Low-cost flat-plate array with squinted beam forDBS reception

A. Henderson, BSc, PhD, CEng, MIEEProf. J.R. James, BSc, PhD, DSc, FEng, FIMA, FIERE, FIEE

Indexing terms: Antennas (microstrip), Satellite links and space communications

Abstract: There is a great deal of interest in thereplacement of reflector antennas by low-costwall-mounted flat-plate antennas for the receptionof satellite signals. The paper describes the extentto which combline microstrip technology can beused to obtain a squinted, circularly polarisedbeam at very low cost. The antenna presentedconsists of a simple travelling wave printed struc-ture comprising a foam/metal sandwich withmetal strip polariser. Performance parameterssuch as bandwidth, dissipative loss and patternquality are examined and the effect of manufac-turing tolerances are discussed. Finally, compari-sons are made with reflector antennas and otherflat-plate designs, with particular reference to theDBS specification; the possibility of introducingbeam control capabilities into the array is con-sidered. It is concluded that the development ofsuitable low-cost substrates is a key issue and elec-tronic beam control raises problems of a morefundamental nature.

1 Introduction

Attention is now being concentrated internationally onthe development of flat-plate antenna arrays which arecapable of receiving satellite transmissions and can bedeployed on a variety of buildings. Typical applicationsinclude data transmission and direct broadcast satelliteTV(DBS). These installations will necessarily be of lowcost, and the market potential is considerable. In general,the technical specifications can be met by reflectorantennas [1] provided that they can be positioned on asuitable wall, but other factors, such as environmentalobjections and performance degradation due to winddamage and weather effects [2], have led to intensifiedresearch into thin planar arrays.

In the long term, it is important that these planararrays be designed with some degree of electronic beamcontrol to accommodate the variety of geographical walllocations and also be capable of addressing a selection ofsatellites. The development of such a sophisticated, yetlow-cost, array would appear to be some years away and,owing to its complexity, is only likely to be achieved inseveral stages. In the immediate future it is realistic toaim for a simpler array with a fixed beam squint in ele-

Paper 5676H (Ell), first received 13th October 1986 and in revisedform 4th June 1987

The authors are with the Wolfson RF Engineering Centre, School ofElectrical Engineering & Science, Royal Military College of Science,Shrivenham, Swindon SN6 8 LA, United Kingdom

IEE PROCEEDINGS, Vol. 134, Pt. H, No. 6, DECEMBER 1987

vation, and some beam control in azimuth. Here wedemonstrate the extent to which microstrip technologycan meet some of these important requirements, such aslow cost, a squinted beam and circular polarisation, priorto assessing its potential for further research into beamcontrol aspects. The demand for a domestic array forDBS reception is an immediate technological challengeand the objective of the present research is to assess thefeasibility of meeting this particular specification.

The paper commences in Section 2 with a summary ofthe DBS antenna requirement, as applied to the flat-platearray, and then Section 3 gives a detailed description ofthe construction and design of the microstrip array, alongwith appropriate measured results. Section 4 criticallyexamines the overall performance of the microstripantenna and compares this to other flat-plate designoptions. Finally, conclusions are stated in Section 5about the suitability of the existing microstrip array forthe DBS market, and areas for future research are identi-fied.

2 DBS flat-plate requirements

The antenna requirements for DBS reception were laiddown in 1977 by the WARC Conference [3] but, sincethat time, further improvements in receiver noise figureshave allowed some relaxation in the antenna gain per-formance [4] for the UK allocated band of 11.7—12.1 GHz. A detailed breakdown of the down-link powerbudget will be given in Section 4.2 indicating the relation-ships between receiver noise figures and antenna gainrequirement. The sidelobe envelope and crosspolarisationlevels were also specified to minimise interference fromother satellites, and are given in Fig. 12 for both the orig-inal WARC specification and a relaxed version whichapplies only to the case of UK reception from a UKsatellite. Overall the antenna electrical specification isvery stringent for a low budget microstrip array. Theprincipal physical requirements for the array are that it islight to handle and of reasonable proportions for fixingto a wall. An in-built beam squint of about 24° allowsvertical fixing with some slight adjustment for differentgeographical locations and azimuth optimisation.

