A 2 by 2 array of fan dipoles was designed as a feed for an existing 9.14 meter diameter reflector. Baffles (conducting fences) were utilized to reduce mutual coupling between dipoles and to improve the feed radiation pattern in the H plane. Significant strut scattering effects are noted in the secondary patterns of the reflector antenna

Bailey, M. C.

Hefner, B. B., Jr.

English

NASA Technical Reports Server

NASA-TM-86300 19840025570
NASA Technical Memorandum 86300
DEVELOPMENT OF A 611 MHz FEED FOR A 9-METER
REFLECTOR
M. C. BAILEY AND B. B. HEFNER J JR.
AUGUST 1984
LIBRARY COpy
OCT 101984
Nl\SI\
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23665
.LANGLEY RESEARCH CENTER
LI8RARY, NASA
HAM~Too.. VIRGINIA
https://ntrs.nasa.gov/search.jsp?R=19840025570 2020-03-20T20:52:15+00:00Z
3 117600520 7536
DEVELOPMENT OF A 611 MHZ FEED
FOR A 9~ETER REFLECTOR
M. C. Ba i 1ey
B. B. Hefner, Jr.
Summary
A 2X2 array of fan-dipoles is designed as a feed for an
existing 9.14 meter diameter reflector. Baffles (conducting
fences) were util ized to reduce mutual coupl ing between
dipoles and to improve the feed radiation pattern in the
H-plane. Significant strut scattering effects were noted in
the secondary patterns of the reflector antenna.
Introduction
A four-element dipole array is designed as a feed to
illuminate a nine-meter diameter paraboloidal reflector at
611 MHz to be used with a UHF radiometer for measuring the
sky background radiation. The reflector is installed on a
2-axis pedestal located near building 1299 and was originally
configured as a cassegrain antenna to be used as a
missile-tracking ground station. For the present
application, the subreflector was removed, the feed opening
in the center of the main reflector replaced by a flat
conducting plate, and the antenna configured as a direct-fed
paraboloid.
Description
The geometry of the reflector is shown in figure 1. The
ratio of the focal-length to diameter is 0.417; therefore,
the feed pattern angular illumination is ±620 • In order to
achieve high beam-efficiency, as required for radiometric
measurements, the antenna must be designed for low-sidelobes;
therefore, the illumination of the reflector must be highly
tapered at the edge. An edge taper between -20dB and -25dB
was selected for the feed design, which should produce a
reflector radiation pattern sidelobe level of -40dB or lower
in the absence of aperture blockage. However, the presence
of the feed and struts in front of the reflector can produce
significant electromagnetic scattering which will result in
increased sidelobes.
A photograph of the feed is shown in figure 2. The feed is a
2X2 array of fan-dipoles excited in-phase by a 4-way power
divider network mounted on the backside of the array as shown
in figure 3. The pertinent dimensions of the final array
design are illustrated in figure 4. The baffles on two edges
and between elements were included as an integral part of the
design to control the pattern shape and to aid in impedance
matching due to reduced interelement coupl ing. The radiating
element is a fan-dipole tuned to 611 MHz at the first
resonance and matched to a 50-ohm coax input. The fan-dipole
was selected since enough parts were already available for
the array assembly and the basic design data had already been
generated earl ier during the development of a broad-band
version of the half-wave fan-dipole [1]. Some adjustment in
the element design was necessary to obtain a good impedance
match within the 608-614 MHz band for the 4-element array.
The final dimensions of the dipole element are shown in
figure 5. The electromagnetic effects of the polystyrene
supports for the dipole elements (shown in figure 2) were
found earlier to be insignificant.
-2-
•
•\',
Design
The basic array geometry <i .e. element spacing) of the 2X2
array was establ ished by array pattern calculations for
linear dipole elements located over an infinite conducting
ground plane. The principal plane patterns for a 2X2 array
of linear dipoles can be calculated from
E-plane:
Ee = sece [cos<~L sine) - cos<~L)J
·sin<~H cose) cos(~S sine) / (1 - cos<~L» <1)
H-plane:
