Thermal and mechanical distortion degrade the RF performance of antennas. The complexity of future communications antennas requires accurate, dimensionally stable antenna reflectors and structures built from materials other than those currently used. The advantages and disadvantages of using carbon fibers in an epoxy matrix are reviewed as well as current reflector fabrications technology and adjustment. The manufacturing sequence and coefficient of thermal expansion of carbon fiber/borosilicate glass composites is described. The construction of a parabolic reflector from this material and the assembling of both reflector and antenna are described. A 3M-aperture-diameter carbon/glass reflector that can be used as a subassembly for large reflectors is depicted. The deployment sequence for a 10.5M-aperture-diameter antenna, final reflector adjustment, and the deployment sequence for large reflectors are also illustrated

Ard, K. E.

Gilger, L. D.

Sharp, G. R.

English

NASA Technical Reports Server

EXTREME PRECISION ANTENNA REFLECTOR STUDY RESULTS 
G. R. Sharp 
NASA Lewis Research Center 
Cleveland, Ohio 
and 
L. D. Gilger and K. E. Ard 
Harris Corporation 
Melbourne, Florida 
Large Space Antenna Systems Technology - 1984 
December 4-6, 1984 
279 
https://ntrs.nasa.gov/search.jsp?R=19850015519 2020-03-20T18:40:08+00:00Z
INTRODUCTION 
The changing thermal environment at geosynchronous orbit can cause antenna 
structures to warp and distort over the twenty-four-hour orbit period. The 
distortion pattern and amplitude also vary seasonally. The mechanical distor-
tion of the antenna reflector and structures in turn can cause deleterious 
effects in the far-field antenna pattern; the main antenna lobe may broaden. 
its amplitude decrease, its aiming point shift, and the antenna sidelobes in-
crease in amplitude. The resulting degradation in RF performance can be a 
serious problem in large complex systems. 
280 
COMMUNICATIONS ANTENNA DESIGN TRENDS 
Communications antennas are now being designed to operate at higher fre-
quencies since the lower frequency bands are becoming crowded. They are also 
being designed to conserve the frequency spectrum by featuring various fre-
quency reuse concepts. Multiple narrow spot beams are used for trunking and 
for scanning or beam hopping. If these beams are at the same frequency, they 
must be isolated by polarization. 
Thus, future communications antennas will be more complex than those at 
present and will require accurate dimensionally stable antenna reflectors and 
structures built from new materials. 
HIGHER OPERATING FREQUENCIES 
LARGE APERTURE DIAMETERS 
COMPLEX ANTENNAS (MULTIPLE FIXED AND 
SCANNING BEAMS) 
281 
282 
EXTREME PRECISION ANTENNA REFLECTOR AND STRUCTURE DESIGN STUDY OBJECTIVE 
The design study objective is shown in the figure below. 
SELECT AND ANALYZE THE BEST MECHANICALI 
THERMAL DESIGN FOR EXTREME PRECISION SPACE-
BASED ANTENNAS FROM 2M TO 15M-APERTURE 
DIAMETER AND UP TO 90 GHZ 
TASK DESCRIPTION AND STATUS 
In early 1982, the Harris Corporation was awarded a contract for an 
"Antenna Design Study for Extreme Precision Antenna Reflectors and Systems 
Using Advanced Materials and Structural Techniques," NAS 3-23249 (Research in 
progress, G. R. Sh2rp, N.ASA Ames, L. D. Gilger and K. E. Ard, Harris CorporCltion). 
That study is nearing completion. Reports have been published on the materials 
assessment, thermal control system assessment and thermal distortion analysis 
techniques tasks. The report on the antenna system design task is nearly com-
p-1ete and materials properties verification testing of the selected materials 
is in progress. 