By considering the design options available for planarstructures, it is evident that many of the desired featuresof the DBS array are incompatible with known technol-ogies. For instance, excellent planar arrays have beenbuilt with a boresight beam using high quality overlaidsubstrate [5] and suspended microstrip [6], all based onparallel fed elements. To convert these to a squintedbeam design would require a much more complicatedfeeder topology. Furthermore there will be little prospectof devising a scanned system without incorporating

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separate phasing for each element in the array. Thedesign presented here, and elsewhere [7, 8], circumventsthese difficulties by using a travelling-wave design on acheap foam substrate but the inherent frequency scanningloss at the band edges is an obvious penalty.

3 Constructional details and design

3.1 ConstructionMost of the previously published flat-plate arrays [5,9-11] have been manufactured from expensive micro-wave plastic substrates, and to ensure that any array cancompete in costs with mass-produced dishes it is essentialthat cheaper materials and simple manufacturing pro-cesses are chosen. The microstrip antenna presented hereis manufactured from a sandwich structure of foammaterials (see Fig. 1) and has a clear cost advantage overpresent plastic materials of about 10 :1 in substrate costs.

Fig. 1 Laminar construction of foam microstrip array

1 Metal groundplane 2 Foam substrate 3 Combline antenna elements 4 Foamspacer 5 Polarising grid 6 Weather-proof cover

The microstrip antenna is constructed from a thinlayer of high quality foam substrate (er = 1.06) and theradiating copper elements are etched from a layer of verythin copper-coated plastic film, which is then adhered tothe foam sheet. The full-size DBS array is built up fromfour subarray units fed individually by coaxial cables,each subarray unit consisting of a fully integratedpackage using 16 comblines with a foam triplate feed.Because the combs are linearly polarised, a polarisingsheet is required to match into the circularly polarisedtransmissions, which can easily be rotated to change thehand of polarisation. For the preliminary experiments, asimple metal plate-type polariser [12] was used, althoughit is feasible to design a flat printed sheet for this purpose,which would be more suitable for mass production.

Finally the antenna can be protected from rainfall by athin plastic box chosen to match the colour of the housewalls.

3.2 Antenna design procedureThe subarray unit was optimised by a combination ofCAD and practical testing procedures. To facilitate thisprocess, the antenna was built up in several design stagesfrom individually optimised subcomponents, as follows:

First, a single combline was designed [16] formaximum overall gain and pattern quality for the 11.7—12.1 GHz band. Secondly, four of these combs were com-bined into a subgroup and fed by an integrated powersplitter. This subgroup corresponds to the dashed box inFig. 2. Testing on this subgroup enabled the foam power

-r-r-r-r

LhH

Fig. 2 Photolithographic mask for subarray unit showing first 33 fin-

gers

The overall dimensions of the unit are 60 x 46 cm. The dashed box is a subgroup

splitter and the unwanted radiation at the microstrip/triplate discontinuity to be examined in detail, along withany mutual coupling effects between the combline ele-mental arrays. The subarray unit was then printed offfrom four subgroup sections with a 16 :1 power splitteras shown in Fig. 3, and a metal plate polariser wasdesigned for this unit, taking into account the squintedmain beam angle and the proximity to the subarray.Finally measurements were made for four subarrays [17]connected by coaxial cables in a 4 x 1 configuration suit-able for the DBS application.

3.2.1 Combline array: The initial layout of the singlecombline array is generated such that the overall antennagain is maximised over the 11.7-12.1 GHz band, takinginto account the scanning losses and line dissipation as afunction of antenna length and line impedance. Compu-tations indicate that a minimum of 40 fingers using a

510 IEE PROCEEDINGS, Vol. 134, Pt. H, No. 6, DECEMBER 1987

4 mm combline width realises the best overall gain figurefor 1.59 mm foam substrate (see Fig. 4). The final designdid, however, require a certain amount of cut-and-tryexperimentation to achieve an acceptable sidelobe level.

Fig. 3 Subarray unit with 16 comblines and integrated feed

24r 11.9

£22

20-

0 20 40 60number of f ingers

Fig. 4 Computed overall gain of combline array with 4 mm feed widthand a line attenuation of 0.1 dB/guide wavelength, for 11.7, 11.9 and12.1 GHz

Frequency scanning losses are included

Detailed measurements of the combline are given inFig. 5 for 11.9 GHz, indicating (-13, -12) dB sidelobelevels with a fairly high level of cross polarisation of— 17.5 dB, principally due to the wide terminal finger ele-ments associated with the low permittivity substrate.

