Antenna Technology at Intelsat

pp. 361-377 361

Pierre NEYRET *

Antenna technology at INTELSAT

Abstract

After a brief summary of the evolution of antenna technology at INTELSAT and of specific constraints for the 1NTELSAT system, the paper presents a reminder of the fundamentals of multi-beam antenna design : reflector size and number of feed elements, spill-over loss, cross-polarization, gain factor of merit (and the equivalent notions of gain • area product and gain • beamwidth product). Also, the paper presents a concise description of the antenna subsystem of INTELSAT V and of the hemi/zone antenna of INTELSAT VI, and provides the references of detailed publications. Then the paper provides highlights of ~NTELSAT research and development on satellite antenna technology, refering to available publications for more detail. Finally, the paper presents a brief summary of the progress in antenna technology for INTELSAT Earth stations.

Key words : Space antenna, Telecommunication satellite, Earth station, Reflector antenna, Multibeam antenna, Techni- cal progress, Evolution, International institution, Coverage. System design.

TECHNIQUE DES ANTENNES INTELSAT

Analyse

Aprks un bref rappel de l'dvolution technique des a n t e n n e s INTELSAT, et des contraintes propres au systbme INTELSAT lui-m~me, l'auteur prdsente un rappel des notions fondamentales pour lYtude des antennes multifaisceaux : dimensions du rdflecteur et nombre d'dldments de la source, pertes par ddbordement, polarisation croisde, facteur de mdrite en gain (et les notions dquivalentes de produit gain • surface de couverture et gain • ouverture angulaire du faisceau). Aussi, l'auteur prdsente une description succincte du sous-systbme antenne d'INTELSAT l Z et de l'antenne

d'INTELSAT Vl pour couverture d'hdmisphbres et de zones, et fournit les rdJ~rences de publications plus ddtailldes. Ensuite, l'auteur effectue un survol du programme de recherche et ddveloppement d'INTELSAT sur la technique des antennes pour satellites, et donne les Mfdrences de publications plus ddtailldes disponibles clans la litt6rature. Enfin, l'auteur prdsente un bref rdsumd du progrbs technique pour les antennes des stations terriennes INTELSAT.

Mots el6s : Antenne spatiale, Satellite t61~communication, Station terrienne, Antenne r6flecteur, Antenne multifaisceau, Progr6s technique, Evolution, Organisme international, Cou- verture, Conception syst6me.

Contents

I. Introduction.

II. System constraints.

III. Fundamentals of multi-beam antenna design.

IV. INTELSAT V antennas.

V. INTELSAT VI hemi/zone antenna.

VI. Highlights OfINTELSAT R & D on satellite antenna technology.

VII. Earth station antenna technology.

References (50 ref.).

I. INTRODUCTION

The progress of antenna technology has been a key factor for the evolution of the INTELSAT system, both for the space segment and the ground segment.

Early Bird (1965) implemented a simple fiat toroid beam which sent most of the satellite RF power into outer space. INTELSAT III (1968) had a global beam sending most of the satellite RF power to the earth's

* International Telecommunications Satellite Organization INTELSAT, 490 l'Enfant Plaza S.W., Washington, D.C. 20024.

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362 1,. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

disk. INTELSAT IV-A (1975) implements two-fold frequency reuse by means of two shaped hemispheric beams with a 27 dB spatial isolation obtained by sidelobe control. INTELSAT V (1981) implements four-fold frequency reuse and in-orbit reconfigurability of two of the beams. INTELSAT VI (scheduled for 1986) will implement six-fold frequency reuse and a larger degree of in-orbit reconfigurability.

The C-band transmit antenna of INTELSAT IV-A consists of a 1.2 • 1.3 metre square reflector fed by a 37-element array ; INTELSAT V uses a 2.4 metre diameter reflector fed by an 89-element array; INTELSAT V I uses a 3.2 metre diameter reflector fed by a 145-element array. The mass of the complete antenna subsystem of INTELSAT V ( m o r e than 100 kg, including the support tower) is approximately one third of the total payload mass. The mass of the complete antenna subsystem of INTELSAT VI (more than 300 kg) is approximately one half of the total payload mass.

Though less spectacular, technical progress for earth station antennas has also been important. Utilization of shaped Cassegrain reflectors provides very high peak gains (gain factors of 70 percent to 75 percent at both transmit and receive frequencies have been measured on some standard A antennas). Monopulse tracking couplers provide accurate pointing. Sophisticated feed sources, corrugated horns, beam waveguides and well-aligned optics provide excellent polarization isolation ; well-aligned optics als0 provide low sidelobes. Furthermore, the combination of beam-waveguide and carrousel mount allows a substantial reduction of the mass of the movable portion of the antenna, and therefore of its manufacturing cost.

II. SYSTEM CONSTRAINTS

The rapid growth of international and domestic satellite communications has been spurred by four key advantages of satellite networks.

a) The cost is independent of the distance.

b) Multiple access is possible.

c) The network configuration can readily be modified to match changing traffic patterns.

d) The system is not limited to point-to-point communications; broadcasting is possible.

Indeed, the success of satellites communications has been so great that the geostationary orbit is becoming congested and coordination between different systems is necessary to keep mutual interference to acceptable levels. Earth stations now are required to meet strict sidelobe specifications. Interference from ground communication networks must also be taken into account.

Because of INTELSAT'S international role, specific constraints apply to the system. Service must be provided to most countries of the world. The density of traffic ranges from very high for Western Europe to very low for South Pacific Islands. The magnitude of traffic requires up to six-fold frequency reuse (INTELSAT VI). Most INTELSAT satellites are located close to the center of the Atlantic, Indian and Pacific Oceans in order to facilitate intercontinental communications, which gives a particular importance to propagation conditions, since many earth stations see the satellite with a relatively low elevation angle. In areas with heavy rainfalls, propagation impair- ments can be severe, particularly for frequencies above 10 GHz.

The number and size of antenna beams radiated by a satellite is a major factor in system trade-offs. Broad antenna beams provide favorable coverages and facilitate interconnection between users, but their low antenna gain requires high power amplifiers for Earth stations (up-link) and for the satellite (down-link). Multiple narrow beams provide high antenna gain and allow multiple frequency re-use. However, utilization of multiple beams has three consequences :

a) Connectivity between users requires either a large number of repeaters or implementation of on- board dynamic switching.

b) Beams must be isolated from each other either by siddobe control, polarization isolation, or a combination of both.

c) Beams must be reconfigurable in order to allow different coverage contours for different orbital locations.

