Note: Descriptions are shown in the official language in which they were submitted.
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- ;. Background of the Invention:
; Field of the Invention:
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This invention relates to the improvement of a steerable aerial
system (an antenna system) including a beam-waveguide feeder system
employing rotationally asymmetric reflectors for the purpose of trans-
mitting electromagnetic waves between a primary feed point and the aerial
; itself through one or more axes of rotation, with particular application to
a microwave aerial for use with a satellite communications system.
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Description of Prior Art:
A previous aerial system for an earth station of a satellite
communication system has been composed of a dual-reflector aerial
such as a Cassegrainian or Gregorian type having a main reflector,
a subreflector and a primary feed for supplying microwave power to the
aerial, which feed is coupled to the dual-reflector aerial by a beam wave-
guide comprising two concave reflectors and two plane reflectors so
disposed as to couple the microwave power between the dual-reflector
aerial, which together with one of the plane reflectors is capable of being
rotated about an elevation axis, and the primary feed located on a fixed
mount low on the aerial system structure, while permitting the dual-
reflector aerial and beam waveguide together to be rotated about an
azimuth axis In an endeavour to minimize power losses from the beam,
undesirable cross-polarisation effects and aerial beam asymmetry, a
beam-waveguide fed aerial system has heretofore employed two rotationa-
lly asymmetric concave paraboloid reflectors in fixed mirror-image rela-
tion to each other and two plane reflectors so mounted as to turn the beam
.; through a right angle without distortion at the axes of rotation of the aerial
system. Such a system using a pair of paraboloid reflectors presupposes
the generation of a parallel beam by these reflectors, whereas in practice,
- due to diffraction effects resulting from the fact that the reflectors used
are not extremely large relative to the wavelength used, there is in effect
a defocussing of the beam which causes spilling over of the microwave
power leading to a reduction in aerial performance and potential hazard
to maintenance staff from stray microwave radiation. In order to
overcome this effect when using such an arrangement of paraboloid
reflectors, or any other geometrically-derived arrangement, it is nece-
ssary to enlarge the reflectors to prevent spillover, thus causing an
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undesirable increase in the size of the structure. Further, the use of
two mirror-image curved reflectors in fixed relationship to each other is
intended to ensure that the undesirable wave distortion introduced by
the first curved reflector is completely cancelled by the second curved
reflector, thereby maintaining aerial beam symmetry and avoiding cross-
polarisation distortion and tracking errors. However, due to the diver-
gence of the beam, the cross-polarisation introduced at the second
reflector is greater than that at the first and cancellation is incomplete
at the second reflector. While this effect is generally not large enough
to cause serious degradation of performance due to noise pick-up or
tracking errors, it does introduce a significant cross-polarised compo-
nent into the transmitted wave which is undesirable, especially in view
of the introduction into satellite communications systems of a method of
frequency-spectrum re-use involving the transmission or reception of a
pai.r of independent signals sharing a common frequency and discriminated
only by having orthogonal polarisations, whether linear or circular.
Summary of the Invention:
An object of the present invention is therefore to improve the
cross-polarisation characteristics of a beam-waveguide fed aerial system,
thereby enhancing its performance, especially when used with a frequency
re-use satellite communications system employing orthogonal wave
polarisations .
Another object of the present invention is to improve the beam
focussing within a beam-waveguide fed aerial system, thereby enabling the
- necessary performance to be obtained within smaller overall dimensions
than would otherwise be possible.
In accordance with the present invention, there is provided a
steerable microwave aerial system wherein the microwave energy is
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conveyed between a moveable aerial portion and a fixed portion containing
a primary feed, by means of a four-reflector beam waveguide, with the
four-reflector beam waveguide together with the moveable aerial portion
being rotatable relative to the fixed portion about a first axis and the
moveable aerial portion being further rotatable about a second axis, the
four-reflector beam waveguide comprising a first curved reflector
mounted on the first axis, a second curved reflector, a third curved ref-
lector mounted on the second axis and a plane reflector also mounted on
the second axis but being rotatable together with the moveable aerial
portion about the said second axis relative to the remainder of the four-
reflector beam waveguide, the reflectors being so disposed and the curved
reflectors being of such curvatures as to reflect without loss due to spill-
over microwave radiation emanating from a focal point of the primary
feed lying on the first axis via the first curved reflector to the second
curved reflector, thence to the third curved reflector, from there along
the second axis to the plane reflector and thence to a focal point of the
moveable aerial portion lying in a plane normal to the second axis, while
the mode of transmission of the microwave radiation is maintained un-
changed at the focal points of the primary feed and the moveable aerial
portion .