3.2.2 Four-comb group and triplate feed: The triplatefeed was the most difficult subcomponent to optimise,requiring careful shaping of the power splitter tracks tominimise both unwanted mode generation within the feedand the dissipative loss. The final track layouts of thetransmission through each track of the feed were made,

1EE PROCEEDINGS, Vol. 134, Pt. H, No. 6, DECEMBER 1987

and the power transmission characteristics are given inFig. 6 for two of the outputs. Small differences in thepower delivery to each track are due to manufacturing

Or

-10

-20

-30-90-80° 60° 40° 20° 20" 40° 60° 80° 90°

Fig. 5 H-plane radiation pattern of40-finger combline

crosspolar

0

co- - 6

reference

ports

11.5 11.7 11.9 12.1

GHz

12.3

Fig. 6 Transmission characteristics of 4 :1 triplate splitter throughtwo ports

tolerances in the feed and mutual coupling between thefour tracks. An average insertion loss of 0.4 dB is indi-cated from these figures although this was worsened by1.0 dB when the combline elemental arrays were attachedto form a four comb group. This was due to the powermismatch at the triplate/air boundary and was reducedby tapering the copper track in the vicinity of the junc-tion and turning up the top plate of the feed to form asmall horn-line structure. The input characteristics of thefour-comb group are given in Fig. 7 showing a good

Or

- 1 0

-20

-30

£ -40

117 11.9 12.1

GHz

Fig. 7 Input characteristics of4-comb group

power match over the band. The £-plane radiation pat-terns for the four-comb group are given in Fig. 8 and thesymmetry in the main beam confirms an adequate powerdistribution within the feed, //-plane co-polar patternsare similar to those of a single combline. Gain measure-ments indicate an insertion loss of 1.0 dB for the triplate

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feed, and a full breakdown of the gain budget is given inSection 4.

3.2.3 The subarray unit: The measured £-plane radi-ation pattern [15] given in Fig. 9 showing ( — 11,

For a normally incident circularly polarised wave, thedesign of the plate polariser requires that the E^ com-ponent, see Fig. 10, is transmitted through the metal(waveguide) plates with a phase shift of 90° relative to the£| component. The dimensions of the plate are chosen

0;

11.7GHz

-60° -40° -20° 0° 20° 40° 60

Fig. 8 E-plcme patterns of 4-comb group

Or

-10

-20

-30-90° -60° -30° 30* 60° 90°

Fig. 9 E-plane of subarray at 11.9 GHz

copolar, crosspolar

-15) dB sidelobe levels and a - 2 7 dB crosspolarisationlevel. The overall gain was 30 dB at 11.9 GHz, with 2 dBfrequency scanning losses at the band edges, indicating a3 dB insertion loss in the 16:1 triplate feed system.

3.2.4 The polarising grid: The metal plate polariser,Fig. 10, was chosen not only for its simplicity of con-

Fig. 10 Metal-plate polariser

struction and design but also for its acceptable band-width performance at main beam. Wire-grid sandwichstructures as described by Warren [13] are preferablestructurally for manufacturing reasons, but tests on asingle layer type indicated a very narrow band response,unsuitable for the DBS application. A wider responsecould be obtained by using multilayer designs, typicallyinvolving meander lines [14], but there may be somepenalty to pay in terms of increased insertion loss, andthis would have to be determined by further research.

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such that b = 0.607 Xo and a = 0.655 Ao (a is the spacingbetween the plates and b the width of the plates) whenthe near-field effects at the plate edges are taken intoaccount. To retain these phase conditions, the entiremetal plate assembly should be tilted by an angle of 24°for the squinted beam antenna. Instead, the plate struc-ture was laid in a plane conformal to the antenna ele-ments, but each plate was tilted by 17.5° to take this intoaccount, and the dimensions of the plates were modifiedslightly by cut-and-try methods to ensure the correctphasing conditions. The correction of 17.5° was derivedfrom elementary geometrical relationships. Even so, theconformal construction does compromise the basic pol-ariser design, by introducing progressive phase errorstowards the edges of the aperture. Additionally, the pol-ariser is located within the near-field region of theantenna itself, and measurements were necessary to deter-mine the optimum separation distance for the best axialratio and minimum insertion loss.

