Polarization RHCPRadiation Hemisferical and isofluxPattern compatibleXPD > 8dB @2-2.3GHz , lei < 65° , V rjJ
Thus, a high coverage is achieved with 3dB above a typicalhemispherical antenna, but which does not require a beamforming network like the classical quadrifilar Kilgus [I] typeantenna. This reduces the number of required parts, and due toits loading, a smaller dimension and higher structural stiffnessis achieved .
II. R EQUIREMENTS
The SAC-D/AQUARIUS orbit is a heliosynchronous LEOorbit at an altitude of 657km. Under these conditions the linkdistance varies from 657km when the ground station is in nadir(0° from the antenna) to 2800km when the ground station risesabove the horizon at an angle of approximately 65°. The freespace loss is about IldB higher at the horizon than at the nadir,and therefore the critical link point is at 65° from the antenna ,where the maximum gain is required.
Abstract- In this paper we describe the design, manufacturingand measurement of an antenna with an isoflux type radiationpattern used in the space segment TT&C subsystem. Inparticular, the antenna corresponds to the flight model of missionSAC-D/AQUARIUS, which is a LEO earth observation satellite.The chosen solution is a loaded bifilar helical antenna, whichpresents a higher gain than a typical hemispherical antenna butwhich doesn 't require the use of a beam forming network like theclassical quadrifilar Kilgus type antenna. This reduces thenumber of required parts, and due to its loading, smallerdimension and higher structural stiffness are achieved.
All space missions have a bidirectional radio link usuallycalled IT&C (Telemetry Tracking and Command). Its threemain functions are to provide telemetry data downlink, providethe ground segment with the signal used to track the satelliteand provide an uplink for sending commands to triggermaneuvers or other predefined programs for the mission.
The TT&C link lies in the S-Band (2-2.3GHz frequencyband) in a large number of satellites. The space segment isrequired to provide the link with an omnidirectional coverage.This means that for any given satellite attitude there is enoughlink margin to transmit and receive data with a given error rate.This requirement forces the space segment antennas to have anideally isotropic radiation pattern. This condition canapproximatel y be achieved by an array of two hemisphericalantennas, which in the case of earth observation satellites isimplemented with one antenna pointing to nadir and the otherone pointing to zenith (taking into account the satellite isstabilized and in nominal attitude).
As a requirement contrary to omnidirectionality, the linkefficiency needs to be maximized when the satellite is innominal attitude and the downlink data rate is higher. This isaccomplished by illuminating I) the earth only and 2) with thesame power density for every direction (isoflux pattern).
A compromise solution between these two extremes waschosen using an antenna with a hemispheric radiation patternbut compatible with an isoflux radiation pattern, placed in atwo-opposing antenna configuration (configured on oppositesides of the satellite).
Simulation was carried out using the MoM [4] method for asimplified model (Fig. I) during the optimization stage. Thesimplification is done by segmenting the helices into squaresection chunks (instead of round). The same radiation patternand impedance results are obtained but the simulation time isreduced from 180 to 20 minutes on a 20Hz processor.
The optimization yields the following geometry for theradiating element: D=25mm, S=56mm y N=3. These valueswere verified by simulating the detailed model shown in Fig. 2
B. Optimization
The optimization variables used to maxnmze the goalfunctions (1)-(2)-(3)-(4) are: D (Helix diameter) , S (Steps pertum), N (Number of turns) as shown in Fig. I .
Figure I. Simulation model, simplified for optimization
O PTIMI ZATION GO AL FUNCTIONS, GOALS AN D RANGE
F UNCTION GOAL RANGE
I: GRHCP >2dBi 2 - 2.3GHz,0 = ±65°, \j t/J
2: XPD > IOdB 2 - 2.3GHz,IOI < 65°,\j t/J
3: G(65°)-G(OO) >3dB 2 - 2.3GHz,\j t/J
4: Sll C) <-3dB 2-2.3GHz
TABLE II.