The isolation and reconfigurability requirements are driving factors for antenna design ; they are the main cause for the complex design of the INTELSAT V and INTELSAT VI C-band antennas. When large antennas are required, as for INTELSAT VI , compatibi- lity with launch vehicles (ARIANE, S.T.S.) further increases the complexity of the antenna.

III. FUNDAMENTALS OF MULTI-BEAM ANTENNA DESIGN

lII.1. Reflector size and number of feed elements.

The size of the reflector is determined mainly by the requirement of greater than 27 dB sidelobe isolation between two co-polar shaped beams. For INTELSAT VI , with the coverage requirements shown in Figure 23, the limiting factor is the 2.15 ~ separation between the Z3 and Z, AOR (Atlantic Ocean Region) zones and between the Zx and Z3 IOR (Indian Ocean Region) zones. This separation must be

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P . N E Y R E T . - A N T E N N A T E C H N O L O G Y A T I N T E L S A T

ZONE~ TH$ S ZO~N/E 2 ~

FIG. 1. ~ Definition of the minimum inter-zone spacing ~ s)~.

Ddfinition de la sdparation minimale ~ s ~ entre deux faisceaux copolaires modelds.

larger than the constituent beams radiated by a single feed element, by a factor of approximately 1.4 times the constituent beamwidth as shown in Figure 1.

Calling ~ s )~ the separation between two isolated co-polar zones, and B the constituent beamwidth, both in degree of angle, we get the relation (1).

(1) s ~ = 1.4 B ~

The constituent beamwidth is approximately related to the reflector diameter D by relation (2).

(2) B ~ = 70 ~ X / D .

N o t e : B ~ corresponds to the - - 4 dB contour of the constituent beamwidth because each element illuminates the reflector with a relatively high edge pedestal.

The required reflector diameter for a given beam separation can thus be determined from relation (3) :

98 ~ 100 o (3) D = ~ X _~ s ~ X.

D. Difonzo [1] has shown that this relation is verified by various reflector sizes respectively compatible with the INTELSAT VI-A, INTELSAT V and INTELSAT VI requirements (Fig. 2).

The number of constituent beams required can be obtained by comparing the solid angle of the Earth's disk to the solid angle of one constituent beams. In

1 ] FREQUENCY - 4 GHz

O �9 "3"

z (2_ b-

100

I I I [ I 1"%.1

2 3 4 5 6 7 8910 REFLECTOR DIAMETER ~ METERS

Fro. 2. ~ Angular separation between the edges of adjacent coverages for 27 dB isolation with various reflector sizes.

Sdparation angulaire entre deux faiseeaux copolaires (isolation : 27 dB) pour diffdrentes dimensions du rdflecteur.

363

practice, however, it is more convenient to utilize projected areas in satellite north-south/east-west coordinates, taking as reference a square area of 1 ~ • 1 ~

The area of the Earth's disk in square degrees is :

7~ (4) al = ~ (17.34~ 2 = 236.15 deg 2.

The area of the hexagon circumscribed to the constituent beam is :

(5) ax = ~ • \ ~ / = 0.65 (B~ 2 deg 2.

These two relations then allow us to obtain the number of adjacent constituent beams required, N :

N - a l 363 ( D ) 2 712 (6) a2 -- (BO) 2 -- 0.074 - (~)-2 �9

The aperture diameter <<d~ of each of the adjacent elements of the feed array must not be larger than the diffraction spot of the reflector in order to avoid the appearance within the coverage area of a ripple caused by inadequate sampling. From Born and Wolf [21 :

(7) d _-< 1.22 X F / D ,

where F is the focal length and D the diameter of the projected aperture, as shown in Figure 3.

\ \\

\x\

\\\x\\\

. . . . . . . ~ ,

F I G . 3. - - O f f s e t r e f l e c t o r c o n f i g u r a t i o n .

Configuration d'un rdflecteur t} source ddealde.

This value ~ d)) of the maximum diameter of the aperture of a feed element depends only on the choice of the F ] D ratio. It is independent of the size of the reflector.

We can verify that this diffraction spot requirement is consistent with the number of constituent beams N of relation (6). Neglecting the beam deviation factor, as in Figure 4, the total aperture size of the feed array (diameter A of the intersection of the focal plane with the cone corresponding to the apparent

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364 I,. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

/

~ " ~ " ~ ' ~ E A R T H

~ * " " - - I C O V E R A G E T F A

I

FIG. 4. - - Aperture size of the feed array, neglecting the beam deviation factor (center-fed antenna).

Dimension de l'ouverture rayonnante du r~seau de sources pri- maires, en ndgtigeant le facteur de ddviation du faisceau (rdflecteur

centre).

diameter of the Earth, 17.34 ~ is :

17.34 ~ (8) A = 2 F t a n 2 - -0 .305F .

The number N ' of the elements d which can fit within the diameter A is :

(9) N ' = 7~ A2[4 75.5 ( D / 2 (3~f~12)(d2]4) 1000 , _ , = 1.02 N ~ N.

The relation (9) indicates that the number of elements N required for 27 dB sidelobe isolation is almost equal to N', which means that their aperture meets the diffraction spot sampling requirement of relation (7).

The antenna designer may elect to utilize a larger number of overlapping constituent beams if he wishes by using smaller elements for the feed array. This facilitates the sidelobe control, but increases the complexity of the power distribution network and of the feed array.

I f the designer has some difficulty in accommodating the size of the reflector (which was the case for INTELSAT VI), he may also elect to keep the same number N of elements and to reduce the reflector size. The constituent beams then become adjacent for instance at their - - 3 dB contour level (they overlap at their - - 4 dB contour level), and the relations (2), (3) and (6) becomes (2'), (3') and (6').

X (2') B ~ = 61 ~ --

D '

X (Y) D = 86 ~

S ~ '

(6') N - (BO) ~ - 0.097 - (sO) 2 .