Fig. 11 gives the measured E- and H-plane patternsfor the subarray/polariser unit at 11.9 GHz. An axialratio of 0.5-0.8 dB was obtained over the band in themain beam although this worsened considerably in thesidelobe regions. An insertion loss of 0.5-0.7 dB wasobtained for an optimum separation distance of 1 cmbetween the antenna and the polariser. It was concludedthat a simple conformal polariser could be constructedfor operation in close proximity to the antenna, providedthat crosspolarisation levels are not too stringent atwider angles. This will be discussed further in Section 4with reference to the DBS specification.

3.2.5 The large DBS array: Four subarray units wereconstructed, and their individual gain performances com-pared in Table 1, which gives a clear indication of the

Table 1: Gain of four identical subarray units showing tol-erance effect

Subarray 1 2 3 4

Gain (dB) 29.5 30.0 28.5 29.5 average = 29.4 dB

manufacturing tolerances. To minimise scanning lossover the frequency band, it was decided to arrange thefour subarrays in a 4 x 1 matrix rather than a 2 x 2matrix thus giving a narrower beam in azimuth than ele-vation and reducing any interference effects between

IEE PROCEEDINGS, Vol. 134, Pt. H, No. 6, DECEMBER 1987

satellite signals. Measurements of the gain and radiationpatterns of the large array were made without the polari-sers, and were subject to limitations on accuracy due to

Table 2: Gain budget for large array at 11.9 G Hz

30° 20° 10° 0° 10° 20° 30°

50" 40" 30" 20° 10

Fig. 11 Circularly-polarised patterns at 11.9 GHz for subarray fittedwith metal-plate polariser

ground reflections and beam alignment errors; the resultsfor the £-plane are given in Fig. 12. A gain figure of33 dB was obtained at 11.9 GHz with 2 dB scanninglosses at the band edges.

i—O—O-O—O—O-O-O—O—i

-10

-20

-90° -60° -30° 30° 60° 90°

Fig. 12 E-plane pattern for large DBS array, showing DBS antennaspecifications for reception

WARC specification for copolar, - x - x - x UK specification for cross-polar, - O - O - O UK specification for copolar.

4 Overall antenna performance

4.1 Gain budget and efficiencyTable 2 gives a budget of the gain performance of thelarge array and its subcomponents at 11.9 GHz, withoutpolarisers. A 2 dB gain reduction is experienced at theband edges owing to the travelling wave design.

The 7.1 dB loss of gain is excessive and can be attrib-uted to several sources (see Table 3). The overall antennaefficiency of about 20% is very poor when compared to60-70% for a small parabolic dish. An examination ofthe gain penalties involved indicate that the use of a

Singlecombline

Four-combsubgroupwith 4 :1feed

Subarray

Large array

Measuredgain, dB

21.0

26.0

30.0

33.0

Predictedgain, dB

a 22.1

6 28.1( = a + 6dB)

C34.1( = 6 + 6dB)

d40.1( = c + 6dB)

Shortfallin gain,dB

1.1

2.1

4.1

7.1

Cause

Nonuniformcurrentdistribution

1 dB due tofeed losses*

3 dB due tocorporate feed3 dB incables andmanufacturingtolerances

• The feed losses in the 4 :1 splitter can be broken down into0.5 dB at the triplate/microstrip junction, 0.4 dB dissipative loss inthe tracks and 0.1 dB power mismatch.

Table 3: Sources of gain reduction for large array

(a) Cable loss and foam combiner = 1.5 dB min I 3 dB in(b) Variation in gain due to manufacturing = 1.5 dB max J total(c) Triplate feed: (i) dissipation =1.5dB

(ii) mismatch = 1.5 dB(d) Errors in combline design, either =1.1 dB

adhesive losses or inaccuracies inradiation mechanisms

Total = 7.1 dB

cheap foam sandwich structure has contributed about2 dB by way of tolerance errors and power mismatchesin feeds and at junctions. The polarising grids contributean additional 0.5-0.7 dB loss.

