Present ground station microwave antennas of the Deep Space Network are of the symmetric dual reflector (cassegrainian) type. An investigation is being made of alternative high-performance offset antenna designs which have a clear aperture (no reflector or structural blockage) with shaped reflector surfaces. A 1.5-m, 32-GHz clear aperture model was built for experimental studies. The unique processes of fabrication, surface measurement, and alignment are described

Carpenter, J.

Chian, C. T.

Rocci, S.

English

NASA Technical Reports Server

N83 14336
IDA Progress Report 42-71 July - September 1982
The Fabrication and Surface Tolerance Measurements of the
JPL Clear Aperture Microwave Antenna
J. Carpenter, S. Rocci, and C. T. Chian
DSN Engineering Section
Present ground station microwave antennas of the Deep Space Network are of the
symmetric dual reflector (cassegrainian) type. An investigation is being made of alterna-
tive high-performance offset antenna designs which have a clear aperture (no reflector or
structure blockage) with shaped reflector surfaces. A 1.5-m, 32-GHz clear aperture model
was built for experimental studies. This article describes the unique processes of fabrica-
tion, surface measurement and alignment.
I. Introduction
Present microwave antennas in the NASA-owned Deep
Space Network (DSN) are of the dual-reflector (cassegrainian)
type. Recently an investigation has been made of alternative
ground station antenna designs to increase the performance
and enable efficient operation at 32 GHz. Significant advances
in the synthesis microwave, analysis and performance optimi-
zation of several designs have been completed since 1978
(Ref. 1). Parallel studies on structural designs of antennas with
symmetrical and asymmetrical reflectors were undertaken. The
clear aperture (sometimes called offset) antenna design wRh
dual-shaped reflectors emerged as a superior design. A 1.5-m
clear aperture antenna model with shaped-reflector surfaces
was selected as a proof-of-design for experimental verification.
The predicted superior performance has a potential 2 to 3 dB
increase in the gain to noise temperature ratio (G/T). This
proof-of-design antenna was to be carefully tested at JPL both
for mechanical stability and for the high-performance electri-
cal (microwave) characteristics. The antenna synthesis and
analysis has predicted a high aperture efficiency of 86%. It was
necessary to obtain a highly accurate surface finish with mini-
mum surface and alignment tolerance to permit accurate esti-
mates of the electrical (microwave) loss at 32 GHz at the 0.5%
level for surface error to wavelength ratio. This accuracy
requirement corresponds to a mechanical setting accuracy at
the 0.04-mm (0.0015-inch) level.
II. Surface Tolerances
The geometry specifications and the design details were
developed in house to meet the needs of the high performance
sought. The subreflector and feedhorn as shown in Figs. 1
and 2 were to be adjustable in X, Y, and Z axes plus a rotation
for further mechanical alignment and microwave testing. The
goal for the main reflector and subreflector surface tolerance
was to be equal to 0.18 mm (0.007 in.) or better. The align-
ment error must be equal to 0.5 mm (0.020 in.) or better for
optimum performance.
104
https://ntrs.nasa.gov/search.jsp?R=19830006065 2020-03-21T04:53:07+00:00Z
III. Fabrication Steps
After the documentation was completed a search was ini-
tiated to locate a vendor with the unique capability to ma-
chine the shaped surface of the reflectors to the specified
tolerance. Among 20 firms contacted, Tempe Precision Air-
craft Co. of Tempe, Arizona, was selected in June 1981, with
a contract completion date of Feb. 1, 1982. The details of the
computer coordinates for machining the reflectors were devel-
oped in house as described in the appendix. Details of the