Brief Description of the Drawings:
An embodiment of the present invention will now be described
in detail by way of example, with reference to the accompanying drawings,
in which:
Figure 1 shows in longitudinal section the construction of a
prior-art dual-reflector aerial system employing a four-reflector beam-
waveguide feed using two plane and two curved reflectors;
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Figures 2(a) and 2(b) show the electric field distributionsexpected from application of classical ray-optical analysis to exist within
the prior-art four-reflector beam waveguide system;
Figures 3(a), 3(b) and 3(c) show the electric field distributions
existing within the prior-art four-reflector beam waveguide system;
Figures 4(a), 4(b) and 4(c) show the electric field distributions
within a four-reflector beam waveguide system constructed according to
the present invention;
Figure 5 shows in longitudinal section an embodiment of the
present invention;
Figure 6 shows in longitudinal section an embodiment of the
present invention, with illustration of the curved ray paths predicted
according to the principles of wave theDry.
Detailed Description of the Preferred Embodiments:
Referring to Figure 1, which shows in longitudinal section a
prior-art beam-waveguide fed aerial system, the primary feed 1 com-
prising an electromagnetic horn, having its aperture marked la in the
figure, is placed with its focal point coincident with focal point F1 ' of the
beam waveguide system. This focal point F1 ' is the image of focal point
F1 of paraboloid reflector 3 as reflected by plane reflector 2. A-A' is
the azimuth axis of the aerial system. The transmitted electromagnetic
wave travels from the primary feed 1 to the first plane reflector 2
where it is directed towards the first paraboloid reflector 3. Since
paraboloid reflector 3 lies obliquely to the incident direction of the
wave, the wave is distorted on reflection. In order to cancel out this
distortion, the second paraboloid reflector 4, lying on the elevation axis
of the aerial system, is made to be a mirror image in plane X-X' of the
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first paraboloid reflector 3. The wave directed to the second paraboloid
reflector 4 from the first paraboloid reflector 3 is thus reflected
towards focal point F2 with the distortions introduced by the two oblique
paraboloid reflectors largely cancelled out due to their symmetrical
disposition. Between the second paraboloid reflector 4 and the focal
point F2 is interposed a second plane reflector 5 which re-directs the
wave to a new focus at F2' which is arranged to coincide with the focal
point of a dual-reflector aerial 6a, 6b which may be of Cassegrainian or
Gregorian type, or one of the constant aperture-phase microwave analogues
of either of these two types. Since the focal points of the primary feed
and the dual-reflector aerial coincide with those of the four-reflector
beam-waveguide feed and the distortions in the beam waveguide system
are largely cancelled out, it is almost as if the primary feed were locat-
ed at the focus of the dual-reflector aerial and an aerial system having
good performance characteristics can be constructed, with the advantage
over other types that the primary feed 1 and the transmitting and receiv-
ing equipment 7 desired to be connected closely with it can be located in
a stationary room at or near ground level thereby allowing easy access
for operation and maintenance, while the aerial can be steered as required
to point towards the satellite. The type of beam-waveguide system des-
cribed above has therefore been used in high-performance aerial systems
having easy access to the transmitting and receiving equipment
The prior-art four-reflector beam waveguide system has,
however, been disadvantageous in that no account is taken of diffraction
effects arising from the fact that in a practical microwave aerial system
the reflectors used in a beam waveguide system feeding it cannot be
greater than about 20 wavelengths across. In general, when an electro-
magnetic wave is reflected from a surface whose dimensions are not
considerably greater than one wavelength, the reflected rays diverge,
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due to the effects of diffraction, from the paths predicted by classical
straight-line ray optics, and it is found that a reflector which by this
theory would be expected to produce a focussed parallel beam in fact
produces a divergent beam. Thus if, as in Figure 1, a paraboloid
reflector 3 is used with the intention of focussing a wave emanating
from its focal point F1 into a parallel beam incident upon a second ref-
lector 4 of equal dimensions, the focussing is imperfect and a part of
the energy, represented by divergent curved rays "a" and "b", inevitably
misses the second reflector, causing what is known as"spillover". This
effect is further compounded on reflection of the beam towards the second
plane reflector 5 due to the divergent, as opposed to parallel, nature of the
beam incident upon the second paraboloid reflector 4 causing further
defocussing. This has deleterious effects on the performance of the aerial
system, especially in respect of beam misalignment, signal loss and
increased noise and susceptibility to interference arising both from within
the beam waveguide and from the resulting increase in aerial sidelobe
level, while the stray microwave energy is potentially hazardous to operat-
ing staff. To combat this effect in order to obtain adequate aerial system
performance, a beam-waveguide fed aerial system constructed according
to the prior art has to use larger reflectors than would otherwise be
necessary, which is disadvantageous in that a larger supporting structure
has to be used and the protective shield within which a beam waveguide is
commonly enclosed is also necessarily enlarged, Further, if the first
paraboloid reflector 3 is arranged obliquely relative to an axially symmet-
ric incident wave hàving the electric-field distribution shown in Figure 2(a),
the reflected wave will no longer be axially symmetric and a cross-
polarised component 10 shown in Figure 2(b) will be superimposed upon
the principal wave 9. By application of straight-line ray optics it is predictedthat placing a second paraboloid reflector 4 in a mirror-image relation-
ship to the first will cause complete cancellation of this distortion and
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it is the intention of a beam waveguide system constructed according to
the prior art so to restore the electric field configuration to that of
Figure 2(a) upon reflection from the second reflector. Now it can be
shown by the mathematical technique known as Spherical Wave Expansion
that the magnitude of the cross-polarised component introduced into a
wave by an oblique asymmetric curved reflector increases as the area
of the reflector illuminated increases (For an exposition of this technique,
reference is made to a paper by R Ludwig Published in the Transactions
of the Institute of Electrical and Electronics Engineers, of New York
U.S.A. Volume AP-19, No. 3, Page 214, March 1971). Since in the
prior-art beam-waveguide system the beam between the two paraboloid
reflectors 3 and 4 in Figure 1 is divergent, a greater area of the second
paraboloid reflector 4 is illuminated than is the case with the first
paraboloid reflector 3 and therefore a larger degree of cross-polarisation
is induced at the second such reflector than was induced at the first.
