Note: Descriptions are shown in the official language in which they were submitted.
33~5
Technical Field
The present invention relates generally to microwave
antennas and, more particularly, to reElector-type microwave
antennas having conical feeds.
Background Art
Conical feeds for reElector-type microwave antennas
have been known for many years. For example, a 1963 article
in ~he Bell System Technical Journal describes the selection
of a conical-horn reflector antenna for use in satell'ite
con~unication ground station (Hines et al., "The Electrical
Characteristics oE The Conical Horn-ReElector Antenna", The
Bell System Technical Journal, July 1963, pp. 1187-1211). A
conical horn-reElector antenna is also described in Dawson
U.S. Patent No, 3,550,142 issued December 22, 1970. Conical
feed horns have also been used with large parabolic dish
antennas.
One oE the problems with a smooth-walled conical horn
reflector antenna is that its radiation pattern envelope
(hereinafter referred to as the "RPE") in the E-plane is
substantially wider than its RPE in the H-plane. When used
in terrestrial communication systems, the wide beamwidth in
~he E-plane can cause interference with signals from other
antennas. Also, when a smooth-walled conical horn is used
as the primary Eeed for a parabolic dish antenna, its
different beamwidths in the E and H-planes make it difficult
to achieve symmetrical illumination of the parabolic dish.
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In Canadian Patent No. 1185696 issued April 16, 1985,
by Knop, Ostertag, Matz and Cheng for "Reflector-Type
Microwave Antennas With Absorber-Lined Conical E'eed," there
is described an improved horn-reflector antenna having a
lining of absorber material within the conical feed horn.
That antenna produces narrower E-plane RPE's, thereby
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s
bringing the E-plane and H-plane RPE's closer together, with-
out significantly degrading other performance characteristics
of the antenna.
_escription of the Invention
It is a primary object of the present invention to provide
an economical and effective way to achieve further narrowing
of the E-plane RPE of a horn-reflector antenna having a conical
feed, without significantly degrading the H-plane RPE or any
other performance characteristic of the antenna. In this
connection, a related object of this invention is to provide
an improved conical feed which is capable of bringing the
RPE ' s in both the E and H-planes even closer together.
It is another important object of this invention to
provide an improved horn-reflector antenna which introduces
only a small gain drop into the microwave syste~ in which it
is used.
It is yet another object of this invention to provide
such an improved horn-reflector antenna which can be effi-
ciently and economically fabricated.
Other objects and advantages of the invention will be
apparent from the following detailed description and the
accompanying drawings.
In accordance with one aspect of the present invention,
certain of the foregoing objects are realized by a horn-
reflector antenna in which the lower end portion of the înside
surface of the conical horn lS formed by a smooth metal wall,
and the balance of the inslde surface of the conical horn is
formed by a layer of absorber material, the surfaces of the
metal wall and the absorber material defining a single con-
tinuous conical surface. The absorber material increases
the Eigen value E and the spherical hybridicity factor Rs
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sufficiently to cause the E-plane and H-plane RPE's to
approach each other.
In accordance with another aspect of the invention, the
cost of a horn-reflector antenna having an absorber lining in
the conical feed horn is reduced by providing the absorber
lining on only the opposed walls of the feed horn that affect
the patterns of the antenna in the horizontal plane.
Brief ~ Drawinqs
FIGURE 1 is a front elevation, partially in section, of a
horn-reflector antenna embodying the present invention;
FIG~ 2 is a vertical section taken along line 2-2 in
FIGURE 1;
FIG. 3 is a perspective view of the antenna illustrated
in FIGURES 1 and 2, with various reference lines superimposed
therein;
FIG. 4 is an enlarged end view of one of the pads of
absorber material used to form an absorber lining in the
conical section of the antenna of FIGURES 1-3;
FIG. 5 is a vertical section, similar to FIG. 2, of a
modified horn-reflector antenna embodying the present inven-
tion; and
FIG. 6 is a section taken generally along line 6-6 in
FIG. 5.