fabrication of the major components are as follows:
A. Main Reflector
This is designed to be 1.5 m in diameter. A 1524 X 1828.8
'X 260-mm (5 ft X 6 ft X 10 in.) aluminum billet was blan-
chard ground and mounted on a three-axis, numerically con-
trolled horizontal milling machine as shown in Fig. 3. The
excess material was removed to form a unique "parabolic"
shape. Since the machine was not large enough to machine the
entire surface, the main reflector was rotated in 90 deg incre-
ments for machining. Each quadrant was machined in three
steps. The first step was rough cut to remove excess material
to within 2.8 mm (0.110 in.) of the finished surface. The sec-
ond step was a cut to remove material to within 0.25 mm
(0.010 in.) as shown in Fig. 4. The third cut produced a
finished machined surface. The surface was spot-checked after
each machining operation to verify the computer program
results. After the surface was machined and the perimeter
trimmed to the final envelope size, the reflector was removed
and hand-finished to remove the tool marks. The main reflec-
tor surface was measured at 727 points at Tempe Precision
Aircraft Co. as shown in Fig. 5 and verified at JPL machine
shop as shown in Fig. 6. The root mean square (rms) error
obtained from the two machines was 0.114 mm (0.0045 in.) as
outlined in Table 1.
B. Subreflector
The subreflector, which is 0.4 m in diameter, was machined
utilizing the same methods and equipment above. An rms error
of 0.114 m (0.0045 in.) was obtained. The X-, Y-, and Z-axis
adjustments were obtained utilizing three micrometer transla-
tion units with 50 mm travel.
IV. Field Alignment and Installation
The complete assembly was optically and mechanically
aligned as shown in Fig. 7 utilizing an optical level (Wild, N-3),
a theodolite (Wild T-2), two master precision levels, a series
of inside micrometers and a precision fabricated template to
maintain the theoretical geometry and to obtain the optimum
signal reception. The three subassemblies were aligned within
0.25 mm (0.010 in.), verified and inspected as shown in Fig. 8.
The assembly was transported to the test range and installed
on an azimuth-elevation positioner located on the roof of the
test building and the alignment was rechecked.
V. Results of Surface Measurements
An attempt was made to compute the rms value of the sur-
face distortions in the Z-axis normal to the reflector surface.
The surface distortions were measured in a local coordinate
system and the reflector surface has a final shape close to a
paraboloid. A FORTRAN computer program CAA-RMS,
described in the appendix, was developed to transform the
local coordinates into a global coordinate system. An rms best
fitting program (half-path length error) was used to calculate
the rms value of the reflector surface distortion. The rms
values of the surface distortion of the main reflector e , were
found to be 0.094 mm (0.0037 in.) without best fitting, and
0.0285 mm (0.001123 in.) with best fitting. The surface devi-
ation of the subreflector e (without best fitting), was found
to be comparable 0.114 mm (0.0045 in.).
Since comparable machining techniques were used through-
out, it is estimated that the fitted rms of the subreflector is
approximately 0.028 mm (0.0011 in.). The net effective rms
of both reflectors together, with best fitting, is thus approxi-
mately or 0.040 mm (0.0015 in.). This yields 0.5%
for the ratio of surface error to wavelength at the 32-GHz
microwave frequency, from the Ruze equation, and a gain
limit frequency near 600 GHz. The 1.5-m clear aperture
antenna model is currently undergoing electrical microwave
tests that will be the subject of future TDA reports.
105
Acknowledgment
The authors acknowledge the technical and administrative assistance given by C. L.
Thoronton (420), F. W. Stoller (355), E. Laumann (350), D. A. Bathker (333), A. Cha