The combination of computer programs and interconnecting programs used in 
the thermal distortion analysis task has consequently been refined at NASA 
LeRC to provide a very accurate tool for calculating the on-orbit variations 
in antenna patterns. 
o TASKS 
MATERIALS ASSESSMENT 
THERMAL CONTROL ASSESSMENT 
THERMAL DISTORTION ANALYSIS TECH. 
ANTENNA SYSTEM DESIGN 
MATERIALS PROPERTIES VERIFICATION 
STATUS 
- COMPLETE 
- COMPLETE 
- COMPLETE 
- COMPLETE 
- IN PROGRESS 
283 
CURRENT ANTENNA REFLECTOR AND STRUCTURE FABRICATION TECHNOLOGY 
Currently, antenna reflectors are constructed using carbon fibers in an 
epoxy matrix in order to minimize weight and the coefficient of thermal expan-
sion and hence antenna thermal distortion. The reflector structures typically 
are of two types: unidirectional plies of carbon/epoxy arranged to give 
planarly isotropic near-zero coefficient of thermal expansion in a solid re-
flector or thin honeycomb sandwich panel reflectors with very thin planarly 
isotropic carbon/epoxy face sheets. After construction, these reflectors are 
fastened to ali ghtwei ght strongback structure that is used to adjust the 
reflector to as nearly a true parabolic shape as possible. 
Other antenna structures such as the antenna feed support towers are con-
structed using carbon/epoxy or carbon /polyimide tubes and structural shapes. 
A 2.7M-aperture-diameter state-of-the-art offset fed parabolic reflector 
was recently constructed by TRW as a part of an advanced 30-GHz proof-of-
concept antenna system. The o. 25-i n. -thi ck refl ector was made from honey-
comb sandwich panel with carbon/epoxy face sheets and a Nomex core. The 
reflector was mounted to a strongback also constructed from carbon/epoxy 
honeycomb sandwi ch pane 1. The strongback was used both to support and for 
final adjustment of the reflector surface. The maximum zero-to-peak deviation 
from a true parabola was 0.020 inches. The RMS deviation from a true parabola 
was 0.0055 inches RMS. When th is ref 1 ector was manufactured, problems were 
encountered with excess adhesion to the molding surface. Therefore, accuracy 
for state-of-the-art manufacturing of a flight-type reflector should be 
perhaps·0.003 inches RMS. 
284 
° DESIGN 
- REFLECTOR - HONEYCOMB SANDWICH PANEL 
- CARBON/GLASS FACE SHEET (PLANAR QUASI-ISOTROPIC 
LAYUP) (+60°, 0°, -60°) 
- CARBON/GLASS EGGCRATE CORE 
- ASSEMBLY BY SLUMP FORMING AND FRIT BONDING 
- ANTENNA STRUCTURE - CARBON/MAGNESIUM OR CARBON/ALUMINUM 
TUBES AND STRUCTURAL SHAPES 
° ACCURACY (FOR A 4-METER REFLECTOR) (TDAS) 
- REFLECTOR - MAXIMUM ZERO-TO-PEAK DEVIATION FROM TRUE PARABOLA 
- .004 INCHES 
- SURFACE ROOT MEAN SQUARE (RMS) DEVIATION FROM 
TRUE PARABOLA - .001 INCHES RMS 
CURRENT ANTENNA REFLECTOR AND STRUCTURE FABRICATION TECHNOLOGY (CONTINUED) 
The advantages of carbon/epoxy construction for antenna reflectors are 
that the uni-directional plies can be tailored and assembled in such a way as 
to yield a near-zero coefficient of thermal expansion (eTE) in the plane of the 
reflector and that the construction technology is well developed. 