~ hile the invention will be described in connection with
certain preferred embodiments, it will be understood that it
is not intended to limit the invention to those particular
embodiments. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included
within the spirit and scope of the invention as defined by the
appended claims.
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Description of_the Invention
Turning now to the drawings and referring first to
FIG~RES 1 and 2, there is illustrated a conical horn-reflector
microwave antenna having a conical section 10 for guiding
microwave signals to a parabolic reflector plate 11. From the
reflector plate 11, the microwave signals are transmitted
through an aperture 12 formed in the front of a cylindrical
section 13 which is attached to both the con ! cal section 10
and the reflector plate 11 to form a completely enclosed
integral antenna structure.
The parabolic reflector plate 11 is a section of a para-
boloid representing a surface of revolution forr~ed by rotating
a parabolic curve about an axis which extends through the
vertex and the focus of ~he parabolic curve. As is well
known, any microwaves originating at the focus of such a
parabolic surface will be reflected by the plate 11 in planar
wavefronts perpendicular to the axis 14. Thus, the conical
section 10 of the illustrative antenna is arranged so that its
apex coincides with the focus of the paraboloid, and so that
the axis 15 of the conical section is perpendicular to the
axis of the paraboloid. With this geometry, a diverging
spherical wave emanating from the conical section 10 and
striking the reflector plate 11 is reflected as a plane wave
which passes through the aperture 12 and is perpendicular to
the axis 14. The cylindrical section 13 serves as a shield
which prevents the reflector plate 11 frorn producing interfer-
ing side and back signals and also helps to capture sorne
spillover energy launched from the conical section feed. It
will be appreciated that the conical section 10, the reflector
plate 11, and the cylindrical shield 13 are usually formed of
conductive metal (though it is only essential that the reflec-
tor plate 11 have a metallic surface).
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To protect the interior of the antenna from both the
weather and stray signals, the top of the reflector plate 11
is covered by a panel 20 attached to the cylindrical shield
13. A radoMe 21 also covers the aperture 12 at the front of
the antenna to provide further protection from the weather.
The inside surface of the cylindrical shield 12 is covered
with an absorber material 22 to absorh stray signals so that
they do not degrade the RPE. Such absorber materials are
well known in the art, and typically comprise a conductive
material such as metal or carbon dispersed throughout a dielec-
tric material having a surface in the form of multiple pyra-
mids or convoluted cones.
In keeping with the present invention, the lower end
portion of the inside surface of the conical feed horn is
formed by a smooth metal wall, and the balance of the inside
surface of the horn is formed by a layer of absorber material,
the surfaces of the metal wall and the absorber material
defining a single continuous conical surface. Thus, in the
illustrative embodiment of FIGS. 1-3, the bottom section lOa
of the conical feed horn 10 has a smooth inside metal surface.
The balance of the inside surface of the conical horn 10 is
formed by an absorber material 30, with the innermost surfaces
of the metal section lOa and the absorber material 30 defining
a single continuous conical surface. To support the absorber
material 30 in the desired position and shape, the metal wall
that forms the lower horn section lOa forms an outwardly
extending shoulder lOb at the top of the section lOa, and then
extends upwardly along the outside surfac2 of the absorber 30.
This forms a continuous conical metal shell lOc along the
entire length of the absorber material 30. At the top of the
absorber material 30, the metal wall forms a second outwardly
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extending shoulder 10d to accommodate a greater thickness oE
the absorber material 22 which lines the shield portion of
the antenna above the conical feed horn.
This recessed arrangement of the absorber material 3
permits further narrowing of the E-plane RPE and/or
reductiotls in the gain drop of the antenna as compared with
the structure shown in the aforementioned copending
application Serial No. 267,267. More specifically, for a
given gain dropr the structure of the present invention
permits the absorber-material to be extended farther down
into the throat of the conical feed horn 10, thereby further
narrowing the E-plane RPE. On the other hand, for a given
RPE (in other words, if it is desired to minimize the gain
drop of the antenna), the metal surface of the section 10
can be extended farther up from the bottom of the conical
feed horn so that the narrowness of the E-plane RPE is
essentially the same as that produced by the structure
described in the issued Canadian Patent No. 1185696, but at
the same time reducing the gain drop relative to that of the
structure described in said copending application.