(333), R. Levy (355), and F. Lansing (355) during the various execution steps of this
work. The support given by the Design and Mechanical Section (356), Quality Assurance
Section (511), and Fabrication Section (664) is also acknowledged.
Reference
1. Cha, A., "Design of a 1.5-m, 32-GHz, Clear Aperture Antenna," IDA Progress Report
42-66, Jet Propulsion Laboratory, Pasadena, Calif., Dec. 1981, pp. 87-93.
106
OF POOR UALITY
Table 1. Results of surface measurements
Surface deviation, mm (in.)
Main reflector
Sub-reflector
RSS
Not
best-fit
0.112
(0.0044)
0.114
(0.0045)
0.160
(0.0063)
Best-fit
0.028
(0.0011)
0.028
s (0.0011)
0.038
= (0.0015)
Half pathlength error, 0.0381 mm (0.0015 in.); surface tolerance loss
s 0.5% of wavelength (32 GHz); gain limit frequency s 600 GHz.
107
ORIGINAL PAGE IS
OF POOR 'QUALITY
'
•-50.39*1
Fig. 1. Geometry of offset clear aperture dual reflector (in cm)
PRESENT SYMMETRIC STATIONS
OFFSET
CLEAR APERTURE
UNDER INVESTIGATION
IN THIS REPORT
Fig. 2. Alternative reflector configurations for very low noise and high-gain ground stations
108
Fig. 3. Milling machine and tooling showing billet
Fig. 4. First rough cut and one quadrant of second cut of
main reflector
Fig. 5. Checking the cms of the completed reflector at
Tempe Precision
109
Fig. 6. Checking the rms of the reflector at JPL machine shop
Fig. 7. Alignment equipment for the clear aperture antenna
110 ORIGINAL PAGH
BLACK AND WHITE PHOTOGRAPH"
-. i
Rg. 8. Axis alignment of the clear aperture antenna
111
ORIGINAL PAGE 1
OF POOR QUALITY
Appendix
Details of Computer Program CAA-RMS
The computer program CAA-RMS, written in FORTRAN,
was developed to transform the measured surface distortions
from a local-coordinate system into a global-coordinate system
as shown in Fig. A-l. The output of this program as shown in
Fig. A-2 is used as input to the JPL RMS-BEST-FITTING
PROGRAM to calculate the best fitting rms value of the reflec-
tor surface distortion.
I. Surface Geometry Equations
The paraboloid is defined by
Z =
where F = focal length
X2 + Y2
 = R*_
4F ~ 4F (A-l)
The measured distortion vector AZ has components U, V,
U = 0.
K = AZ • sin 0
W - AZ • cosfl )
(A-5)
The projection of the distortion vector on the normal has
components:
* l =
3 2 3
where
(A-6)
The slope is defined by
+ Y2
AR 2F
The normal vector at any point on the paraboloid is
N =
where
) * - - (A3)3j c , c,c
j = V4JT2 + 4Y2 + 16F2
(A-2)
The coordinate transformation from the local coordinate
j, ylt Zj) to the global coordinate (X, Y, Z):
X = x.
Y = Yoff + yi cosO -
Z = Zoff + yl sin0 +
sinfl
cos
(A-4)
The length of the projected vector
PL = v^2 + P\ +
When the normal is projected in dZ direction:
PL PL
IdZI =
cos (SLOPE)
(A-7)
cos
(A-8)
A listing of the program is given in Fig. A-3.
II. Sample Calculation
A. Input
Focal length of fitting paraboloid, F = 1342.39 mm (52.85
in.)
Offset of local coordinate system:
Yoff = 1434.08 mm (56.46 in.)
112
Z = 383.03 mm (15.08 in.) B. Output
RMS values of the surface distortion:
Angle between local and global systems:
0.094 mm (0.0037 in.) (without fitting)
B = 28.073 deg
0.0331 mm (0.001303 in.) (with best fitting)
The surface distortions, AZ, measured in local coordinates,
are read in (a total of 400) The printout of this example is shown in Fig. A-4.
113
POOR QUALfTY
PARABOLOID
GLOBAL SYSTEM
Fig. A-1. Local and global coordinates
INPUT J
^
COORDINATE TRANSFORMATIONS
PROJEa DISTORTION VECTORS,
MEASURED IN LOCAL COORDINATES,
ON NORMAL
TRANSFORM NORMAL COMPONENT
OF DISTORTION VECTORS
INTO GLOBAL COORDINATES
J PL RMS
BEST FITTING PROGRAM
BEST FITTING RMS VALUE
OF SURFACE DISTORTIONS
Fig.A-2. Row chart of rms computer steps
114
s _, ORIGINAL PAGE 13
^ OF ;Pt50R? QUALITY
4 8 3 3 f c * M
1
2
3
II
5
6
7
3
9
13
1 1
12
13
14
15
IS
17
13
1?
20
21
22
23
P4
25
26
27
23
29
30
3 1
32
33
i*H
35
36
3 7
3fl
39
43
4 1
42
4 3
14
45
46
47
43
49
f3
51
52
53
54
55
Ei
57
5B
59
60
61
62
63
