns of the overlapping beams. Figures 12(a), (b), and (c) show theantenna patterns formed with the group A amplitude
weights, group B amplitude weights, and combined
A + B weights, respectively. The gain deviation
against power division ratio (d) of groups A and B
covers the range from d = 0.0 (Group A) to d = 1.0(Group B), as shown in Fig. 13. These results confirm
that the overlapping beams meet all the expected
requirements.
To determine the power distribution flexibility
NTT Technical Review

Special Feature

among beams, we calculated the RF power shortagedue to HPA saturation; the virtual power distribution
ratios were generated at random. The probability of: 43.5 dBi: 33.5 dBi

Elevation (degrees)

1

0

–1
–1

0

power shortage, as determined from 1000 trials, isshown in Fig. 14. For comparison, the characteristics
obtained with the conventional focal feeder using 3
MPAs and the case of not using overlapping beams
are also shown in the figure. Because increasing the
RF power shortage reduces the flexibility of RF
power distribution among the beams, the results confirm that our new technique greatly improves the RF
power distribution flexibility among beams.
Finally, we compared the weight of the conventional focal feed system and our new feed system; weconsidered the elements forming the feed systems
shown in Table 3. Weights of the basic circuits weretaken from previous reports [8]-[10]. Our new feedsystem offers almost the same performance as the1

Azimuth (degrees)

0.1: 33.5 dBi: 43.5 dBi

Elevation (degrees)

1

Gain deviation (dB)(a) Group A
0.05

0

–0.05

0
–0.1
0

–1

0.25

0.5

0.75

1

Power division ratio, d
–1

0

1

Fig. 13. Calculated coverage gain deviation versus
power division ratio.

Azimuth (degrees)(b) Group B

100: 33.5 dBi: 43.5 dBi

Without overlap

80
Probability (%)

Elevation (degrees)

1

0

–1

3 MPAs60

40
With overlap
and combined
20

–1

0

1

Azimuth (degrees)(c) Combined A + B

Fig. 12. Example of calculated antenna beam patterns(Area: D, contours: 43.5 and 33.5 dBi).

Vol. 5 No. 1 Jan. 2007

0
0

2

4

6

8

10

RF power shortage (%)

Fig. 14. Calculated RF power shortage against virtual
power distribution.

67

Special Feature

Table 3. Comparison of components and weights of feed systems.
Configuration

Unit weight

Number of items
Focal feeder with MPAs

New feeder

Feed element

0.15

104

64

Hybrid for output hybrid network

0.3

192

0

HPA

1

104

64

BFN and Hybrid for input hybrid
network for focal feeder

10

1

—

BFN for new feeder

30

—

1

Total weight

—

187.2

103.6
(units: kg)

focal feed system with an MPA but it weighs much
less.

References[1]

6. Conclusion
We investigated two novel techniques—a multibeam antenna feed system that uses the beam-groupconcept and a beam-forming method that combines
beams from different beam groups—for a lightweight
onboard multibeam antenna feed system that offers
RF power distribution among beams. Our beamgroup concept improves the antenna gain over theentire coverage area compared with the conventional
array-fed reflector antenna. Our beam-forming
method combines beams from different beam groups
and so improves the RF power distribution flexibility
among beams.
We confirmed the validity of these methods by calculating antenna patterns for a 69-beam antenna. The
antenna patterns formed by our feeder satisfy theantenna performance requirements. Weight estimations showed that our multibeam antenna feed system
is much lighter than the equivalent conventional
multibeam antenna feed systems while still satisfying
the antenna performance requirements.