IV. OPTIMIZATION OF RADIATION PARAMETERS
The first step in the design process is to achieve ageometrical configuration which meets or exceeds the radiationrequirements (Table I) even under the constraints imposed bymaximum allowed dimensions, realizable design, materials,etc.
D. Materials and Mechanical Constraints
• Dielectric media: PTFE
• White PTFE radome for thermal protection.
• Max. Antenna height: 200mm
• 17mm CZ)< helix diameter < 30mm C)
• Flange fastener: 60mm
• Coaxial fastening mast diameter: Yz inch
• Helix wire diameter: 3mm
A. Goal Functions
The goal functions, goals, and their range used In theoptimization process are listed in (Table II)
III. BASIC DESIGN
The designed antenna is of the 3-turn, dielectric loaded,bifilar helical antenna type.
The basic design was conceived during development stage,working on a multi-tum bifilar helical antenna. A bifilar designwas chosen because of its reduced complexity as compared tothe two-tum quadrifililar Kilgus [I] type antenna. The same"saddle" type radiation pattern was sought. As opposed to theKilgus type antenna , which uses vacuum inside the helices,PTFE dielectric loading was used. In this way, the helices werestructurally supported , and its size could be reduced (for thesame given number of turns). The only reference availableabout loaded bifilar helicoids is [2], but it is a 6.5-turn antennaoperating at X-Band and with another type of dielectric. Thehelices are fed by means of a split balun [3]. In this way thecentral coaxial feed acts as a mast, supporting the radiatingelement. The whole antenna configuration is shown in Fig. 1.
I Radiating element without impedance matching.
Figure 2. Detailed model for simulation and coordinate reference system
This design resulted in a low manufacturing cost. Materialsused include SAE 64 Bronze for the radiating element, 100%PTFE for the Radome and Dielectric Load, 6061-T6Aluminium for the flange, AISI 304 Steel bolts and MIL rated4 holes flange mount SMA female connectors. Themanufactured flight model is shown in Fig. 4.
V. IMPEDANC E MACTHING
The radiating element and the impedance matching areuncoupled in the chosen antenna configuration. This allows foran uncoupled treatment of both problems.
The required bandwidth percentage of 15% of the centralfrequency (300MHz) is relatively high for this type of radiator.On the other hand, the radiating element resonates with abandwidth of about 5%, and is displaced from the 2-2.3GHzband due to the optimization being aimed at the radiationpattern. Thus, a great effort is needed to match the impedance .
This implies that several impedance matching stages arerequired to reach the 300MHz bandwidth. The only degree offreedom left was the inner core of the feeding coaxial.
Impedance matching synthesis techniques like the Binomialmethod, Chebyshev method, etc. are available for n-transitionimpedance transformers. But they are based on the assumptionof the load being real and constant in frequency , which is faraway from the typical antenna impedance.
The solution was found by brute-force searching for thecombination of transitions required to reach the objective,based on a transmission line model which was previouslyestimated by experimental measurement and a transmissionline model of the complex input impedance of the antenna portmeasured over a frequency range. The final result is a 6transition adapter shown in Fig. 3.
VSWRMeasurementANT·IAR·SOI(Ne\Wolk~Agilent E8362 B Celibr ation polt1)
2,05 2,076 2,1 2.15FrecllfllCYGHz
2.25 2.3
Figure 7. VSWR Measurement
C. Measurement versus simulation I[
VIII. A NTENA PLACEMENT PERFORMANCE
There are two main types of distortions affecting themounted antenna performance. The first and most importantone is the array factor, which appears between both antennasand gives rise to nulls arounde = 90°, and reduces the coveragepercentage of the space segment. The second one is caused bythe reflections on the structure due to the back radiation ofeachelement. This distorts some areas of the front side of theradiation pattern on the zenith and nadir sides respectively.Thus, polarization isolation and gain are reduced whencompared to the isolated condition.