For INTELSAT VI, it also has been necessary to utilize the beam-sharing technique (low level excita- tion of the same element for two different beams by means of a hybrid coupler) in order to get adequate sidelobe isolation in the most critical areas.

I f the reflector size is not a limiting constraint, the designer also may elect to increase it. The antenna will need a larger number of feed elements, but sidelobe control will be easier, and the need for beam- sharing may be eliminated. Also, the contours of the different beams can match more closely the coverage requirement ; for narrow zone beams, the gain advantage can be significant. One must keep in mind, however, that the smaller size of the constituent beams will increase the distortion of their pattern by defocalization of those beams scanned towards the edge of the Earth.

The main degree of freedom for the antenna designer is the choice of the F]D (focal length to reflector diameter ratio). This value is usually chosen in the 0.8 to 1.3 range, depending on the coverage and antenna size requirements. Figure 5 illustrates

3.2 NEER REFLECIOIt F/D - 11.9 4-08 CONTOURS

I

i l I i

P.F.FLEC(OR F O C U S I 0 ' O F S C A N I

3 .2 NEIER REFLECIOR F/O " 1 . 4 4-DB CONTOURS I I i i i 1 i ; i i i ~ i i

I I

I [ ~ I i I i i 1 i I I i t

FIG. 5. - - Impact of the reflector F[D on the scanning distortion of the component beam.

- - 4 dB contours for a 3.2 m reflector with F]D = 0.9 (top) and F]D = 1.4 (bottom).

Influence du rapport F] D du rdflecteur sur la distorsion de bala- yage du pinceau ~ldmentaire.

Contours gt - - 4 dB pour un rdflecteur de 3,2 m de rapport FID = 0,9 (figure du haut) et F]D = 1,4 (figure du bas).

how utilization of a large F[D ratio improves the antenna design by reducing the defocalization distortion for beams scanned towards the edge of the Earth. However, the aperture of the feed array increases with the F[D ratio, and a longer focal length may be more difficult to accommodate on the satell i te; also, the mass of the antenna will be increased. F[D = 1.3 is considered to be the highest practical value, since it would not make sense to have a feed array aperture almost as large as the reflector itself. F]D = 0.8 is the lowest practical value, since it corresponds to a feed element aperture of one wavelength, which is the lowest acceptable value to keep these elements relatively free from unwanted polarization degradation by proximity coupling effects.

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III.2. Spifl-over loss.

The spill-over loss is the amount of RF energy radiated by the feed array which does not intercept the reflector, as shown on Figure 6. Antenna computer

FIG. 6. -- Spill-over loss for a narrow zone beam.

Pertes par d~bordement pour un faisceau de zone relativement ~troit.

programs take the spill-over loss into account when calculating the antenna gain, but th~.y often do not provid~ explicitly th~ value of the spill-over loss, which is useful information. For narrow zone beams, the reflector is located in the far-field zone of the feed array (Fig. 6 ) ; therefore, the spill-over loss can b~ obtained by calculating the pattern of the feed array, and then taking the ratio of the integral of the fraction of the pattern intercepting the reflector to the integral of th~ complete pattern. For the large hemispheric beams of INTELSAT VI, the reflector is not in the far field of the feed array, but in the near- field, which m~ans that the energy radiated by the array remain a cylindrical wave all the way to the reflector (Fig. 7).

FIG. 7. -- Case of the INTELSAT VI hemispheric beams ; reflector in the near-field of the feed array.

Cas des faisceaux h(misphdriques d'INTELSAT-VI, O~ le rdflecteur est situd dans le champ proche de la source.

Therefore, there is no spill-over loss, but there is instead a defocussing loss, because the incident energy is not a spherical wave. Various authors [3], [4], [5] have investigated ways of reducing the spill-over loss by using high efficiency elements. A high efficiency element is an element where the energy is spread as uniformly as possible across its aperture by means of mode-control in order to obtain a narrower element pattern. As a result, the element radiates less energy in the direction of the grating lobes of the array, and the spill-over loss is reduced, as shown in Figure 8.

10

20

30

t" /

I I t

I

J

ST/ ~DARD

~ HI 3H FFV :IENC'~

I v ", '1 I

0

FIG. 8. - - Spill-over loss reduction using high-efficiency elements.

L" emploi d'dl~ments rayonnants d' efficacit~ dlev~e r~duit les pertes par d~bordement.

I11.3. Cross-polarization.

In linear polarization, an offset reflector configuration does generate a cross-polarized field in its plane of asymmetry. For the INTELSAT C-band antennas operating in circular polarization, an offset reflector configuration does not generate a cross-polarized field for a feed element at the focus ; instead in the plane of asymmetry, a squint occurs between the right-hand and the left-hand signals [6], [7], [8]. Adatia and Rudge [9] have derived an approx ima te

formula (which in fact proved to be very accurate) that gives the beam squint angle simply as :

(;~ sin 0o) (10) tF s = arcsin - -

4 nF

Therefore, if the feed array is well focussed (with its phase center located at the reflector focal point), it is the only source of cross-polarized energy.

Each element of the array considered separately (that is removed from the array) radiates a cross- polarized signal, the pattern of which is the product of two contributions : the aperture contribution and the polarizer contribution.

The aperture contribution comes from the fact that the field distribution in the radiating aperture

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366 P. NEYRET. - A N T E N N A T E C H N O L O G Y AT INTELSAT

does not correspond to a hybrid mode. The amplitude of the cross-polarized energy is radiated along a revolution pattern with a null on the boresight axis, but its phase corresponds to a (( twice-the-tilt-angle law>>; therefore, the energy radiated in any two orthogonal planes are in opposition of phase.

The polarizer contribution radiates an equiphase cross-polarization pattern with a maximum on the boresight axis. The cross polarization energy comes both from imperfections in the polarizers and from VSWR (voltage standing wave ratio) mismatches. There are three kinds of polarizer imperfections : the misalignment (error relative to the required 45 ~ direction), the resistive loss, and the phase error relative to the required 90 ~ phase shift. The misalignment error (0.15 dB of axial ratio for one degree error) can be made negligible ; the resistive losserror is small for a silver-plated polarizer �9 the phase error (0.15 dB of axial ratio for one degree of phase error [10]) is the dominant one. As a result, the cross-polarized wave generated by the polarizer imperfection is in quadrature of phase with the co- polarized wave. The combined VSWR mismatches of the radiation aperture and of the orthogonal port of the input transducer tee results in radiating a cross-polarized reflected wave. The phase of this wave depends on the electrical length of the element. This contribution can be made small by using a very well matched radiating aperture and by using hybrids or Wilkinson couplers in the power distribution network.