The drawbacks of carbon/epoxy construction are listed below. The most 
serious of these are the non-linear coefficient of thermal expansion (re-
flector operational temperatures at geosynchronous orbit range from -260oF 
to +1750 F) and permanent creep that occurs as construction stresses are 
relieved. 
o ADVANTAGES OF CARBON/EPOXY CONSTRUCTION 
- NEAR-ZERO COEFFICIENT OF THERMAL EXPANSION (CTE) 
- DEVELOPED TECHNOLOGY 
o DISADVANTAGES OF CARBON/EPOXY CONSTRUCTION 
- NON-LINEAR COEFFICIENT OF THERMAL EXPANSION WITH RESPECT TO 
TEMPERATURE 
- EFFECT OF ABSORBED MOISTURE AND SUBSEQUENT DRY-OUT IN SPAr,E 
ON FINAL REFLECTOR SHAPE AND FINAL COEFFICIENT OF THERMAL 
EXPANSION 
- MICRO-CRACKING - ALLOWS DIMENSIONAL CHANGES FROM FORMED 
SHAPE TO NEW SHAPE 
- CREEP - IF UNDER CONSTANT STRESS WILL TEND TO RELIEVE 
STRESS AND MOVE TO NEW POSITION 
285 
CURRENT SPACE ANTENNA REFLECTOR FABRICATION TECHNOLOGY 
The method of fabrication for present-day carbon/epoxy antenna reflectors 
is illustrated. First, a convex plaster mold is prepared of the desired sur-
face contour. Subsequent ly, a plaster concave and then a high- temperature 
convex mold are prepared. The antenna shell is cured then on a high-tempera-
ture convex mold. It is then fitted to the plaster mold whereon final adjust-
ment of the reflector surface takes place. 
286 
30-GHZ 9-FT DIA. MBA OFFSET REFLECTOR FABRICATION-
FLOlAJ DIAGRAM 
1A. IllEfP ROUGH COAT 
OF'LASTER 
• 
2A. SWEE' fiNISH COAT OF 3". PREPARE MOLD AND 
PLASTER APPl Y MOLD RELEASIE 
~ ·-~~J~t~.~_~s:?_u 
ANOAOOMOlD ERATUREFI8RE ROUGH TRIM AND ROUGH TRIM 
tA. CAST A PLASTER RELEASE GLASS/EPOXY 
PATTERN TOOL 
.... INST"ll SHElL ON 
MASTER PLASTER 
PATTERN AND 8QIiIK) R_ 
REFLECTOR FINAL ADJUSTMENT 
The reflector is shown after final adjustment to the master mold. Thi s 
was accomplished by permanently deforming the reflector surface by moving it 
with respect to a strongback frame and then locking the reflective surface in 
place. 
287 
COEFFICIENT OF THERMAL EXPANSION OF CARBON/EPOXY (HARRIS CORPORATION) 
Note that while at room temperature, the coefficient of thermal expansion 
(CTE) is +0.3, at the typical lower expected operating temperature of -260oF 
the CTE drops to -0.7. In a situation wherein the unprotected reflector face 
is coming out of shadow and into sunlight, reflector temperatures can typi-
cally simultaneously vary from -20oF to -190oF. Over the range, the CTE 
could instantaneously vary from -0.5 to +0.05 over the reflector. Thus, the 
reflector thermal distortion and hence the antenna pattern can be affected by 
the non-linear CTE properties of the carbon/epoxy composite. 
. 
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288 
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COEF. OF THERMAL EXP. OF GFRE 
(QUASI-ISOTROPIC TDRSS RIB LAMINATE) 
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250 
REQUIRED ANTENNA REFLECTOR AND STRUCTURE FABRICATION TECHNOLOGY 
When more accurate reflectors are needed and if the dimensional stability 
prob lems of carbon/epoxy construct ion are to be overcome, new matrices and 
fabrication technology will be required. Both glass and polyimide matrices 
were considered for carbon-filament-reinforced composite reflectors. 