E. = sin<~H cose) cos<~S sine) <2)
where L is the length of the dipole, H is the height of the
dipole above the ground plane, and S is the dipole spacing
<all dimensions specified in wavelengths). Using equations 1
and 2, the element spacing was varied in order to achieve the
desired reflector illumination. The calculated patterns of a
2X2 array of linear dipoles is shown in figure 6 for element
spacings of 0.50A in the E-plane and 0.55A in the H-plane.
These equations and calculations are not intended to
accurately model the radiation patterns for an array of
fan-dipoles, but are intended to be used as an initial
starting point for the emperical design of the fan-dipole
array; although, the main beam <i.e. the reflector
illumination function) should not differ greatly from these
calculations.
The fan-dipoles were initially tuned to the desired band
using existing data and then final tuning was accomplished by
trimming the individual dipole arms of each element in the
array with the other three elements terminated in 50 ohms.
The input voltage-standing-wave-ratio <VSWR) for each element
of the array is shown by the dashed curve in figure 7. The
sol id curve in figure 7 shows the input VSWR of the array
with the four elements combined through the power divider
network.
Since the initial array design did not contain baffles or
other techniques for reduction of mutual coupl ing, the VSWR
increase within the design band is attributed to array
coupling effects. The measurements in figure 8 confirm the
existence of coupling levels of sufficient magnitude to cause
-3-
a significant array mismatch. Baffles (conducting fences)
centered between the array elements were empirically designed
to reduce the interelement coupl ing, as verified by the
measurements in figure 9. The effect of the fences upon the
input VSWR is shown in figure 10. The presence of the fences
cause the individual elements to be tuned to a lower
frequency; however, the combined array VSWR within the
resonant band of the dipoles shows an improvement due to
reduction of interelement mutual coupl ing.
Before retuning the dipoles to the desired frequency band,
radiation patterns of the array with cross-baffles were
• measured. These patterns are shown in figure 11. The
E-plane pattern of the feed array indicates a very highly
tapered reflector illumination with very low spillover and
back-radiation; however, the H-plane feed pattern requires
some improvement in order to achieve the desired reflector
illumination. The H-plane feed pattern improvement was
accomplished by install ing baffles (conducting fences) on two
edges of the ground plane and optimizing the baffle spacing
to obtain the feed radiation patterns shown in figure 12.
With the cross-baffles and H-plane edge-baffles installed, an
attempt was made to tune the array to the 608-614 MHz band by
shortening the dipole elements. The VSWR results are shown
in figure 13. Further improvement in the array impedance
matching would require additional empirical iteration of the
dipole height (length of balun) and dipole length. Lack of
time did not allow further design iterations.
Reflector Radiation Patterns
The radiation pattern in the far-field of an aperture can be
calculated from the Fourier transform of the tangential
electric field distribution in the aperture. The e and.
components of the radiation fields for the aperture are
computed from
e- jPr ~x sin.]Ee = jP cos. + P y2.r
e- jPr
[py cost Sin.]E. = jP - Px2~r
(3)
(4)
-4-
•where Px and P y are defined as
Px = SJ Eax(x,y) ej~(x sine cos. + y sine sin.) dx dy
Sa
Py = 5f Eay(x,y) ej~(x sine cos. + y sine sin.) dx dy
Sa
In equations 3 and 4, Eax and Eay are the x,y components
of the aperture electric field which, for a reflector, is
obtained by the geometrical optics projection of the feed
radiation pattern off the reflector surface onto a plane
perpendicular to the z-axis of the reflector. Sa is the
projected area of the reflector, (r,e,.) are the usual
spherical coordinates of the far-field observation point, and
~=21(/).. where).. is the wavelength at the operating frequency.
Equations 3 and 4 give very good results, provided that there