4.2 Noise figures and pattern qualityAs shown in Section 4.1, the microstrip DBS antenna hashigh dissipative losses and the possibility of additionalsystem penalties due to increased thermal noise is nowexamined. Rainger et al. [15] have presented a com-prehensive downlink power budget for DBS reception inthe UK from which the systems performance of themicrostrip array can be projected. The figures are sum-marised in Table 4 for the large array and two sizes of

Table 4: Comparison of antenna types and projectedcarrier-to-noise ratios (C/IM) at 11.9 GHz: NF = noisefigure

0.9 metredish

0.5-4.56

38.0

14.017.520.7

0.6 metredish

0.5-8.07

35.5

10.514.017.2

Large microstriparray

5.2-4.50

32.3

9.012.214.9

Antenna dissipation, dBAperture capture area,dB m2

Antenna gain, dBTotal C/N, dB:8 dB NF receiver5 dB NF receiver2.5 dB NF receiver

These figures assume [15](a) 1.4 dB rain loss(b) 0.3 dB galactic noise(c) 160 W transmitted power(d) antenna is modelled as a simple attenuator situated beforereceiver.For C-MAC system, picture qualities are 'fair' for C/N > 9 andgood' for C/N > 12

reflector dish. The high attenuation of the printed array isonly a significant noise source when a low-noise receiveris used.

The microstrip array at midband operation is capable

1EE PROCEEDINGS, Vol. 134, Pt. H, No. 6, DECEMBER 1987 513

of satisfying the carrier to noise (C/N) criterion provideda low-noise receiver is used. When frequency scanninglosses are taken into account at the band edges, the gradeof performance is not acceptable for a UK domesticsystem, and some amplification would be necessary toboost the C/N ratio by introducing FET devices inadvance of the feed structure.

For reception in the UK, the antenna co-polarpattern, Fig. 12, meets the UK pattern specification inthe £-plane but the crosspolarisation performance,derived from axial ratio measurements, is poor in the H-plane, Fig. 11, particularly at wide angles where groundreflections could introduce interference from other satel-lites. This isolation problem is chiefly a consequence ofusing cheap foam materials with a squinted beam designand it may be necessary to apply further effort to theredesign of the polarising sheet and combline elementsfor an improved wider angle response. Alternatively oneshould consider other forms of flat-plate array withcleaner patterns, but, as described below, their full capa-bilities when optimised are still unclear.

4.3 Alternative flat-plate designsTo date, information on the axial ratios of various flat-plate arrays is sparse, particularly for those using polar-ising grids. The suspended-substrate array of Rammos etal. [6] has an acceptable gain figure of 35.2 dBi over theUK DBS band and a — 40 dB crosspolarisation level isquoted for the linearly polarised version. This array, withfurther redesign, should be capable of using circularlypolarised (CP) elements directly, but figures are not gen-erally available for this configuration as yet. As men-tioned previously, this array, like all parallel fed arrays,has the disadvantage of being more difficult to adapt forbeam steering operations than a microstrip travelling-wave type. Nevertheless, provided the necessary toler-ances and CP characteristics can be controlled, theRammos array is a good proposition for the immediatefixed beam DBS market, although generally it will not beconformal to the wall of the building.

Recently, a series of fixed beam arrays have beenpublished using improved microstrip CP elements config-ured either in pairs [10, 11] or with parasitic overlays[9]. These have a wider CP bandwidth performance andlower axial ratios at wide angle, and as such deservefurther examination for the UK DBS requirements. Thearrays reported to date have lower gains than thatdemanded by the UK DBS specification and have beenconstructed on expensive substrates; some degradationseems likely should these techniques be applied to highergain squinted arrays on cheap foam substrates.

5 ConclusionsThis investigation has established that the domestic DBSrequirement for a fixed squinted beam conformal array isindeed a challenging specification when severe substratecost constraints are applied. The limitations in per-formance observed in the foam microstrip array such aslower efficiency, poorer pattern quality and scanning lossare a consequence of the array topology, the squintedbeam and the low permittivity substrate. The poorpattern quality for this squinted beam design remains acritical issue requiring much more research into the wideangle characteristics of the combline and the polariser forthis particular antenna. The tolerance effects associatedwith foam substrates subjected to extremes of tem-perature are also important, and the final choice ofantenna configuration for a commercial product willdepend largely on this factor.