Because of their respective phase characteristics, the aperture and the polarizer contributions to the cross-polarized radiation (the non-hybrid mode aperture distribution and the non-90 ~ phase-shift polarizer) add (field addition) in one diagonal plane, subtract (field subtraction) in the other diagonal plane and add quadratically (addition of two orthogonal vectors in the complex plane) in the principal planes.

When the radiating element is inserted in the array, proximity couplings with other elements of the array can significantly increase the cross-polarized radiation. The mechanism of proximity coupling effects is illustrated on Figures 9 and 10, assuming, for illustra- tion purposes, that only one element is powered (<( excited >)), the adjacent elements being terminated on matched loads.

! COUPLED

i = , ~ ~FUNDAMENTAL MODE

EXCITED ELEMENT

t-------- COUPLED

FUNDAMENTAL MODE I

REFLECTED HIGHER ORDER MODES (CROSS-POLARIZATION)

DIRECT RADIATION OF EXCITED ELEMENT (COPOLAR SIGNAL)

REFLECTED HIGHER ORDER MODES (CROSS-POLARIZATION)

FIG. 9. -- Proximity effects for a single excited element set in an array of passive loaded adjacent elements.

Effets de proximitd pour un dldment alimentd unique insdr~ clans un rdseau d'dldments adjacents non alimentds et charges.

COUPLED MODAL CONTENT OF ELEMENT COUPLED APERTURE (LOADED)

WEAKER COUPLING AREA

STRONGER COUPLING AREA

FUNDAMENTAL HIGHER ORDER MODE MODE

EXCITED ELEMENT

FIG. 10. -- Modes induced by proximity couplings into the aperture of adjacent elements.

Le couplage par e frets de proximitd induit une superposition de modes dans l'ouverture des dldments adjacents.

Figure 10 shows that the co-polar signal of the excited element creates a stronger coupling in the closest area of an adjacent element than in its most remote area. This uneven coupling generates a co- polar asymmetric field in the aperture of the adjacent element, which can be considered as the superposition of a fundamental mode (which enters into the adjacent element and gets absorbed by the matched load), and of higher order modes, which cannot enter into the adjacent element (they are at cut-off). These higher order modes are therefore reflected and radiated in the opposite sense of polarization, that is cross- polarized energy (Fig. 9). There is usually a clear correlation between the energy levels measured on the terminated ports of the adjacent elements and the levels of cross-polarized radiation.

An experimental study of proximity coupling effects [11], [12], [13] demonstrated that polarization degradations are minimum when the distribution of the co-polar field across the element aperture is well tapered and null on the edge of the element, which is the case of the Potter horn [14]. I f the co- polar field across the aperture is more uniform, and not null on its edges, the cross-polarized levels significantly increase, as shown in Figure 11.

The Potter horn is a multi-mode element combining 85 percent of fundamental TE 11 mode power and 15

-3 dB CO-PO I.. CONTOUR

(J) O,7~APERTURE eal POTTERHORN

0 dB REFERENCE: COPOLAR BEAM PEAK

FIG. 11. -- Cross-polarization contours for a single excited element set in an hexagonal array of adjacent loaded

elements.

Contours en polarisation croisde pour un ElEment alimentd unique ins~r~ dans un r~seau hexagonal d'~ldments charges.

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P . N E Y R E T . - A N T E N N A T E C H N O L O G Y A T I N ' I E L S A T 367

percent of TM 11 mode power in phase in its aperture, creating a quasi-hybrid distribution. The method to calculate the amplitude and phase of the Xi 11 mode excited by a step in the waveguide has been determined by W. J. English [15]. TE I1 and TM 11 modes have different propagation velocities, and therefore they cannot stay exactly in phase in the frequency bandwidth ; a 30 ~ phase error is acceptable. It is necessary to design the step-mode transducer so as to avoid exciting other modes. Modes which cannot be excited by reason of symmetry, like the TE 31 mode, become dangerous at their cut-off frequency [12]; since their propagation velocity becomes infinite, the plane of the step transducer becomes electrically identical to the aperture plane, and a resonance occurs. For the TE 31 mode, proximity couplings with the adjacent elements of the array substantially increase this resonance, probably because the hexagonal structure of the array matches its field symmetry. Therefore, the net result is, in linear polarization, an unwanted broadening of the E plane (Fig. 12), and, in circular

E , / ~ ' ~ , E, I / / I \ ~, 1-20da 1-2Oda I I

i I I I I I I I I I

60 ~ 30 ~ 0 ~ 30 ~ 60 ~ 60 ~ 30 ~ 0 ~ 30 ~ 60 r

(a) NORMAL PATTERNS (b) E-PLANE BROADENING AT TE 31 CUTOFF

FIG. 12. -- E-plane broadening for a Potter horn at the cut-off frequency of the TE 31 mode (linear polarization).

Elargissement du diagramme plan E d'un cornet de Potter la frdquence de coupure du mode TE 31 (polarisation rectiligne).

polarization, a substantial increase of the cross- polarized radiation away from the boresight axis.

Figure 13 illustrates this phenomenon for a Potter horn element all by itself (solid line) and set in an array of adjacent elements terminated by matched loads (dotted lines). The INTELSAT VI C-band antennas are equipped with Potter horn elements, and HAC has designed a multi-stage step mode transducer

-20 dB I I I I t I I I I ~ i ~ TE31 MODE CUTOFF

t I t ~ . / �9 ~0 dB ~ t .o I I I* ", �9

-40 dB ~ 1 5.8 5.9 6.0 6.1 6.2 8.3 6,4 6.5 6.6

GHz

FIG. 13. -- Worst cross-polarization level within the -- 1.25 dB eopolar contour of a Potter horn (solid line : single element by itself ; dotted line : same element inserted in an hexagonal

array of loaded elements).