The effect on transmission is shown in Figures 3 (a), 3 (b) and 3 (c), where
Figure 3(a) shows the transverse electric field distribution of the axially
symmetric wave incident upon the first curved reflector 3 of a beam
waveguide system constructed according to Figure 1 wherein the first
reflector 2 is plane. On renection from the first paraboloid reflector 3
towards the second paraboloid reflector 4 the wave is distorted due to the
oblique disposition of the first paraboloid reflector and contains a cross-
polarised component 10 as shown in Figure 3(b) superimposed upon the
principal wave 9. However, instead of introducing an equal and opposite
compensatory distortion to the wave as intended, as shown above the
second paraboloid reflector 4 introduces a distortion which is greater than
that produced by the first paraboloid reflector 3 and the result is to cause
` in effect an over-compensation leading to the wave reflected from the
second paraboloid reflector 4 towards the second plane reflector 5 contain-
ing a residual cross-polarised component 10 as illustrated in Figure 3(c).
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This causes an undesirable asymmetry in the beam radiated by the aerial
system, a degradation due to the spurious generation of higher transmi-
ssion modes of the performance of any tracking system which operates
by detecting such modes, and especially, in the event that the aerial is to
be used in a communications system where different signals at the same
frequency are distinguished by having orthogonal polarisations, undesirab-
le mixing of the orthogonally-polarised signals leading to a degradation
in the performance of the communications system taken as a whole.
The present invention is thus aimed at the application to a four-reflector
beam waveguide of new methods whereby spillover within the reflector
system is reduced to a minimum while making the most economical use
of the reflector area, and cancellation of the aberrations introduced by the
oblique curved reflectors is improved. The improved beam-waveguide fed
aerial system and the methods used to obtain these improvements will now
be described by means of the drawings.
An embodiment of the present invention is shown in longitudinal
section in Figure 5. While this embodiment is described herein as it is
applied to a transmitting aerial system, it is also applicable to a receiv-
ing aerial system, An electromagnetic wave generated by a transmitter
located in the communications equipment room 7 is radiated from an
electromagnetic horn 1 such that it forms an axially symmetric spherical
wave with its apparent origin at focal point F1 ' . The wave is then reflec-
ted through an angle of ninety degrees by an offset hyperboloid reflector
8 which is so shaped that an exact, predetermined amount of distortion is
introduced into the wave. Subsequently the wave is further reflected by
a pair of offset ellipsoid reflectors 9 and 10 which are arranged to be
mirror images of each other. By further reflection at a plane reflector
5 the wave is brought to a focus at focal point F2' which is arranged to
coincide with the focal point of the dual-reflector aerial 6a, 6b with the
result that the wave reflected onto the main reflector 6a from the
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subreflector 6b is then reflected along the axis of the main reflector in
the form of a narrow-beam plane wave.
Since even a so-called geostationary satellite moves periodica-
lly with respect to a point on the earth's surface, it is necessary to change
the direction to which the axis of the main reflector, and hence that of the
transmitted beam, points. It is possible to do this without distorting the
transmitted beam by ensuring that the axes of rotation of the aerial system
act at points within the beam-waveguide feeder at which the beam is
axially symmetric. Thus in Figure 5 if main reflector 6a, subreflector
6b and plane reflector 5 are kept in fixed relation to each other and are
together rotated about a horizontal axis B-B', it is possible to alter the
angle of elevation of the transmitted beam without distortion since the wave
incident on the plane reflector from ellipsoid reflector 10 is arranged to
have symmetry about axis B-B' and the plane reflector also has symmetry
about this axis, Further, since the wave emanating from the horn 1 has
axial symmetry it is possible to rotate the whole system of reflectors
together about a vertical axis A-A' relative to the fixed horn and equip-
ment room 7 without distorting the transmitted wave, provided only that
the spatial relationship between reflectors 8, 9 and 10 and axis B-B' is
kept fixed. Thus steering of the aerial beam in both azimuth and elevation
axes is possible with this embodiment of the present invention without
degradation of performance.