For these reasons, it seems likely that the first gener-ation of flat-plate arrays will have a conventional bore-sight design with antenna elements printed onto plasticsubstrate(s) rather than onto foam and possibly employ-ing low loss feeding structures, such as waveguide, tomeet gain and pattern quality specifications. The devel-opment of a cheap plastic material with temperature sta-bility is imperative enabling a greater design flexibilityand control in manufacture. One manufactured flat platearray has already been publicised [18] and it seems onlya matter of time before these devices appear on themarket.

Some provision for electronic beam scanning by incor-porating active devices into the feed structures would bea major breakthrough, enabling the array to be installedfully conformal to the wall. The difficulties encounteredin this investigation with controlling pattern quality forsquinted beams raise some doubts as to whether thisobjective can be achieved without considerable effort andit will remain a challenge for the next few years.

6 AcknowledgmentsWe would like to thank P. Haskins of RMCS for thedetailed measurements undertaken during this project,and acknowledge the financial support of the BritishBroadcasting Corporation during the initial phase of ourwork.

7 References1 CLARRICOATS, P.J.B.: 'Reflector antennas for satellite systems —

a review'. 14th European Microwave Conference, Liege, Belgium,10-13 September 1984, pp. 9-20

2 ITOH, K., OGAWA, Y., OHMIYA, M., and SASAKI, M.: 'Adapt-ability experiments of satellite broadcasting antenna systems insnowy district'. Proc. ISAP, Kyoto, Japan, 1985, pp. 201-204

3 International Telecommunications Union Radio Regulations,Appendix 30, Geneva, Switzerland, 1979

4 MILLARD, G. H., and GANDY, C : 'Antennas for direct receptionof broadcasts from satellites', Proc. ICAP, Norwich, England, 1983,pp. 365-368

5 DUBOST, G., and VINATIER, C : 'Large bandwidth and high gainarray of flat folded dipoles acting at 12 GHz', Proc. ICAP, Norwich,1983, pp. 145-149

6 RAMMOS, E.: 'Suspended substrate line antenna fits 12 GHz satel-lite application', Microwave Syst. News, 1984,14, pp. 111-125

7 HENDERSON, A., JAMES, J.R., HALL, P.S., STOTT, J.H., andBOARDMAN, D.H.: 'Investigation of a cost-constrained 12 GHz"flat-plate" antenna for DBS', Proc. ICAP, Warwick, England, 1985,pp. 108-112

8 HENDERSON, A., and JAMES, J.R.: 'Improved microstrip flat-plate array for domestic reception', IEEE AP-S International Sym-posium Digest, Philadelphia, June 8-13, 1986, pp. 565-568

9 HORI, T , and NAKAJIMA, N.: 'Broadcast circularly polarisedmicrostrip array with co-planar feed', Trans. Inst. Electron. &Commun. Eng. Jpn. Part B, 1985, 68, pp. 515-522 (in Japanese)

10 HANEISHI, M., and TAKAZAWA, H. 'Broadband CP planararray composed of a pair of dielectric resonator antennas', Electron.Lett., 1985, 21, (10), pp. 437-439

11 HANEISHI, M., YOSHIDA, S., and GOTO, N.: 'A broadbandmicrostrip array composed of circularly polarised microstripantenna', IEEE AP-S, International Symposium Digest, May,160-163

12 LAIT, A.J.: 'Broadband circular polarisers', Marconi Rev., 1969, 32,pp. 159-184

13 WARREN, K.A.J.: 'A planar antenna circular polarisation con-verted utilising printed circuit technology', Marconi Rev., 1980, 43,pp. 176-184

14 YOUNG, L., ROBINSON, L.A, and HACKING, C.A.: 'Meander-line polariser', IEEE Trans, 1973, AP-21, pp. 376-378

15 RAINGER, P. et al: 'Satellite broadcasting' (John Wiley and Sons,Chichester, 1985)

16 JAMES, J. R., HALL, P.S., and WOOD, C : 'Microstrip antennatheory and design' (Peter Peregrinus Ltd, Stevenage, UK, 1981)chap.5

17 FORREST, J.R.: 'Assessing antennas for small SATCOM termin-als', Microwave Syst. News, 1981, 11, pp. 77-100.

18 'Flat is now beautiful', Electron, and Power, 1987, 134, (2), p. 227

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