Niveau contrapolaire le plus ~lev~ rencontr~ ?z l'int&ieur du contour copolaire ~ - - 1,25 dB d'un cornet de Potter (trait plein : Eldment isold ; trait pointill~ : dl~ment insdrd dans un

r~seau hexagonal de sources charg~es).

which carefully avoids the cut-off frequency of hazar- dous modes [16].

There is, therefore, a fundamental conflict between gain and polarization performance of a multi-beam antenna, since high efficiency elements (near uniform field distribution) provide low spill-over loss, while low efficiency element (Potter horns) provide low cross-polarization levels. Fortunately, the array factor improves the overall polarization performance of the antenna [13], [17], since it adds the field of each element on the boresight axis (where the co-polar level is maximum and the cross-polar level is minimum) and subtracts them in directions where the ratio of co-polar energy to cross-polar energy is less important (Fig. 14). Then, while most of the co-polar energy intercepts the reflector, a large fraction of the cross-polar energy does not, as shown on Figure 15. Finally, the reflector provides more

OdB

- 1 0 d B

- 2 0 dB

- 3 0 dB

- 4 0 dB

- 5 0 dB

- - - - - - : S INGLE E L E M E N T PATTERNS

: PATTERNS OF COMPLETE FEED A R R A Y

FIG. 14. -- Polarization isolation improvement by the array factor.

AmHioration de la puretd de polarisation par l'effet de rdseau.

: COPOLAR PATTERN

- - - - - - : CROSS-POLAR PATTERN

FIo. 15. -- Polarization isolation improvement by reflector spill-over.

Amdlioration de la puretd de polarisation par ddbordement en dehors du rEflecteur.

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368 P. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

gain for the co-polar energy (which is an equiphase wave) than for the cross-polar energy (which is a wave with a complex phase distribution).

III.4. Gain factor of merit.

The coverage directivity Dr of an ideal beam (radiated by a lossless antenna of infinite size) is the ratio of the area of the isotropic sphere to the area S of the coverage. If the coverage area is counted in square degrees in satellite coordinates (taking as reference a square area of 1 o north-south by 1 o east- west), the area of the isotropic sphere is 41.253 square degrees ; therefore, the directivity of the ideal beam is :

41253 (11) Dc = S '

that is, in decibels ( d B ) :

(12) DcdB = 46.15 dB - - 10 log (S).

Relation (13) gives the directivity for an ideal elliptical beam of principal diameters a and b, and relation (14) gives the directivity for an ideal circular beam of diameter c :

(13) DcdB = 47.2 dB - - 10 log (ab),

0 4 ) DcdB = 47.2 dB - - 20 log (c).

The area of a shaped beam has to be either calculated using a computer or measured using a planimeter.

The gain of a real antenna is substantially lower. A reflector of finite size radiates a beam with rounded edges and sidelobes, and losses occur, mostly by :

a) Resistive losses in the feed array and on the reflector.

b) Spill-over loss (usually taken into account by the antenna computer program).

c) Pointing losses (misalignment error, spacecraft attitude control error, thermoelastic distortions of the reflector).

d) Scattering by surface roughness of the reflector.

e) VSWR mismatches.

The ratio between the directivity Dr of the ideal beam and the real gain G~ of the antenna (that is the minimum gain within the coverage area) is the gain factor of merit m of the antenna.

(15) G~ = m De.

The gain factor of merit m characterizes the quality of the gain performance of the antenna. It must be noted that antenna engineers tend to consider m at the actual input of the antenna, while system engineers tend to consider m at the interface between the antenna and the repeater subsystem. The difference (the feeder line loss) is small for a C-band rigid wave-

guide run but it can be significant for a coaxial feeder line or for K-band waveguide runs including sections of flexible waveguide. Also, antenna engineers almost always refer the gain to the actual beamwidth of the antenna pattern, while system engineers tend to consider the specified coverage on the Ear th ; the difference is significant for narrow zone beams, particularly for satellites with relatively large attitude control errors.

Some authors call the gain factor of merit efficiency, which, of course, must not be mistaken for the aperture illumination efficiency as defined by Sciambi [18], and refer it to the peak gain, rather than the coverage gain [19], [20]; they assume that the coverage is the - - 3 dB contour. While this is a perfectly valid assumption for judging the gain performance of an antenna, it must be kept in mind that the - - 3 dB contour is not necessarily the one providing the highest gain for a given coverage area.

Some authors prefer to utilize the notions of gain x area product instead :

(16) gain x area = GcS = mDr

When elliptical or circular beams are considered, other authors [19], [20] prefer to utilize the gain • beamwidth product :

(17 a) gain x bw = Gcab = mDcab,

(17 b) gain / bw = Gcc 2 = m D c c2.

It must be kept in mind that, for a given antenna, the ratio between the gain x beamwidth product and the gain x area product is :

gain x beamwidth 4 (18) - - 1.05 dB.

gain x area

Therefore, one must not be mistaken for the other ; highly respected engineers have been known to fall into this trap. For a well focussed single circular beam, for example, the INTELSAT V west-spot K- band beam, the following values are obtained [19] :

(19) Peakgain x - - 3 dBbeamwidth = 35,330

(20) - - 3 dB gain x beamwidth = 17,665

(21) - - 3 dB gain x area = 13,874

(22) Factor of merit : m = 33 .6~ = - - 4.7 dB.

The values in relation (19) to (22) are computed data that include only the spill over loss, without resistive, pointing, scattering, mismatch, or feeder line loss.

It is more difficult to give an order of magnitude of the gain factor of merit for a shaped-beam antenna. The beam shaping provides a substantial gain advantage, but the resistive loss is higher, particularly for reconfigurable antennas ; defocussing losses are significant for beams scanned towards the edge of the Earth. The gain factors of merit for the different beams of the INTELSAT VI hemi-zone antenna are presented in Section V of this paper.

ANN. T~LF.COMMUN., 40, n ~ 7-8, 1985 8/17

P. NEYRET. - A N T E N N A T E C H N O L O G Y AT INTELSAT 369

IV. INTELSAT V ANTENNAS

The INTELSAT V spacecraft is shown in Figures 16 and 17. Its antenna subsystem is described in detail in references [21] and [22]. The satellite includes two C-band multi-beam ~ hemi/zone >) antennas, the larger one for transmission of the down link signals

(3.7-4.07 GHz) and a smaller, scaled antenna for reception of the uplink signal (5.925-6.3 GHz). It also includes two C-band global coverage horns, (4.07-4.2 GHz) and 6.3-6.425 GHz respectively) two K-band (11-14 GHz) spot beam antennas, and antennas for tracking, telemetry and command. Some satellites also include an L-band (1.5-1.6 GHz) four-helix array for maritime communications.