° DESIGN 
- REFLECTOR - HONEYCOMB SANDWICH PANEL 
- CARBON/EPOXY OR CARBON/POLYIMIDE FACE 
SHEETS (PLANAR QUASI-ISOTROPIC 
(+600, 00. -600 PLIES) 
- NOMEX OR ALUMINUM HONEYCOMB CORE 
- ANTENNA STRUCTURE - CARBON/EPOXY OR 
CARBON/POLYIMIDE TUBES AND STRUCTURAL SHAPES 
° ACCURACY (FOR A 3-METER REFLECTOR) 
- REFLECTOR - MAXIMUM ZERO-TO-PEAK DEVIATION FROM 
TRUE PARABOLA .020 INCHES 
- SURFACE ROOT MEAN SQUARE (RMS) 
DEVIATION FROM TRUE PARABOLA - 0.0055 
INCHES RMS 
289 
REQUIRED ANTENNA REFLECTOR AND STRUCTURE FABRICATION TECHNOLOGY (CONTINUED) 
Glass was chosen as the favored matrix because carbon/glass composites can 
be constructed with near-zero coefficients of linear expansion (eTE) and they 
are impervious to moisture and dryout distortion. Near optical-type stability 
can be obtained since creep values are low in comparison to carbon/epoxy. 
Also, extreme precision surfaces can be ground or lapped on the hard stable 
surface if required. The carbon/glass also has good resistance to particle 
and ultra-violet radiation and a wider operating temperature range. 
However, the carbon/glass used to construct a reflector is processed at 
high (2,OoooF) temperatures. Also, additional development is needed on the 
processes for manufacturing and assembling the materials. 
° ADVANTAGES OF CARBON/GLASS CONSTRUCTION 
- NEAR-ZERO COEFFICIENT OF THERMAL EXPANSION 
- IMPERVIOUS TO MOISTURE AND DRYOUT DISTORTION 
o DISADVANTAGES OF CARBON/GLASS CONSTRUCTION 
- HIGH (2.000oF) GLASS TILE SUB-ASSEMBLY FORMING TEMPERATURE 
- MATERIALS PROCESSING AND ASSEMBLY NEED DEVELOPMENT AND TESTING 
290 
CARBON/GLASS MANUFACTURING SEQUENCE 
In a fashion very similar to that for carbon/epoxy, the raw fiber is in-
filtrated and coated with the matrix (in this case powdered glass). After 
drying, it is cut just like carbon/epoxy prepreg (carbon fibers or cloth that 
have been i nfi ltrated with epoxy but not cured). The p 1 i es are then stacked 
in the desired configuration and the volatiles removed. The final step is 
high -temperature pressing into the finished form (ref. 1). 
SUPPLY SPOOL 
LOAD AND 
PRESS 
-
BURNOUT 
TAPE LAYUP PROCESSING 
---
TAKE-UP DRUM 
~ 
• STACK ~ 
CUT 
291 
COEFFICIENT OF THERMAL EXPANSION (CTE) OF CARBON/GLASS 
The O/90-ply orientation HM carbon fiber/borosilicate glass matrix 
composite CTE changes very little as a function of temperature below OOC as 
compared to carbon/epoxy (see figure for coefficient of thermal expansion of 
carbon/epoxy - Harris Corporation) and even compared to other pure glasses 
(ULE, Zerodur and fused silica). 
292 
THERMAL EXPANSION OF CONTINUOUS FIBER COMPOSITES RELATIVE TO CURRENT 
MATERIALS 
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60 
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300 420 
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TEMPERATURE 
CARBON/GLASS REFLECTOR ASSEMBLY 
A parabolic reflector of carbon/gl ass would be constructed of honeycomb 
sandwich panel. The face sheets would be made of tiles laid up in a planarly 
isotropic pattern (+600 , 00 , -600 ). The reflective surface would be of 
random-oriented fibers. The core would be carbon/glass eggcrate construc-
tion. The reflector would be assembled using the tiles and eggcrate core and 
then heated to slump-form and frit-bond (lower temperature melting ground 
glass particles) the parts together. Final contour machining of the random 
fiber surface should assure accuracies of 0.004 in zero-to-peak deviation from 
a true parabola and RMS deviaton of less than 0.001 inches. 