The lining 30 may be formed from conventional absorber
materials, one example of which is AAP-ML-73 absorber made
by Advanced Absorber Products Inc., 4 Poplar Street,
Amesbury, Maine. This absorber material has a flat surface,
as illustrated in FIG. 4 (in contrast to the pyramidal or
conical surface of the absorbed used in the shield), and is
about 3/8 inches thick. The absorber material may be
secured to the metal walls of the antenna by means of an
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adhesive. When the exemplary absorber material identified
above is employed, .is is preferably cut into a multiplicity
of relatlvely small pads which can be butted against each
other to form a continuous layer of absorber material over
the curvilinear surface to
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which it is applied. This multiplicity of pads is
illustrated by the grid patterns shown in FIGS. 1-3.
In accordance with a further aspect of the present
invetltiorl, the absorber material 30 is provided only on the
two diametrically opposed regions of the interior walls oE
the conical horn 10 that affect the patterns of the antenna
in the horizontal plane. In terrestrial communication
systems, the only significant patterns of the antenlla are
those taken in the horizontal plane, which is the ~-Z plane
in FIG. 3. That is, ~or a horizontally polarized signal,
the Y-Z plane is the E-plane, and the X-Z plane is the H-
plane; for a vertically polarized signal, the Y-Z plane is
the H-plane, and the X-Z plane is the E-plane. The portions
of the conical feed horn 10 that principally affect the E-
plane RPE (of a horizontally polarized signal) are the left
and right hand walls o~ the horn through which the X-Y plane
extends. Thus, as illustrated in FIG. 5, the absorber
material 30 can be limited to diametrically opposed regions
40 of the inside sur~ace of the eed horn. Restricting the
absorber material in this manner reduces the cost of the
antenna by reducing both the amount of absorber material
required and the labor required to install the absorber
lining within the conical horn~
When the absorber material 30 does not ext~nd around
the entire circumference of the horn 10, the absorber can be
recessed (flush mounted) into the horn wall in the two
regions 40 so as to maintain a single continuous conical
surface on the inside of the horn 10. Alternatively, the
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metal wall can form the entire conical surface, as in the
structure described in the issued Canadian Patent ~o.
1185696, and the absorber material 30 applied only to the
limited regions 40 on the inner surface thereof. These
constructions will not
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offer the full advantages o~ the recessed absorber
arrangement illustrated in FIGS. 1-3, but they reduce the
manufacturing cost of the antenna.
As described in the aforementioned issued Canadian
Patent No. 1185696, the absorber material 30 within the
conical section 10 causes the field distribution within the
cone to taper off more sharply adjacent to the inside
surface of the cone, due to the fact that the wall impedance
of the absorber lining tends to force the perpendicular E
field to zero. Furthermore, it does this while abstracting
only a small fraction of the passing microwave energy
propagating through the cone.
There is a substantial difference in the taper or
dropoff of the field distributions in the E and H-planes in
the absence of the absorber material 30. With the absorber
material 30 in the horn, the E-plane field distribution
tapers off much more sharply, approaching that of the H-
plane field, while there is only a slight degradation in the
H-plane taper which brings it even closer to the E-plane
field. In the theoretically ideal situationr the profile of
the E-plane field distribution would coincide with that of
the H-plane. In actual practice, this theoretically ideal
condition can only be approximated, though the approximation
is c]oser with the present invention than with the structure
described in issued Canadian Patent No. 1185696.