The INTELSAT V spacecraft configuration has been optimized for a single launch using expandable launch vehicles (Atlas-Centaur and Ariane). The tall tower on top of the satellite would penalize a multiple launch using STS, since the STS pricing policy does take into account the occupied kngth of the STS cargo bay.

FIG. 16. -- INTELSAT-V antenna subsystem arrangement (with L-band maritime antenna).

Configuration du sous-systdme d'antenne du satellite INTELSAT V (avee antenne bande L pour service maritime).

IV.1. INTELSAT V multi-beam antenna.

The hemi/zone antenna radiates two fixed hemispheric beams in polarization ~< A , (left-hand up- link, right-hand down-link), and two reconfigurable zone beams in polarization << B >> (right-hand up-link, left-hand down-link). The four beams are isolated from each other by 27 dB. The coverage areas of these beams are shown in Figure 18 ; the zone beam coverages are modified in the Indian Ocean Region by means of a s.~t of two-way coaxial RF switches. The 3 ~ s~paration between the two hemispheric beams has required the utilization of a reflector of 2.44 m diameter (transmit antenna) associated with a 89-element source. The aperture of the multi- element source is shown in Figure 19 a, and the triplate power distribution network in Figure 19 b.

FIG. 17. - - INTELSAT-V spacecraft under test.

Un satellite INTELSAT V ell essaL

9/17 ANN. T~L~COMMUN., 40, n ~ 7-8, 1985

370 1,. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

FZG. 18. - - INTELSAT-V hemi/zone coverage for the Atlantic Ocean Region and the Indian Ocean Region.

Couvertures hdmi/zone du satellite INTELSAT V pour l'Ocdan Atlantique et pour l'Ocdan Indien.

FIG. 19. - - INTELSAT-V multi-element primary source : a) radiating aperture ; b) printed circuit of the triplate power

distribution network.

Source primaire multi-dldment du satellite INTELSAT-V " a) ouverture rayonnante ; b) circuit triplaque du rdseau de rdpartition

d'dnergie.

Obtention of an adequate stability of the amplitude/ phase distribution in the temperature range has required a lot of efforts. The radiating element shown in Figure 20 is particularly compact, owing to the utilization of a square waveguide septum polarizer [23] and coaxial hook coupling probes [22]. However, the polarization isolation characteristics of the square aperture are not very favorable [11], [12], and FACC had to design experimentally a set of jumper clips and corner equalizers in order to get satisfactory performance [24].

FIG. 20. - - INTELSAT-V radiating element.

Eldment rayonnant INTELSAT V,

IV.2. INTELSAT V spot beam antennas.

INTELSAT V has two K-band steerable spot beam antennas. The west spot antenna radiates a circular beam and the east spot beam radiates an elliptical beam ; both beams are in opposite linear polarizations. Each antenna is a transmit/receive antenna, the transmit and the receive signals being radiated in orthogonal linear polarizations. The west spot antenna is an offset parabolic reflector antenna with a circular aperture fed by a circular corrugated horn. The east spot antenna utilizes a shaped double curva- ture reflector (nonparabolic surface) to generate an elliptical beam with a circular aperture by means of a phase error in one principal plane [25], [26], [27].

It is interesting to compare the gain factor of merit of the two antennas. The east spot antenna provides a gain at repeater interface of 33.3 dB (including 0.7 dB of losses) within a 3.2 ~ • 1.8 ~ elliptical beam [28] ; therefore, its gain factor of merit is 23.4 700 = - - 6.3 dB. The west spot antenna provides a gain at repeater interface of 37.6 dB (including 0.7 dB of losses) within a 1.6 ~ circular beam [28], [19] ; therefore, its gain factor of merit is 28.1% = - - 5.5 dB. The utilization of the shaped reflector technique results in a 0.8 dB gain penalty, the corresponding energy going into the increased sidelobe level created by the imperfect focalization. The more recent Dragone technique using dual offset cylindrical reflectors with a circular corrugated feed horn in order to radiate an elliptical beam [29] avoids this penalty ; the gain factor of merit obtained with the improved ECS antenna [19], [30] is similar to the one obtained with the well-focussed INTELSAT V west spot antenna.

IV.3. IMTELSAT V beam pointing error.

The antenna beam pointing errors for INTELSAT V are approximately • 0.25 ~ along the roll and pitch axis and + 0.7 ~ along the yaw axis. Reference [31] provides the detailed contributions of the attitude control, the thermal distortion, the alignment error, and orbital errors.

ANN. T~LI~COMMUN., 40, n ~ 7-8, 1985 10/17


V. INTELSAT VI H E M I / Z O N E ANTENNA

The INTELSAT-VI spacecraft is shown in Figures 21 and 22. Its antenna subsystem is described in detail

FIG. 22. - - Artist concept of INTELSAT-VI.

INTELSAT VI (dessin).

FiG. 21. - - INTELSAT-V[ antenna configuration.

Configuration des antennes INTELSAT VI.

FIG. 23. - - INTELSAT-VI coverages (solid lines : specified coverages ; dotted lines : computed - - 3 dB beam widths).

Couvertures du satellite INTELSAT-VI (trait plein : couvertures spdcifides ; pointilld : contours ~ 3 dB calculds).

in reference [16]. Like INTELSATV, INTELSATVI includes two C-band hemi/zone antennas, two K- band spot-beam antennas, global coverage horns and TTr antennas. The INTELSAT Vl" spacecraft is compatible with the STS and Ariane launch vehicles.