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293 
CARBON/GLASS ANTENNA ASSEMBLY 
The carbon/glass antenna reflector is shown here in an offset configura-
tion typical of space communications applications. 
RULEC lO~ A5SV 
294 
EXTREME PREC IS ION 
ANTENNA REFLECTOR 
FOLOING JOINT 
SIDE STRUT 
!.IOUNTlHG PLAl'rORU 
EXTREME PRECISION DEPLOYABLE REFLECTOR CONCEPT 
The 3M-aperture-diameter carbon/glass reflector can be used as a sub-
assembly for larger reflectors. Preliminary design studies resulted in this 
concept for a 10. 5-meter offset-fed preci s ion antenna. Because carbon/gl ass 
has such a low CTE in combination with excellent dimensional stabilty, it was 
determined that a deployable antenna back-up truss or structure would not be 
necessary for this concept. 
295 
DEPLOYABLE REFLECTOR DEPLOYMENT SEQUENCE 
The deployment sequence for the 10. 5M -aperture-di ameter antenna is shown 
here. The upper panel is first lifted clear of the central stack. It is then 
rotated into position. A jackscrew is then used to lower the panel into 
place where locking mechanisms will be used to lock it to the central panel. 
Each subsequent panel is deployed in a similar sequence. 
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296 
1/ 
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II 
1/ 
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1/ 
DEPLOYABLE REFLECTOR DEPLOYMENT SEQUENCE 
When all panels have been deployed and locked in place, micro-jacks at the 
panel edges can be used for final reflector adjustment. This can be done on 
either a one-time or continual basis. 
297 
DEPLOYMENT SEQUENCE FOR LARGER REFLECTORS 
Panels can be assembled as sub-stacks in the central stack rather than as 
single panels. Then, from the initial central stack, a single panel can be 
used to swing out two more panels (a three-pedal sequence) in addition to 
itself. A sequence of such deployments could result in a l6-meter deployable 
reflector. A six-pedal sequence is also shown. Here, a single central panel 
would deploy a stack of six panels from which the sequences of two and three 
panels could be deployed. 
The design study demonstrated that the great advantage of this method of 
solid reflector panel deployment was that the packing density in the launch 
vehicle could be many times that of other deployment methods such as radial or 
pie-shaped panel deployments which require long narrow stacks for larger diam-
eter reflectors. 
298 
DOUBLE RING 
16-METER DIAMETER 
TRI-RING 
22-METER DIAMETER 
THREE-PEDAL 
SEQUENCE 
SIX-PEDAL 
SEQUENCE 
EXTREME PRECISION ANTENNA FUTURE PLANS 
It is planned to design and build an engineering model of a small ex-
tremely precise antenna (such as mi~ht be suitable for a model of a 60-GHz 
intersatellite link antenna) in FY 86-'87. The antenna would use standard 
lightweight materials for the reflector support structure. However, the re-
flector would be constructed from carbon/glass subpanels and thus would act as 
a pathfinder for lightweight carbon/glass reflector technology. 
o PROGRAM - ENGINEERING MODEL DEVELOPMENT 
o DESCRIPTION - 1 1/2 TO 3-M APERTURE DIAMETER 
- 57 GHz 
- DIRECT FED (SINGLE HORN) 
- FLIGHT-TYPE REFLECTOR, ENGINEERING MODEL 
STRUCTURE 
o SCHEDULE - FY 186 TO 188 
o ESTIMATED RESOURCES - 1 TO 2 M 
o MAJOR TECHNICAL CHALLENGE - PRECISION REFLECTOR CONSTRUCTION 
299 
REFERENCE 
1. K. M. Prewo and E. J. Minford: Graphite Fiber Reinforced Thermoplastic Glass 
Matrix Composites for Use at 1.000oF. Proceedings of the 16th National SAMPE 
Conference, Anaheim, CA, April 21-23, 1981. 
300 


Extreme Precision Antenna Reflector Study Results

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