64
65
66
67
69
69
70
3PKI «S
iK C D . C A A - R h S
C PKO&KAH COMPUTED IHT D E L T A t FKrtORS OF C L E A * A P E 3 A T U R E R iFLECTO*
C WHICH HAS A SHAPED SURFACE CLOSt TO A P A R A 8 3 L O i n
C THF OUTPUT F ITS HE RMS B E S T F ITTIN6 P R O G R A M SO T H A T A BFST
C - THE SUhFACE bAb A C T U A L L Y MEASURED IN A L O C A L C O O R D I N A T E S Y S T E M
C A B O U T ITS X AKJ i 28.073 DE3KEES
C ' OFFiET IN T 56.46 INCHES FK3M THE PARA80L3I3S ' S Y S T E M
C OFFSET IN Z 15.08 INCHES F S O M THE P A R A B O L O I D S ' S Y S T E M
C
C C O N S T A N T S
C
REAL w i « N 2 » N 3
A R E A : i .0
F: 52.65 » FOCAL LENGTH CF F ITT ING PARABOLOID
YOFF: 56.46 9 Y OFFSET OF L O C A L COORDS S Y S T E M
ZOFF: 15. OB 3 Z O F F S E T OF LOCAL COORDS SYSIE"
X * O T = 28.073* 0.01745329252
NP: 0
DIMENSION 10(000). X K 4 0 0 ) . Y ldOO), Z 1 I 4 C O > > ZERRI400I
PRINT 400P
4000 FOHMAI £• ID X Y Z ZZ ZERR V
S y C2 PI P2 P3 P. T A N G 32 '»
PRINT 4100
4100 F O R M A T i '
» ' n Nl N2 N3 * . / / »
C
C R E A D IN D A T A
C
DO 50 1:1.400
HEAD 1000. 10(1). «!(!). till), 21(1), Z E R R f l )
1000 F O R M A T I)
IF I IIU N .EO.O) GO TO 51
NP=NP+ 1
X: XUD
Y: Y O F F » Y 1 ( I > * C O S ( X R O T ) - Z 1 ( I > »S IN ( X R C T >
ZZ= ZuFF » Zl (I ) « C O S ( X H O T) » YHI )» S I N ( X R O T I
Z : C X * X » Y * Y ) / C 4 . « F )
C Nl.N2.N3 = COMPONENTS 3F THE NORMAL TO P A R A B 3 L O I O
Cl: S'lRU 4 . « X * X » 4 . » Y » Y » 16.«F*F>
Nl= -2 .0«X/C1
N2: -? .»Y/C1
N3= 4.»F/ri
C I'.V.H : COMPONENTS CF THE DISTORSICN V E C T O R
u: 0.
v: - 2 E R R C D * S I N ( X R O T )
h: Z E K R ( I ) * CUS«nOT)
C P R O J E C T P I S T O S T I O N V E C T O R ON N O R M A L
C 2: U* Nl* V* N2« h*N3
Pi: C?* HI
P2: C?* N2
P3: C?* IM3
PL: S J R T t P l * P l » P2*P2* P3«P3J
C SLOPF OK IANRFNT ANILE TO P A R A B O L O I D
SLOPE: SUPT««<» Y * Y > / « 7 . * F )
SLOPE: A T » N » b L O P t >
O/: PL' C O S I S L O P E )
JANG: SLOPE* 57.2957705131
C
HKINT 2 r)Or, in<I) ,K, Y. Z. Z Z . s Z E R « < n . i/.U. C2. Pl.P2.P3. JL.
t T A N t > . 1Z
PRINT 2100. n. M. N?. N3
2100 F O K M A I ( 69X .FR .2 .3FB .4 )
2000 F O R M A T t I5.4F9.4. F7. 4 .RF8 .4. F 9.4)
W R I T E (10,3000) X . Y . D Z . A K F A . I O ( I >
3000 F O R M A T C2F10. 3 . 30 X.F 1C. 4 . F 10 . 2. 11 0>
50 CONTINUE
51 CONTINUE
PRINT 5000. NP
5000 F O R M A T C1HO. •••**• - '.IS.* - POINTS PROSESSFO*)
S T O P
ESO
N K K . C A A / L O C A L - D A I A
Fig. A-3. CAA-RMS program listing
115
'*
C L L A K A P L ^ A T U r t E - A ORIGIN Y OFFSET =
HuNCH = 0 O H T I O N = 0 P F S T F I T = 0 iHLOSS = 0 NO CF POINTS = 13*
F O C 4 L LENGTH = l . ^ M 2 3 9 - M < "52.S5U- IN)
C N S T J : .00000 CNST2 = .OOOnO CNSI3 - .00000
TOFF = .OOJ 20FF r .000 XXO T - .000
INPUT O l b T O R T I O N S O B T A I N F O A N A L Y T I C A L L Y - O P T I S M 0
N O R M A L A N A L Y S I S f U R r.-JPUT D A T A
MNin iZAI ION OF KHS W I T H R E S P F C T TO THE DESIGN P A R A B O L O I D
KKS OF 1/2 L A M B D A W F I C H T E D PY A K F A S - .09-rH ( .0037-IN)
K1MIMIZA1 ION OF RHS wl TH R E S P E C T TO FOCAL LENGTH CHANGE
rtNb Of 1/2 L A M B O A ue iGHTEO flY A K E A S = .03-flf< « .OOt^m-IMI
NtU F O C A L LENGTH : 1.3«30fl-H « ^2 .«7A- IN>
O t v T A T I O N OF THE n E A N - I /? L A M B D A r - .00007 762-HH
X C O O R D I N A T E OF V E R T E X r - .20M-CM ( -.OPO-INJ
Y C O O K Q 1 N A T E OF V E R T E X = .1B6-CM < .191-INI
I C O O R D I N A T E OF V E R T E X r .OQ?-CM ( .P17-IN)
R O T S M O N A B O U T X A X I S = . O O I I S ' - K A O I A N S
R O T A T I O N A B O U T Y A X I S = . 000577-SADI Af4S
ENGLISH UNITS USED FOR ME ASUKE1EN I S AND C A L C U L A T I O N S
P L O T T I N G C O N T R O L C A R D D A T A - MG = 0 Ntf = -1 .0000
PRINT*
Fig. A-4. Sample results of CAA-RMS program
116


The fabrication and surface tolerance measurements of the JPL clear aperture microwave antenna

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