The INTELSAT VI hemi/zone antenna radiates two fixed hemispheric beams in polarization A and four reconfigurable zone beams in polarization B. Figure 23 shows the specified coverage areas and the actual - - 3 dB beamwidth of the different beams. The

different shapes of the zone beams for the three ocean regions are obtained by feeding the elements from three different power distribution networks (Atlantic, Indian, Pacific) through a set of 3-way coaxial switches, as shown on Figures 24 and 25. For the power distribution network and part of the switches themselves, HAC utilizes the square coaxial s q u a r e a x technology. Each power distribution network is a masterpiece of metal machining, as shown on Figure 26. The reflector feed-array geometry generates

11/17 ANN. T~LI~COMMUN., 40, n ~ 7-8, 1985

372 P. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

F I G . 2 4 . - - Hemi/zone network implementation.

Ddtctil de la source primaire pour couverture hdmisphdrique et de zones.

F t G . 2 5 . - - 3-way squareax switch.

Commutateur d 3 voies d ligne coaxiale/eoaxiale carrde.

FIG. 27. - - Radiating dement (Potter horn).

Eldment rayonnant (cornet de Potter).

F I G . 2 6 . - - East hemi squareax network.

Rdpartiteur pour hdmisphOre Est en ligne coaxiale carrde.

unwanted cross-polarization for the beams scanned towards the edge of the Earth because they are not well focused. Therefore, it has been necessary to utilize Potter horns virtually free of cross-polarization as radiating elements (Fig. 27), at the expense of increased spill-over loss. Figure 28 provides the gain factor of merit for the actual - - 3 dB beamwidths of the INTELSAT-V[ C-Band hemi/zone antenna. The data shown includes all losses, which range f rom 1.0 dB to 1.4 dB depending on the beam. The abscissa

FfG. 28. - - Gain factor of merit (at repeater interface) referred to the - - 3 dB computed contours.

Facteur de mdrite en gain (d l'interface rdpdteur) par rapport aux contours ~ - - 3 dB calculds.

gives the directivity of the ideal beams as defined by relation (12), and also, for convenience, the equivalent diameters of circular beams which would have the same areas in square degrees using relation (14).

ANN. T~L~COMmJN., 40, n ~ 7-8, 1985 12/17


It can be seen that the broad hemi beams have a factor of merit of 25 % to 28 70 ( - - 6 dB to - - 5.5 dB), the medium size zone beams have factors of merit of 20 70 to 23 % ( - - 6.5 dB to - - 7 dB), and the narrowest beams have factors of merit ranging from 11% to 20 ~o ( - - 9.5 dB to - - 7 dB). The two points shown in a circle are not to be included in the compari- son, because for them a coupler diverts a substantial portion of the energy into a resistive load in order to meet the isolation requirements ; it was necessary to reduce the gain for those two bzams, because the coverage areas for the reconfigurable zone beams 2 and 4 are much smaller in the Pacific Ocean region than in the Atlantic or Indian Ocean regions. Figure 29

F ~ G . 3 0 . - - 3-dimensional plot of the East hemi beam.

Reprdsentation tridimensionnelle du faisceau pour hdmisph&e Est.

FIG. 29. - - Gain~factor of meritS(at repeater interface) referred to the specified coverages.

Facteur de mdrite en gain~(& l'interface rdpdteur) par rapport aux contours sp~cifids.

provides the gain factor for the same INTELSAT VI beams taking as reference the specified coverage areas on the ground ; it illustrates the influence of zig-zag coverage requirements on the antenna gain and also illustrates that the actual antenna beam cannot closely follow the contour of the coverage requirement for narrow zone beams like the zone 1 beam in the Atlantic Ocean Region.

Figure 30 shows a 3-dimension representation of the East hemispheric beam ; details of the design optimization procedures for beam shaping are given in [32].

VI. H I G H L I G H T S OF INTELSAT R & D ON SATELLITE ANTENNA TECHNOLOGY

Because INTELSAT has to provide worldwide service, a large fraction of INTELSAT antenna R & O is focussed on reconfigurable multi-beam antenna technology. Most of INTELSAT R • D work is contracted out to industry.

The effort first concentrated on establishing the fundamental trade-offs involved in multi-beam antenna

design. Also, the possibility to operate with relatively large bandwidths (using a compensation polarizer and dielectric-loaded horns) has been demonstrated. The results are described in a comprehensive paper by P. Foldes [33]. It then bzcame necessary to develop variable power dividers and variable phase shifters with relatively low loss, low mass, compact size, low D.C. power requirement, and stable performance both in the temp:rature range and in the frequency bandwidth. Inszrtion losses of 0.25 dB for a Faraday rotator VPD and of 0.6 dB for a dual toroid ferrite phase shifter have been measured [34]. Work now in progress aims at providing a larger degree of reconfigurability than INTELSAT-VI without incurring an excessive penalty in mass and complexity [35].

When large reflectors are utilized, either to allow more frequency reuse, or to provide more antenna gain in high density traffic areas, or in the long term for future systems with scanning/hopping beams, the scanning loss becomes prohibitive with single reflector antennas. Therefore, a large amount of INTELSAT R & D work has been performed on dual- reflector technology. The optimal feed location for offset cassegrain antennas has been established [36] ; offset bi-focal reflectors have been investigated [37]. A major improvement has been obtained with the front-fed offset Cassegrain configuration, which provides excellent scanning performance with a compact configuration [38 a ; 38 b]. Independently from INTEL- SAT. C. Dragone of Bell Labs came to the same design [39] (Fig. 31). Both studies, as well as the study of an offset-fed design performed by FACC in view of the ACTS satellite [40], concur to demonstrate that excellent scanning performance can be achieved with dual-reflector antennas, but that a very large subreflector is a necessary condition, at least in the plane of scan. When this dual-reflector geometry is used in conjunction with a multi-element source, the grating lobe of tile array could create performance

13/17 ANN. T~LI~COMMUN., 40, n ~ 7-8, 1985

374 P. NEYRET, - ANTENNA TECHNOLOGY AT INTELSAT

Fio . 31. - - Front - fed offset Cassegra in an t enna configurat ion.

Configuration d'une antenne Cassegrain ~t source d~calde illumination frontale.

degradations if it intercepts the main reflector, or another area of the large subreflector, and this problem must be carefully addressed by the designer. Other non-INTELSAT work of relevance includes a paper by Vu giving in a simple manner the geometry conditions for low linear cross-polarization with offset Cassegrain and Gregorian antennas [41], and a paper by Clarricoats on the effect of depolarization by proximity couplings for a dual-reflector antenna with a multi-element source [42].

