U.S. patent number 3,747,111 [Application Number 05/182,399] was granted by the patent office on 1973-07-17 for composite antenna feed.
Invention is credited to James C. Administrator of the National Aeronautics and Space Fletcher, Vito Joseph Jakstys, N/A.
United States Patent |
3,747,111 |
Fletcher , et al. |
July 17, 1973 |
COMPOSITE ANTENNA FEED
Abstract
A composite antenna feed subsystem concentrated in a small area
at the prime focus of the parabola of a satellite parabolic
reflector accomodates a plurality of frequency bands. The arrays
comprising the subsystem are mounted on the top cover of a
communication module. A multimode horn is arranged at the center of
the subsystem axis which functions at X- and C-band frequencies,
and a cross-array consisting of individual elements form the S-band
feed, with one arm of the S-band array containing an element
mutually shared with the L-band array. Provision is also made for
UHF frequencies, and a dipole arrangement for VHF frequencies is
arranged around the S-band arms.
Inventors: |
Fletcher; James C. Administrator of
the National Aeronautics and Space (N/A), N/A
(Cupertino, CA), Jakstys; Vito Joseph |
Family
ID: |
22668301 |
Appl.
No.: |
05/182,399 |
Filed: |
September 21, 1971 |
Current U.S.
Class: |
343/725; 343/797;
343/893; 343/729; 343/803 |
Current CPC
Class: |
H01Q
21/00 (20130101) |
Current International
Class: |
H01Q
21/00 (20060101); H01q 021/00 () |
Field of
Search: |
;343/725,726,727,728,729,730,797,893,717,798,803 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Chatmon, Jr.; Saxfield
Claims
I claim:
1. A composite antenna feed subsystem operative at a plurality of
frequency bands comprising:
a first feed shared by first and second frequency bands,
a second feed for a third frequency band having an array of four
arms arranged in orthogonal turnstile relationship centered around
the center of the first feed,
a third feed for a fourth frequency band having a plurality of
radiator elements arranged in a square form array centered around
the center of the first feed, individual elements of the third feed
being interposed between adjacent arms of the second feed,
a fourth feed for a fifth frequency band having a plurality of
elements arranged in a line array and including an element mutually
shared with an arm of the second feed, and
a fifth feed for a sixth frequency band having an array of elements
arranged near the ends of the arms of the second feed located
outermost from the center of the first feed, the first, second,
third, fourth and fifth feeds being mounted to a common ground
plane structure.
2. The composite antenna feed subsystem recited in claim 1 wherein
the first feed comprises a multimode horn centered at the prime
axis of the subsystem.
3. The composite antenna feed subsystem recited in claim 2 wherein
the second feed comprises a plurality of crossed-dipole elements in
each arm of the array.
4. The composite antenna feed subsystem recited in claim 3 wherein
the fourth feed comprises a plurality of crossed-dipole elements
arranged in a line array parallel to two opposite arms of the
second feed.
5. The composite antenna feed subsystem recited in claim 4 wherein
the fourth feed further comprises a single crossed-dipole element
mounted adjoining the line array.
6. The composite antenna feed subsystem recited in claim 5 wherein
the fifth feed array of elements comprises dipoles arranged near
the outermost end of each of the arms of the second feed.
7. The composite antenna feed subsystem recited in claim 1 for use
in an EVM satellite as an interface between the transponder and
parabolic reflector, wherein the common ground plane structure is
the top cover of the EVM satellite.
8. A composite feed subsystem mounted on the top cover of an EVM of
a satellite for use as an interface between a transponder and
parabolic reflector comprising:
a plurality of feed arrays for different frequency bands mounted
about the prime axis of the EVM top cover,
a VHF feed array having a dipole element mounted at substantially
the center and near the edge of each side of the EVM top cover;
wherein one of the plurality of feed arrays comprises four arms
arranged in orthogonal turnstile relationship about the prime axis,
each arm having an outermost end located substantially at the
center and near the edge of a different side of a substantially
rectangular EVM top cover, the VHF feed having first and second
coacting radiating elements mounted near said outermost ends of
each of the four arms to form four dipoles.
9. The composite antenna feed subsystem recited in claim 9 wherein
each of the arms of said one of the plurality of feed arrays
comprises a plurality of crossed-dipole elements.
10. The composite antenna feed subsystem recited in claim 9 wherein
at least a portion of the first and second radiating elements
located nearest the arms of said one of the plurality of feed
arrays is surrounded by a coaxial cable.
