Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
1998-10-06
2002-12-17
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S7810CA, C343S786000
Reexamination Certificate
active
06496156
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to RF receiving antennas, including feeds for such antennas.
BACKGROUND OF THE INVENTION
A typical known receiving antenna includes a parabolic reflector and a corresponding feed horn to guide energy received from a transmitting antenna into a circular waveguide. The energy propagates through the waveguide to an orthomode transducer, which simultaneously extracts horizontally and vertically polarized energy. Such antennas are used in many microwave communications applications, including ground relays and geosynchronous communications satellites, which simultaneously transmit both vertically polarized linear signals and horizontally polarized linear signals on the same frequency allocation. In such applications, it is advantageous to use a receiving antenna that can simultaneously receive both of the respective polarizations, thereby reducing cost complexity and minimizing the space required at the facility at which the receiving antenna is installed.
Referring to
FIG. 1
, a known Newtonian feed antenna system
20
is configured to receive respective horizontally and vertically polarized signals
22
and
24
from a geosynchronous communications satellite transmitter (not shown) along an axis
26
of the antenna
20
. The antenna system
20
generally includes a true parabolic reflector
28
and a feed assembly
30
. The reflector
28
includes a parabolic arc, which causes the respective signals
22
and
24
to reflect from the surface of the reflector
28
towards a focal point
32
, as best depicted in FIG.
2
. The feed assembly
30
includes a circular feed horn
34
, circular waveguide
36
and orthomode transducer (not shown). The feed assembly
30
is supported by a feed assembly support
38
, such that the feed horn
34
is supported at the focal point
32
. Thus, the respective signals
22
and
24
that are directed towards the focal point
32
from the reflector
28
are conveyed down the feed horn
34
to the waveguide
36
, where they are extracted by the orthomode transducer for processing by further receiving circuitry (not shown). In this manner, a single feed antenna is provided with dual-polarization capability.
The dual polarization capability of the antenna
20
, however, presents a problem in that the E-field of a linearly polarized energy distribution across the aperture of a typical feed horn is different in respective vertical and horizontal planes.
FIG. 4
shows a vertically polarized E-field
46
at an aperture
42
defined by a rim
44
of the circular feed horn
34
. For ease of illustration, the aperture
42
is depicted as having respective orthogonal X-, Y- and Z-axes, with the X- and Y-axes being coplanar with the aperture
42
and the Z-axis being perpendicular to and passing through the center of the aperture
42
. As shown in
FIG. 4A
, the magnitude of the E-field
46
is fairly uniform along the X-axis (vertical plane) and terminates at full strength at the rim
44
. As shown in
FIG. 4B
, the magnitude of the E-field
46
along the Y-axis (horizontal plane) is maximum at the Z-axis and terminates to zero at the rim
44
.
As depicted in
FIGS. 5A and 5B
, the differing E-field
46
across the aperture
42
produces a horn radiation gain pattern
48
having a beam width (&THgr;X) as measured in the vertical plane and a beam width (&THgr;Y) as measured in the horizontal plane, which are respectively different. In the vertical plane, where the E-field
46
across the aperture is larger (from rim to rim), the resulting beam width (&THgr;X) of the horn radiation gain pattern
48
is narrower. In the horizontal plane, where the E-field
40
across the aperture
42
is smaller (zero at each rim), the resulting beam width (&THgr;Y) of the horn radiation gain pattern
48
is broader.
Referring to
FIGS. 5A and 5B
, the horn radiation gain pattern
48
produced by the feed horn
34
is directed towards the surface of the reflector
28
and appears on the reflector
28
in the form of a gain contour
50
(depicted in FIG.
6
). The gain contour
50
represents an ideal level of equal gain, typically {fraction (1/100)}th of the peak gain, i.e., −20 dB from the peak gain. The gain contour
50
is optimally coextensive with a rim
52
of the reflector
28
, such that the gain measured from the Z-axis to the rim
52
of the reflector
28
decreases gradually enough that the reflector
28
is fully utilized, while still increasing quickly enough that a substantial amount of energy is not radiated outside the reflector rim
52
and lost behind the reflector
28
.
As depicted in
FIG. 6
, however, the gain contour
50
is not coextensive with the reflector rim
52
. Rather, the gain contour
50
is elliptical in shape, the gain along the X-axis (vertical plane) to decrease too quickly, thereby “underfeeding” the reflector
28
along the X-axis. This mismatch also causes the gain along the Y-axis (horizontal plane) to decrease too gradually, thereby “overfeeding” the reflector
28
along the Y-axis. Because the reflector
28
is “underfed” along the vertical plane, a resulting reflector radiation gain pattern
54
along the vertical plane has a beam width (&phgr;X) that is too broad (as depicted in FIG.
7
), producing a less than ideal antenna gain. Because the reflector
28
is “overfed” along the horizontal plane, the resulting reflector radiation gain pattern
54
along the horizontal plane has a beam width (&phgr;Y) that is relatively narrow (as depicted in FIG.
7
), but a substantial amount of energy is lost behind the reflector
28
, producing a less than ideal antenna gain.
Typically, the feed aperture
42
is sized to adjust the respective breadths of the horn radiation gain pattern
48
as measured in the respective vertical and horizontal planes, i.e., the size of the feed aperture
42
is increased or decreased to respectively narrow or broaden the horn radiation gain pattern
48
in both the vertical and horizontal planes. Because the feed aperture
42
is circular, however, the breadth of the horn radiation gain pattern cannot be adjusted independently for the respective vertical and horizontal planes. Instead, the ideal breadth of the horn radiation pattern in the respective planes and, thus, the ideal gain in the respective planes, must be compromised. Such a problem occurs not only in antenna assemblies such as the antenna system
20
, but in any antenna system that employs a circular feed horn to receive a linearly polarized signal.
FIG. 8
depicts a rectangular feed horn
60
, which addresses this problem. A vertically polarized E-field
66
is shown at an aperture
62
defined by a rectangular rim
64
of the feed horn
60
. For ease of illustration, the aperture
62
is depicted as having respective orthogonal X-, Y- and Z-axes, with the E-field
66
generally polarized parallel and perpendicular to the X- and Y-axes, respectively. The X- and Y-axes are generally coplanar with the aperture
62
and the Z-axis is generally perpendicular to and passes through the center of the aperture
62
. As with the circular feed horn
34
, the magnitude of the E-field
66
is fairly uniform along the X-axis (vertical plane) and terminates at full strength at the rim
64
(depicted in FIG.
8
A), and the magnitude of the E-field along the Y-axis (horizontal plane) is maximum at the Z-axis and terminates to zero at the rim
64
(depicted in FIG.
8
B).
Unlike the circular feed horn
34
, however, the dimensions of the rectangular feed horn
60
can be adjusted to independently vary the breadth of the horn radiation gain pattern in the respective vertical and horizontal planes. That is, the feed horn
60
has dimensions (a) and (b) in the respective vertical and horizontal planes, which can be independently varied to adjust the horn radiation gain pattern in the respective vertical and horizontal planes. Although the E-field
66
along the horizontal plane terminates to zero at the rim
64
, thereby generally creating a broad antenna radiation gain pattern along the horizontal plane, dimensi
Hosono Kazuo
Karp Arthur
Lusignan Bruce B.
Takagi Tohru
Mitsubishi Electric & Electronics USA, Inc.
Wimer Michael C.
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