INTELSAT R • D o n multi-beam antennas also includes long-term efforts. Studies of microstrip antenna technology in dual circular polarization have yielded encouraging results for one element by itself, but degradation of the polarization performance for a multi-element array are severe, because proximity coupling effects are not yet under control. Research work on active antenna aims at providing a better understanding of the problems associated with the final broadband amplification of a composite signal including transmission techniques as diverse as SS-TDMA, FDMA, SCPC and several TV signals. Advan- ced research on reconfigurable direct radiating arrays has also been initiated ; Figure 32 shows the computed power repartition across the aperture of a direct radiating array antenna that would radiate the same zone beams as INTELSAT-VI [43].

Work in other areas of spacecraft antenna technology includes the successful development of a tracking antenna for microwave inter-satellite links [44], as well as the development of a shaped global beam antenna using an array of corrugated horns [45]. Significant research and development effort is also under way in areas as different as unfurlable offset reflector antennas with good polarization and sidelobe characteristics, the prediction of antenna pattern degradation by scattering on the spacecraft structure, and near field testing. For near-field testing of a multi- beam antenna, the grating lobe of the multi-element source can sometimes intercept the analyzer probe and behave as an interfering signal; this phenomenon

Fie. 32. -- Power distribution in the aperture of a reconfigurable direct radiating array antenna with the same zone beam coverages as INr[LSAr VI (Atlantic, Indian and Pacific ocean regions).

Rdpartition de la puissance clans l'ouverture d'une antenne rdseau reconfigurable produisant les m~mes faisceaux de zone

qU'INTELSAT V1 (ocdans Atlantique, lndien, Pacifique).

must be taken into account in order to reconstruct exact far-field patterns.

Though this is out of the scope of this paper, let us mention that INXELSAT also devotes a substantial research and development effort to propagation studies. Propagation phenomena are particularly important for INXELSAX satellites because many countries access INTELSAT spacecraft with low elevation angles.

VII. EARTH STATION ANTENNA TECHNOLOGY

Important progress has also been accomplished in the field of Earth station antenna technology since the first transmission with Early Bird (1965). Cassegrain optics are now almost universally utilized, and reflector shaping increases the gain and improves the sidelobes by means of a near uniform aperture distribution with reduced subreflector blockage, as shown on Figure 33 [46]. As a result, peak gain factors of merit (e f f ic iency) of 70 ~ to 75 ~ at both transmit and receive frequency bands are achievable. Beam waveguides have been introduced in 1971 and, together with the carrousel mount, they provide a reduction of the mass of the antenna which must move to track the satellite ; that makes the antenna structure less expensive to build, and that also allow a much more convenient access for the Rr equipments. Figure 34 shows a typical INTELSAT standard A antenna. Dual-polarization feeds with corrugated or multi- mode horns have been developed in time for operation with INTELSAT-V [47], and compensation of depolarization effects caused by heavy rainfalls can be achieved by means of rotating quarter-wave and half-wave polarizcrs. The monopulse technique provides excellent

ANN. TI~L~COMMUN., 40, n ~ 7-8, 1985 14/17


F~G. 33.--Reflector aperture illumination for a shaped Cassegrain antenna.

Illumination du rdftecteur pour une antenne Cassegrain ~ r~ftec- teurs con form,s.

FxG. 35. - - Typical INTELSAT standard B antenna (Wonkifong).

Antenne INTELSAT typique de classe B.

FIG. 34. - - Typical INTELSAT standard A antenna (Pleumeur-Bodou 4).

Antenne INTELSAT typique de classe A

tracking accuracy. Smaller, Standard B Earth stations (Fig. 35) have been introduced for users with lower volume of traffic, and K-band Standard C have been introduced for very large users.

The most recent progress for large Earth stations include dual-band operation combining C-band (6/ 4 GHz) and K-band (14/11 GHz) on the same antenna (an antenna of this type has recently been installed at Roaring Creek, USA), and wideband (800 MHz) Standard A antennas (two antennas of this type are under construction for KDD, Japan). Both techniques have been developed under INTELSAT R • D contracts. Also, efforts have been made by the industry in order to improve the wide-angle sidelobe performance [481, [49].

Smaller earth stations have been introduced for domestic communications using leased INTELSAT trans- ponders, and a wide use of small Earth stations with

antenna diameters in the range of 5 to 13 meters at C-band, and 3.5 to 8 meters at K-band is expected in connection with INTELSAT Business Services, VISTA and other new services. Very small antennas (less than 1 meter diameter) will be utilized for the INTELNET service, for which the implementation of spread- spectrum techniques provides protection against interference from other satellites of from terrestrial communication networks. Smaller earth station antennas have a lower peak gain factor of merit (efficiency), usually no more than 65~ . Technical progress in the field of small Earth station antennas really started with the introduction of the Torus antenna allowing simultaneous communications with several satellites at different orbital locations [50], and recent advances are mostly the results of industry efforts spurred by the development of domestic communications. INTEL- SAT R & O in this field aims at a reduction of the cost of small Earth stations for dual-circular operation.

This paper would not be complete if the test methods of Earth stations were not mentioned. INTELSAT of Earth stations were not mentioned. INTELSAT researc research and development developed all the antenna measurement techniques using a satellite as source of signal [10]. Present efforts are directed towards the utilization of the moon (for G]T measurements) and of the Sun (for wide-angle sidelobe envelope) as signal source for small Earth stations.

A C K N O W L E D G E M E N T S .

The author o f this paper wishes to express his great respect for all the engineers who made possible the technical advances described herein, particularly the

15/17 ANN. T~LI~COMMUN., 40, n ~ 7-8, 1985

376

designers o f the INTELSAT V and INTELSAT VI multi- beam antennas.

The author also wishes to thank Dr. W. J. English,

J . P . Berges and E. M e y e r f r o m INTELSAT f o r their

P. NEYRET. - ANTENNA TECHNOLOGY AT INTELSAT

help in the preparation o f this paper, as well as COMSAT, FACC, HAC and TELSPACE f o r providing artwork.

Manuscri t re fu le 20 ddcembre 1984,

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17/17 ANN. T~L~COMMUN., 40, n ~ 7-8, 1985

Antenna Technology at Intelsat

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