68

M. Ueba, K. Ohata, and J. Mitsugi, “Next-generation Broadband
Mobile Satellite Communication System in the Ubiquitous Network
Era,” NTT Technical Review, Vol. 5, No. 1, pp. 34-44, 2007.
[2] K. Nakahira, K. Kobayashi, and M. Ueba, “Resource Management
and QoS Control Scheme in a Multibeam Satellite Communication
System,” NTT Technical Review, Vol. 5, No. 1, pp. 45-51, 2007.
[3] S. Harada, A. Meguro, and M. Ueba, “Tendon-reinforced Ultralight
Large Antenna Reflector,” NTT Technical Review, Vol. 5, No. 1, pp.
70-77, 2007.
[4] R. J. Mailloux, “Phased Array Antenna Handbook,” Artech House,
1994.
[5] S. Egami and M. Kawai, “An Adaptive Multiple Beam System Concept,” IEEE Journal on Selected Areas in Communications, Vol.
SAC-5, No. 4, pp. 630-636, May 1987.
[6] T. Ohira, Y. Suzuki, H. Ogawa, and H. Kamitsuna, “Megalithic
Microwave Signal Processing for Phased-Array Beam Forming andSteering,” IEEE Trans. on MTT, Vol. 45, No. 12, pp. 2324-2332, Dec.
1997.
[7] K. Tokunaga, H. Tsunoda, H. Shoki, and M. Okumura, “Design Optimization of Phased Array Fed Reflector Antennas for Mobile Communication Satellites,” 17th AIAA ICSSC, AIAA-98-1223, pp. 105109, Feb. 1998, Yokohama, Japan.
[8] K. Ueno, M. Atobe, T. Ide, Y. Suzuki, and M. Okumura, “ETS-VIII
S-Band Antenna Feed System and Electrical Properties,” Proc. of theInternational Symposium on Space Technology, Bibliographic details
2000, Vol. 22ND/2, pp. 1493-1498, 2000.
[9] F. Ishitsuka, N. Iwasaki, T. Ohira, Y. Suzuki, and Y. Ando, “A compact three-dimensional packaging technique using super-fine-pitch
coaxial connector arrays for on-board satellite equipment,” IEEE
Transactions on Advanced Packaging, Vol. 23, No. 3, pp. 530-537,
Aug. 2000.
[10] http://www.mitsubishielectric.co.jp /society/space/

NTT Technical Review

Special Feature

Yoshinori Suzuki

Kiyoshi Kobayashi

Research Engineer, Satellite Communication
Systems Group, Wireless Access Systems Project, NTT Access Network Service Systems Laboratories.
He received the B.E. in communication engineering, the M.E. and Ph.D. degrees in electricaland communication engineering from Tohoku
University, Miyagi, in 1993, 1995, and 2005,
respectively. He joined NTT Wireless Systems
Laboratories in 1995. Since then, he has been
engaged in R&D of multibeam antenna feed systems for communication satellites. He is currently working on future mobile satellite communication systems. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE) of Japan.

Senior Research Engineer, Supervisor, Satellite Communication Systems Group, WirelessAccess Systems Project, NTT Access Network
Service Systems Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Tokyo Universityof Science, Tokyo, in 1987, 1989, and 2004,
respectively. He joined NTT Radio Communication Systems Laboratories in 1989. Since then, he
has been engaged in R&D of digital signal processing algorithms and their implementation
techniques including modulation/demodulation,synchronization control, and diversity for satellite and personal wireless communication systems. He is currently working on future mobile
satellite communication systems. He is a memberof IEEE and IEICE.

Akira Meguro
Associate Senior Engineer, Engineering Test
Satellite VIII Project, Japan Aerospace Exploration Agency (JAXA), Office of Space Applications.
He received the B.E. and M.E. degrees inmechanical engineering from Keio University,
Tokyo, and the Ph.D. degree in aerospace engineering from the University of Tokyo, Tokyo, in
1983, 1985, and 2004, respectively. He joined
NTT in 1985. His main field is R&D of spacecraft structures. In particular, he has been studying large deployable antenna reflectors on board
communication satellites. He has worked for several satellite projects such as ETS-VI, VSOP, NStar c, and ETS-VIII. He moved to Japan Aerospace Exploration Agency in 2006.

Vol. 5 No. 1 Jan. 2007

Masazumi Ueba
Senior Research Engineer, Supervisor, Group
Leader, Satellite Communication Systems
Group, Wireless Access Systems Project, NTT
Access Network Service Systems Laboratories.
He received the B.E. and M.S. degrees in aeronautical engineering, and the Dr.Eng. degree forwork on the design methodology of highly accurate antenna pointing system from the Universityof Tokyo, Tokyo, in 1982, 1984, and 1996,
respectively. He joined the Yokosuka ElectricalCommunication Laboratories of Nippon Telegraph and Telephone Public Corporation (nowNTT) in 1984. He has been engaged in researchon the dynamics of antenna pointing control systems of satellites and shape control systems forlarge antenna reflectors. He is currently researching technologies for next-generation mobile
satellite communication systems. He is a memberof IEICE, the Japan Society for Aeronautical andSpace Sciences, and the American Institute of
Aeronautics and Astronautics.

69