11. In an antenna system, a first VHF integral radiating element
comprising:
a first length extending in a perpendicular direction relative to a
ground plane of reference,
a second length extending from the end of the first length in a
direction which forms an obtuse angle with the first length,
a third length extending from the end of the second length in a
direction substantially perpendicular to the first length,
a dielectric support mounted between a portion of the third length
nearest its free end and the ground plane,
the first, second and third lengths and the dielectric support
defining a plane substantially perpendicular to the ground
plane.
12. The antenna system as recited in claim 11 wherein the ground
plane of reference comprises the top cover of an EVM satellite, and
further comprising:
a second VHF integral radiating element spaced from and arranged in
mirror image relationship to the first radiating element, operative
to function therewith as a dipole.
13. The antenna system as recited in claim 12 further comprising a
feed array corresponding to another frequency band interposed
between the spaced first and second radiating elements, and coaxial
cables surrounding portions of the first and second radiating
elements to shield the latter from the interposed feed array.
Description
ORIGIN OF THE INVENTION
The invention described herein was made in the performance of work
under a NASA contract and is subject to the provisions of Section
305 of the National Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 U.S.C. 2457).
BACKGROUND OF INVENTION
1. Field of Invention
The invention relates to a prime focus antenna feed subsystem which
serves as the interface between the multimode, multi-frequency
subsystem of a transponder and a parabolic reflector of the
antenna. It has particular utility in the field of antenna feeds at
microwave communication wavelengths and for broadband RF field
measurement systems, especially in conjunction with the NASA
Applications Technology Satellite (ATS).
2. Prior Art
Multi-frequency feed subsystems having simultaneous transmit and
receive functions are known in the prior art but those involved
relate to multiple antennas of the same type such as multiple
horns. Prior art feed subsystems also require rotating joints and
mechanical devices for beam pointing.
SUMMARY OF THE INVENTION
The apparatus according to this invention provides several feeds
designed to perform a diversity of functions. Thus, applicant
provides an antenna feed subsystem capable of handling X-band,
C-band, S-band, L-band, UHF and VHF frequencies.
For example, the S-band feed provides incremental beam scanning in
two orthogonal planes and also simultaneous lobing (monopulse)
operation. The L-band feed generates a fan-shaped beam by
illuminating the entire reflector with uniform phase in one plane
and only a portion of the reflector in the orthogonal plane. The
feed elements are arranged in such a manner that minimum
interaction results between the various feeds.
The feed is concentrated in a small area at the prime focus of the
parabola of the parabolic reflector, and exhibits a high degree of
radiation efficiency and minimal interaction between elements. The
disclosed embodiment shows the composite feed system comprising an
integral part of the top cover of the communications subsystem of
the ATS F&G satellite, providing simplicity of assembly and
test and the shortest possible RF cable lengths.
The composite feed subsystem comprises a multimode horn at the
center of the subsystem axis which functions at X- and C-band
frequencies. A plurality of arms consisting of individual elements
form the S-band array and are centered about the multimode horn,
with one arm of the S-band array containing an element mutually
shared with the L-band array. Provision is also made for UHF and
VHF frequencies, the VHF portion comprising a particular array also
considered to be part of the invention.
The disclosed composite feed subsystem provides certain
improvements over prior art multi-frequency arrays, such as:
1. Repeatability of electrical performance of like elements
2. Low cost fabrication
3. Minimum weight
4. High reliability necessary of spacecraft hardware
5. Broad frequency coverage
6. Medium density packaging
7. Use of simple radiating elements
The invention provides a broad frequency spectrum (UHF to X-band)
of transmit/receive capability, while eliminating the rotating
joints and mechanical devices commonly associated with prior art
feed systems for beam pointing.
A BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of the communications module showing
the composite feed system according to the invention, mounted on
the cover thereof;
FIG. 2 is an isometric view showing the VHF feed element in greater
detail;
FIG. 3 is an isometric view showing a typical arrangement of the
crossed-dipole elements comprising the S- and L-band and UHF arrays
of the composite feed system.