•
A......1m
•
Figure 10. Coverage Diagram - Normalized Gain
The solid angle for which the gain is higher than a givenvalue (and there is enough link margin) is also computed. Thecoverage percentage is computed using (1). The Coveragepercentage vs. min. Gain value are shown in Fig. 11
B. Coverage
A simple way of visualizing the coverage is by means ofthe radiation pattern over the whole sphere. This diagramincluding nominal and emergency zones is shown in Fig. 10.
Cove rage (dB)Antenna: SOl , f=2.2GHz, PO 2 sources, configuration: 1
Figure 9. Array placement on SAC-D IAQUARIUS platform
100100" ..o~...
i i
,;, ;,\x:t· 100 -65 -60
Figure 8. Far-Field Measurement versus Simulation
.s
-10 ············f
A. Simulation Method
Due to the large electrical dimension of the problem,traditional electromagnetic simulation methods such as FDTD,FEM and MoM are not capable of dealing with the antennasplaced on the satellite platform (Fig. 9). Thus, the physicaloptics (PO) method implemented in the FEKO [3] softwarewas used in combination with MoM.
The isolated antenna radiation pattern obtained by MoMsimulation was used as a point source for each of the twoantennas placed on the satellite platform for physical opticssimulation of the array and the whole structure.
In telemetry antennas, the omnidirectional coverage can beevaluated by means of Isotropy [7], defined by (2). A typicalvalue for this applications is 1=0.8 . Isotropy is shown as theshaded area under the curve in Fig. 11.
G=1
1= fC(G)dG(;=0
(2)
IX. CONCLUSION
The complete design, manufacturing and measurementprocess of a flight model isoflux radiation pattern is described.
This design reduces the number of required parts whencompared to a classical quadrifilar antenna, shows highermechanical stiffness. It is built of low cost materials and isattractive to scientific missions.
The proposed solution satisfies the typical TT&Csubsystem requirements of LEO satellites. Mounted antennaperformance is also evaluated, providing the link engineer withthe expected flight parameters.
D. Polarization Loss
An important parameter for the link budget is thepolarization loss [8] between space segment and earth segment,
defined by (3), where Pt y Pr are the polarization ratios for
the transmitting and receiving antenna respectively, and If/ is
the tilt angle between the semimajor axes of the polarizationelipses.
A CKNOW LEDGM ENTS
We would like to thank CONAE (Argentine Space Agency)for providing the resources through the National Space Plan toaccomplish this work and for allowing its publication. Also wewould like to thank the Technology Transfer Division ManagerJ. Sanz for supporting new projects. Special regards to theAntenna Group and all the people at Instituto Argentino deRadioastronomia who made this possible.
The polarization losses assum ing AR=ldB (typical value)for the ground station and a tilt angle of If/ =90° (worst case)
are shown in Fig. 12
I' = 1+ Pt2P; + 2PtPr cos(21f/)
(I + Pt2 )(1 + p;) (3)
REFERENCES
[I] C. C. Kilgus , "Shaped-Conical Radiation Pattern Performance of theBackfire Quadrifilar Helix", IEEE Transactions on Antenna andPropag ation , May 1975, pp. 392-397.
[2] Ye Yun Shang , Li Quan Ming ; "A new type of X-band data transmis sionantenna used on CBERS-I satellite", IEEE Antennas Propagation andEM Theory , 2003. Proceedings 2003 6th Internati onal Sympos ium, 28Oct.-I Nov. 2003 Page(s ):I - 6.
[3] S. Silver, "Microwave Antenna Theory and Design", McGraw-Hili ,New York, 1949, p246 .
[4] FEKO Comprehensive EM Solution s http ://www .feko.info
[7] K. Koob, "Isotrop y and antenn a gain," in 1974 International IEEE/AP-SSymposium Program and Digest, Georgia Institut e of Technology,Atlanta, GA., June , 1974, pp. 87-88 .
[8] Microwave Antenna Measurement, 1.S. Hollis , T. 1. Lyon, L. Clayton,Scientific-Atlanta, Inc. Atlant a, Georgi a, USA, July 1970, Chapter 3.
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