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Mathematically, the operation of the feed horn can be
characteri~ed as follows. If we let E~ (r, ~, ~) and
E~ (r, ~, ~) be the polar and azimuthal components of
electric ield (with the origin at the apex of the cone, and
and ~ the polar and azimuthal angles, respectively) then,
it can be shown that they can be mathematically expressed
as:
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345
(1) E9 (r,~,0) = A f(w) C05
(2) E0 (r,~ A g(w) sin~
where
(3) A = Eo exp(-jkr)/kr
Eo - Arbitrary driving constant, k = 2 ~ ~, ~ = free space
operating wavelength and the functions f(w) and g(w) are given
by:
(4) f(w) = Jl(X)/X + RsJ'l(X)
15) g(w) = ~SJl(X)/X+J'l(X)
with
(6) X = E ~/aO
(7) JltX) = Bessel function of Order 1, argument X
(8) J'l(X) = Derivitive of Jl(X) with respect to X
One then notes that the fields are uniquely known for the
range of 0 ~ 6 aO and 0 ~ 0 '- 360 if the parameters E (the
Eigen value) and Rs (the spherical hybridicity factor) are
known~ These parameters are uniquely determined by the nature
of the conical wall material.
No Absorber
For no absorber present one can show that E = 1.84 and
Rs = 0, thus giving:
(~) f(w) = Jl(1.84 0/aO)/(1.84 ~/aO)
(10) g(w) = J'1(1.84 ~/aO)
where a~plitude distributions (in dB nornalized to on axis,
0= 0) are shown as the solid lines in Fig. 6 (Note: E-plane
= -201Ogl0/ f(w)/f(w)l w = 0/ H plane = -201Ogl01 g(w3/g(w)/
w = 0/3.
Perfect Absorber
For the perfect absorber case (also a corrugated horn with
quarter wave teeth) it can be shown thak E = 2.39, Rs = +1,
_g_
.
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thus giving
(11) f(w) = g(w) = JO (2.39 ~/ aO), perfect absorber
where the identity
(12) Jl(X)/X + J 1(~) o
has been used, with Jo(X) = Bessel function of order zero,
argument X. One notes that the dB plot of (11) is virtually
identical to that of (10), thus showing that the H-plane of
the smooth wall and perfect absorber wall are virtually identi-
cal. Also, for this perfect absorber case, we then see that
the E-plane is identical to the H-plane.
Actual Absorber
An actual absorber has E difEering from the no absorber
case of 1.84 and the perfect absorber case of 2~39, with a
hybridicity factor, Rs, neither zero (no absorber) or unity
(perfect absorber). In general both will be complex with
finite loss in the absorber.
The RPE improvements described above can be achieved over
a relatively wide frequency band. For example, the improve-
ments described above for the antenna illustrated in FIGS. 1-3
can be realized over the common carrier frequency bands co~-
monly referred to as the 4 GH2, 6 GHz and 11 GHz bands.
Absorber materials are generally characteriæed by three
parameters: thickness, dielectric constant, and loss tangent.
The absorber used in the present invention must have a thick-
ness and loss tangent sufficient to suppress undesirable
surface (slow) waves. Such surface waves can be readily
generated at the transition fror.l the metallic portion of the
inside surface of the cone wall to the absorber lined portion
of the cone wall, but these waves are attenuated by the
absorber so that they do not interfere with the desired field
pattern of the energy striking the reflector plate 11. The
end result is that all the improvements descri~ed above are
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attained without producing any undesirable distortion in the
field patterns. The narrowing E-plane effect can, in ~act, be
achieved with zero loss tangent material, but with no loss the
surface waves are not attenuated and the operating bandwidth
is reduced. Consequently, it is preferred to use an absorber
materal with some loss.
Although the invention has been described with particular
reference to a horn reflector antenna, it will be appreciated
that the invention can also be used to advantage in a pri~ary
feed horn for a dish~type antenna. Indeed, in the latter
application the substantially equal main beam widths in the E
and H planes provided by the absorber lined feed horn are
particularly advantageous because they provide symmetrical
illumination of the parabolic dish. The consequent approxi-
mately equal second patterns with their reduced sidelobes,
over a wide bandwidth, and with negligible gain loss, are also
important in this primary feed horn application.
Although the invention has thus far been described with
particular reference to a conical feed horn feeding a reflector
antenna, it can be appreciated that use of absorber lining on
pyramidal (or other shapes) feed horns feeding a reflector
antenna will produce the same desirable effect [i.e., narrow-
ing of the E plane RPE to make it approximately e~ual to the
H-plane RPE).