DETAILED DESCRIPTION OF THE DISCLOSURE
FIG. 1 is an isometric view of an Earth Viewing Module (EVM)
showing the communications subsystem location. The composite feed
system according to the invention is mounted on the ground plane of
the communications module and comprises a prime focus antenna feed
subsystem which serves as the interface between the multimode,
multi-frequency transponder subsystem and the parabolic reflector
in the ATS F&G satellite. The subsystem is capable of handling
X-band, C-band, S-band, L-band, UHF and VHF frequencies
concentrated in a small area at the prime focus of the parabola,
with a high degree of radiation efficiency and minimal interaction
between elements. The frequency ranges associated with the bands
are:
Bands Frequency Range X 7.25 - 8.4 GHz C 5.9 - 6.4 GHz S 1.8 - 2.3
GHz L 1.5 - 1.65 GHz UHF 840 - 860 MHz VHF 136 - 150 MHz
The X-band feed should generate an extremely narrow high-gain beam
on the order of 0.3.degree. to provide accurate antenna pointing
and evaluation of reflector performance. The antenna pointing may
be achieved in a linearly polarized monopulse system incorporated
in the X-band feed.
The X-band feed comprises a center-mounted multimode horn 1 having
thin metal walls arranged in a square horn configuration. The
radiating aperture is of reduced cross-section to allow for closer
spacing of the elements comprising the S-band array described
hereafter. The reduced section is dielectric-loaded to support the
necessary modes and to shape the primary radiation pattern.
The manner of feed excitation is not shown herein because it does
not relate to the composite feed subsystem comprising the
invention. This statement also applies to the excitation of other
band and frequency feeds described hereafter. For illustrative
purposes, however, the horn may be excited from a conventional
four-port comparator consisting of waveguide hybrids through a
four-port rectangular-to-square transition to generate the modes
needed for monopulse operation within the horn structure, the
combination of the modes producing the sum and difference primary
patterns.
The C-band feed makes use of a unique design wherein the X-band,
multi-mode horn is shared at C-band frequencies. This provides
coincidence between the C-band beam and the X-band sum channel
beam. The C-band feed uses the high-gain of the reflector to
receive low-level signals radiating from the earth in the
5,900-6,400 MHz frequency band. Two orthogonal, linear and circular
polarizations are incorporated to allow analysis of potential Radio
Frequency Interference (RFI) problems. At C-band frequencies, the
feed may be excited through two shunt ports (not shown) located on
opposite walls of the horn, and the horn is thus excited with two
orthogonal modes.
Multimode horn 1, shared between the X- and C-bands, is shown in
FIG. 1 as being centered at the center of the composite feed
system. The S-band feed portion of the composite feed system
comprises an array consisting of 32 cavity-backed crossed-dipole
turnstile elements mounted in a cross-array coincident with the
composite feed axis. With respect to FIG. 1, the S-band array
comprises arms 2, 3, 4 and 5 arranged in orthogonal relationship in
the same plane about the composite feed axis. The innermost
elements of each arm may form a monopulse feed which can be used
for single satellite tracking, although this is not a specific
requirement of the composite feed system. The illustrated
cross-array arrangement of the 32 turnstile elements comprising the
S-band array provides the advantages of simplicity and cost
reduction.
The S-band feed enables generation of beams to provide
communications between low-altitude satellites including Apollo and
Nimbus. Because of variations in the orbits of these satellites,
the feed must be designed to generate incrementally scanned beams
to permit simultaneous communications to the two satellites. In the
S-band array, each of the 32 turnstile elements is connected to a
diplexer and separate transmit and receive switching networks which
enable a remote selection of feed elements to provide an
independent feed-phase center for generating 32 secondary beams for
the communications coverage in two orthogonal planes from the
composite feed system axis, in an approximate range of
.+-.7.5.degree. .
In order to locate all feed elements in the focal plane, the S-band
array is elevated above the ground plane as shown in FIG. 1. The
S-band diplexers and switching matrices (not shown) are located
inside the communications module to provide a location having good
thermal control for solid state components.
The third element 6 of arm 3 of the S-band array is shared with
elements comprising the L-band array. FIG. 1 shows the L-band array
as comprising a total of seven cavity-backed, crossed-dipole
elements mounted in a line-array 7 to form the fan beam, and a
separate cavity-backed, crossed-dipole element 8 to form the pencil
beam. Line array 7 is parallel to arms 2 and 4 of the S-band array,
and perpendicular to arms 3 and 5 thereof. The individual L-band
elements are similar to individual S-band elements except that they
are larger in size as shown in FIG. 1. In order to provide identity
of elements comprising the L-band array, the third innermost
element 6 of the S-band array is of the same size as the remaining
elements comprising line array 7 and element 8 producing the pencil
beam.
The L-band feed array illuminates the reflector with an elliptical
primary pattern to produce a normal amplitude and constant phase
characteristics in one plane, while in the orthogonal plane, only a
portion of the reflector is illuminated with a constant phase
energy. Asymmetric reflector illumination generates a fan-shaped
secondary beam having an approximate half-power beamwidth defined
by the wedge of 7.5.degree. by 1.1.degree.. Because of the
displacement of the feed from the composite feed axis, the
secondary beam is squinted approximately 3.degree. off the antenna
axis, and beam steering is accomplished by tilting the spacecraft.