are no objects (such as feed and struts) in front of the
reflector to cause electromagnetic scattering. However,
calculations using equations 3 and 4 can be supplemented with
the Geometrical Theory of Diffraction [2,3] to account for
the effects of feed and strut scattering.
A reflector antenna computer code [4] was used to perform
radiation pattern calculations for the geometry of figure 1
using the feed illumination patterns of figure 12. The
calculated radiation patterns in figure 14 for the unblocked
aperture show very low sidelobes; however, when the feed and
strut scattering effects were included, the sidelobes
increased to -32dB in the E-plane and the 450 -plane and to
-22dB in the H-plane, as shown in figure 15. Upon
examination of the scatter patterns of figures 16 and 17 for
the feed and struts, it was determined that the major
contribution to high sidelobes in the H-plane is due to strut
scatter; whereas, sidelobe levels in the E-plane and
450 -plane were primarily due to scatter from the feed.
Rotation of the feed 450 with-respect-to the struts would
r'esu 1 tin stru t scat ter patterns as shown in figure 18 and
reflector radiation patterns as shown in figure 19. Thus the
preferred configuration is one in which the feed is polarized
450 with-respect-to the strut projections in the aperture
plane.
-5-
n
I
Conclusions
A 611 MHz 2X2 array of fan-dipoles has been designed as a
feed for an existing 9-meter reflector. It was demonstrated
that conducting baffles could be used to reduce interelement
mutual coupl ing (thus aiding in the input impedance matching>
and to reduce sidelobes in the H-plane of the feed pattern.
Calculated results for reflector radiation patterns show
sidelobes of -22dB to -26dB due to strut scattering.
References
[ll Bailey, M. C.: uBroad-Band Half-Wave Dipole,u IEEE
Transactions on Antennas and Propagation, Vol. AP-32,
April 1984, pp. 410-412.
[2l Kouyoumjian, R. G. and Pathak, P. H.: HA Uniform
Geometrical Theory of Diffraction for an Edge in a
Perfectly Conducting Surface,u Proc. IEEE, Vol. 62,
pp. 1448-1461, 1974.
[3] Pathak, P. H. and Kouyoumjian, R. G.: HAn Analysis of
the Radiation from Apertures in Curved Surfaces by the
Geometrical Theory of Diffraction,u Proc. IEEE, Vol. 62,
pp. 1438-1447, 1974.
[4] Rudduck, R. C. and Chang, Y. C.: -Numerical
Electromagnetic Code - Reflector Antenna Code
NEC-REF(VERSION 2) Part I: User;s Manual ,u Ohio State
University Technical Report 712242-16 (713742), December
1982.
-6-
•
y•
•
x
IE------- 914------~
f
r
325
4-
137
-.;
•X
Figure 1. aeometryof 9-meter paraboloidal reflector
<dimensions in centimeters).
•
Flgllf'P 3.
c
Power' divider networl< for 611 MHz fft9d.
•
T----' "'---'7, ,
' "
..
\ , , ,
30.48 1- !' , •,....\ ) ..~, , , ,
, \ , \1. ___ 1 1._---'
2.54...- ~
~~
~---., t'---7
\ , , ,
\ , \ I
~( t20.3230.48 '. . '1 ' \
,
, , \
, \ , ,L ___ ~ L _ __ .Jl
1+-12.7~ ~
7.62
l< 30.48 + 30.48 ~
~~120 7~ 15 024+- 12.7-r-10 0161
2.06 ........
T
10. 16
-1. .....__................... .... _
Figur. 4. Dim.n~ional drawing of 2X2 array with baffl.~
(dim.n~ion~ in c.ntim.ters).
•12.7
Ti.... ~~~~I ....,....... ~~"':I .. ~.,
.. ~ I
I ... ~ 0
7 • 6 2 I ~.., .. 60:
1, ~~ .... _I I,~' ..... " II " ... 'I ~ ~ ... Iy .. ~
0.159
"
12.065
13.46 -----~
20 • 3 2 ---------.;...~I
.... 1 1.....L0.9525
'"P"I ~~to~
-
~.358
DIA .
)
• 358-......
OD ~
\\..- 50 ohm coax
Figure 5. Detail dimensions of final dipolp design
<dimensions in cpntimptprs).
..
l·
Figure 6. Calculated radiation patterns for 2X2 array of
linear dipoles located over an infinite conducting
ground plane (dipole length = 0.3 wavelengths,
dipole height = 0.25 wavelengths).
•4
,
, INITIAL DES IGN
, (no baffles)
,
,
\
,
3 ,
,
,
- - - individual
0:::
combined3:
en
> ,
,
,
\
\ ,,.
\ ~
\ I
\ I
\ I
\ I
\
\
\
1
500 600 700
Frequency (MHz)
Figure 7. Measured VSWR for 2X2 array without baffles .
""
INITIAL DESIGN
( no baffles)
600 700
Frequency (MHz)
E- plane
-- H-plane
-30 L.J.~~..J-l~""""'...J....J....L"""''''''''...L..''''''''''''''''''
500
0')
r:::
c..
::::J
o
U
-
Figure 8. Measured interelement coupling for 2X2 array
without baffles.
..
•--
-
Q')
c:
C5...
:J
o
U
600
Frequency (MHz)
700
•
•
Figure 9. Measured interelement coupl ing for 2X2 array
with cross-baffles.
700600
Frequency (MHz)
4
,
,
INITIAL DESIGN !,
, (cross-baffle's),
,
,
,
,
,
3 ,,
,
,
- -- individual
a::::
,
combined3:
en
>
I
J
I
~,--
2 I
~
I
I
I
I
Figure 10. Measured VSWR for 2X2 array with cross-baffles
before final dipole tuning.
•
Figure 11. Measured radiation patterns of 2X2 array
with cross-baffles.
•l
Figur. 12. M.asur.d radiation patt.rns of ZX2 array
with cross-baffles and edge-baffles.
.
- - - individual
combined
FINAL DESIGN
(cross-baffles)
( edge-baffles)I
,
,
,
I
,
I
,
,
,
,
,
\
,
\
\
\ ~ - -
\ ~,
\ ,
, I
, --~;...--.........
, -' -----
1.......a..................L...L...............-'-.&--I.............. .......~
500 600 700
Frequency (MHz)
•
Figure 13. Measured VSWR for 2X2 array with cross-baffles
and edge-baffles after final dipole tuning •
l•
--- H-plane
- - - - E-plane
-40 LO--J1--...!---L-~~lO~...a..---'-"""-"""~20
Angle (deg)
--(I)
e:::
-
-
Figure 14. Calculated reflector radiation patterns at 611 MHz
using fe.d patterns of figure 11.
H-plane
- - - - E-plane
- 45° plane
o-............-..--~ .......-,....-r-.....,.--r-...,
1
I'
~ ,~~
I\'-"'~~' r--- ,--~, '",~,1 ~ ~~ -"
, ' \ ,- - ~
, I \ I-40L-....L_J-...a~~~_~....L.........--'_.......~
o 10 20
Angle (deg)
-
-(I,)
0::
CQ
"'C
-
• Figure 15. C~lcul~t.d refl~ctor r~diation p~tterns at 611 MHz
including feed and strut scattering effects.
•O,.......,..-~...,.-...----..-..,....--r-..,.........,........
-.
Q) .
.~
10
-Q)
a:::
-40.........._ ......--a.. ......_~_""_...... ..",
.0 10 20
Angle (deg)
Figure 16. Calculated scatter pattern for feed blockage. ..
,
0
•
H-plane
- - - - E-plane
-
45° plane
-10
-co
-c
-
'-Q)
~
a..
-20
.~
n;
-Q)
e:::
"
-30 '", ,
'\ \
\ \
\ ,
-40
0 10 20
Angle (deg)
• Figure 17. Calculated scatter patterns for struts •
- -
--- H-plane
- - E-plane
- --- 45° plane
~
... --=- -------
----
-400L--L_.L-~~---::1~0~-....I..---'-------;20
Angle (deg)
cc
"C
-Q,)
e::
-
-
Figu~e 18. Calculated scatter patterns for struts with feed
polarized 450 to strut projections in aperture plane.
H-plane
- - - - E-plane
--45° plane-
10
Angle (deg)
".,....,-~,
, "
", ... --- ,~ ,
, "...1""""""--.....,
o~.....- .....- ......- ....................- ....- .....-...-....
-40..........._""--...... ...... ..........._ ..
o
-10
-co
"C
-
Ii-
Q,)
~
CL
-20
Q,)
.~
-ro
-Q,)
c:::
,
,
Figure 19. Calculated reflector radiation patterns at 611 MHz
including feed and strut scattering effects with feed
polarized 450 to strut projections in aperture plane.
i.,-,
.'
1;' Report No.
, NASA 'fM-86300
2. Government Acc8$$ion No. 3. Recipient's Catalog No.
4. Title and Subtitle
DEVELOPMENT OF A 611 MHZ FEED
FOR A 9-ME'IER REFLECTOR
7. Author(s)
M. C. Bailey and B. B. Hefner, Jr.
9. Performing Organization Name and Address
NASA Langley Research Center
Hampton, Virginia 23665
12. Sponsoring Agency Name and Address
National A~ronautics and Space Adminstration
Washington, DC 20546
15. Supplementary Notes
16. Abstract
5. Report Date
August 1984
6. Performing Organization Code
506-58-23-01
8. Performing Organization Report No.
10. WOf'k Unit No.
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
,
A 2X2 array of fan-dipoles is designed as a feed for an existing
9.14 meter diameter reflector. Baffles (conducting fences) were
utilized to reduce mutual coupling between dipoles and to improve
the feed radiation pattern in the H-plane. Significant strut
scattering effects were noted in the secondary patterns of the
reflector antenna.
17. Key Words (Suggested by Author(s))
Antennas Scattering
Arrays Feeds
Dipoles UHF
Coupling
Reflector
18. Distribution Statement
Unclassified - Unlimited
Subject Category 32
.,
19. Security Oassif. (of this report)
Unc lass i fled
20. Security Classif. (of this page)
Unc lass ifled
21. No. of Pages
26
22. Price
A03
N-305 For sale by the National Technical Information Service. Springfield. Virginia 22161 NASA-Langley, 1984

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; 3 1176 00520 7536


Development of a 611 MHz feed for a 9-meter reflector

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