The pencil beam, produced by element 8, is symmetrical and as
discussed above, is generated by using a separate single-cavity
backed, crossed-dipole element to minimize switching losses.
The L-band feed illuminates the reflector in a manner which
produces an elliptical beam designed to cover (one-half) of the
hemisphere from synchronous altitude, i.e., the North Atlantic.
This beam is required for the PLACE Experiment which will provide
communications to aircraft traversing this high-density route.
Because of the uncertainty in performance of the aircraft
equipment, the high-gain pencil beam is provided as a backup.
The UHF feed comprises four cavity-backed crossed-dipole radiators
9, 10, 11 and 12 which form an array clustered around the center of
the composite feed axis. The crossed-dipole radiators 9 through 12
are interposed between arms 2 through 5 of the S-band array. The
elements are reduced in size to approximately 0.4 wavelength in
order to allow for closer spacing between the phase centers, thus
obtaining more favorable reflector illumination.
UHF feed generates a circularly polarized on-axis beam, which is
coincident with the X-band monopulse axis. The on-axis beam
eliminates asymmetry, which is always a problem in feed design. The
UHF feed illuminates the reflector to provide a symmetrical beam on
the order of approximately 3.degree.. This finds particular utility
in the experiment designed to transmit educational television
signals to remote areas of Asia, including the country of
India.
A VHF feed also comprises part of the composite feed system to
provide capability of monopulse acquisition of the ground station
and a high-gain link for certain telemetry and command data. As
shown in FIG. 1, it comprises eight quarter-wavelength (.lambda./4)
loops or stubs 13 through 20 arranged to comprise four dipoles
mounted near the outermost ends of adjacent arms of the S-band
array. Thus, loops 19 and 20 comprise a dipole element arranged
near the outermost end of arm 4, for example.
FIG. 2 shows in greater detail, the elements comprising the VHF
feed. Each radiating element (20, for example) extends
perpendicularly upward from the ground plane in a plane parallel to
the wall of the adjacent S-band array for length 21, which is
substantially equal to the height of the top edge of the wall above
the ground plane, as shown in FIGS. 1 and 2. The radiating element
20 then angles out and up from the wall for length 22, and then
extends out from the wall in a perpendicular direction thereto, and
in a plane parallel to the ground plane for length 23. Length 21
and a portion of length 22 of the radiating element, are surrounded
by a coaxial cable to to prevent interference with the adjacent
S-band array arm. Dielectric support 24 functions to support the
outer portion of length 23 from the ground plane. Radiating element
20, in association with radiating element 19, thus forms a dipole
element. The remaining pairs of radiating elements are structurally
and functionally similar to radiating elements 19 and 20 and, for
this reason, are not described herein in detail.
A typical arrangement of the crossed-dipole elements comprising the
S- and L-band and UHF arrays is shown in FIG. 3 of the drawings.
The S- and L-band arrays may comprise an integral unit having walls
and separating partitions defining plurality of cavities housing
the crossed-dipole elements. Lip 35 extends within and around the
cavity to support fiberglass board 36 which is securely attached to
the lips. The radiating arms 37 are mounted in orthogonal
relationship to each other to form the crossed-dipole elements. It
is intended that FIG. 3 serve only as one illustrative embodiment
of a conventional crossed-dipole design and it is to be understood
that other equivalent structures may be substituted therefor.
The composite feed has a fixed relationship with the prime
(longitudinal) axis (axis perpendicular to the EVM cover and also
coincident with the antenna system axis). Adjustment capability is
provided at the junction of the EVM support truss and parabolic
reflector hub to properly relate the feeds and the reflector.
Further, to properly locate the feed phase center at the focal
plane of the reflector, axial adjustment is provided at the
junction of each truss leg and the reflector hub. Adjustment is
also provided to accurately align the focal plane and focal axis of
the reflector with respect to the feeds. The adjustment elements
are not shown because they are conventional in the art and are
discussed herein merely to indicate that such adjustments may be
provided.
The composite feed system provides for the elimination of all
rotating joints and mechanical devices in the feed system for beam
pointing. Specific applications of a composite antenna may not
require one or more of the bands or frequencies disclosed herein,
and the corresponding elements may therefore be eliminated, thus
allowing further optimization of the remaining feeds without
departing from the scope of the invention.
* * * * *