As stated above, in the present invention the curved reflector
9 shown in Figure 5 is an ellipsoid. If straight-line ray optics could be
applied, a wave emanating from its first focal point Fl would be brought
to a focus at its second focal point F3 as shown by the produced straight-
line rays "c" and "d". The beam thus reflected from this reflector would
therefore be convergent. However, since this reflector is in the order of
20 wavelengths across, classical straight-line ray optics does not apply
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other than approximately and the beam is found to diverge from the
straight-line ray paths as drawn. Since the wave reflected from such
an ellipsoid thus has opposing convergent and divergent tendencies, it
is therefore possible by selection of a suitable curvature for reflector
9 to be able to arrange that a second reflector of the same size can be
placed at such a distance from the first that it is fully illuminated by
the reflected wave without spillover. Such an arrangement is shown in
Figure S where reflectors 9 and 10 are both ellipsoids being mirror
images of each other and so placed that reflector 9 if illuminated by a
wave emanating from its focal point Fl will illuminate the whole area of
reflector 10 without spillover, while reflector 10 if illuminated from its
focal point F2 will in turn similarly fully illuminate reflector 9 without
spillover. The system is therefore fully reciprocal and yet allows fullest
use to be made of the whole area of each reflector in both transmission
and reception modes. In aerial systems parlance this arrangement of
reflectors is said to be "efficient", Figure 6, which shows a further
longitudinal section of this embodiment of the present invention, with
the numbering of the several parts following that of Figure 5, shows a
notable feature of this invention, namely that the combination of conver-
gent and divergent properties within the one beam causes the beam pass-
ing between reflectors 9 and 10 and delineated by curved rays "e" and
"f" to exhibit a pronounced waist at a point mid-way between the two
reflectors when their geometric relationship is as described above.
Further, by this means efficient illumination without spillover of the
plane reflector 5 is also assured, The two ellipsoid reflectors 9 and 10
are arranged to be mirror images of each other about a plane mid-way
between them and normal to their mutual axis in order to take the fullest
advantage of the inherent tendency for such a geometrically symmetric
pair of oblique curved reflectors to cancel out at the second the aberrations
introduced into the electromagnetic wave by the first.
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However in practice, even with the use of ellipsoid reflectors somebeam divergence is unavoidable so, as shown above, this pair of
reflectors does not have true electromagnetic symmetry and the trans-
mitted wave contains an undesirable residual cross-polarised compo-
nent, Replacement of the first plane reflector 2 of Figure 1 by the
hyperboloid curved reflector 8 of Figure 5 introduces the ability to
control the cross-polarised component actively rather than to rely
passively on natural cancellation of this unwanted wave. In Figure 4(a)
is shown the electric field distribution of the axially syrnmetric wave
incident upon the oblique h,yperboloid reflector 8 of Figure 5 from the
primary horn 1, where it is seen that at this point no cross-polarised
component exists. Since the curved reflector 8 does not have symmetry
about axis A-A' a cross-polarised component 10 in Figure 4(b) is super-
imposed upon the principal component 9. By careful selection of the
curvature of the hyperboloid reflector 8, this cross-component is made
to have an exact, predetermined value. The wave is reflected from the
hyperboloid reflector 8 towards the first oblique ellipsoid reflector 9,
which upon reflecting the wave towards the second oblique ellipsoid
reflector 10 in turn introduces further cross-polarisation distortion due
to its oblique disposition. As shown in Figure 4(c), the sum total of the
cross-polarised component 10 is now therefore greater than that which
would be produced by reflector 9 of Figure 5 acting alone. The distorted
wave then impinges upon the second oblique ellipsoid reflector 10 and is
directed towards the plane reflector 5. Since the cross-polarisation
distortion introduced by the second oblique ellipsoid reflector is greater
than that introduced by the first oblique ellipsoid reflector acting alone,
as explained above, it is possible by arranging for the hyperboloid
reflector 8 to add the correct amount of distortion to that of the first
ellipsoid reflector 9 to secure complete cancellation of the aggregate
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cross-polarised component by the second ellipsoid reflector 10.
Thus the wave incident upon the plane reflector 5 from the second oblique
ellipsoid reflector 10 contains no cross-polarised component and its electric
field distribution is as in Figure 4(a) Since the plane reflector 5 has
symmetry about axis B-B' it introduces no cross-polarisation distortion
and the axially symmetric beam radiated by the primary horn 1 is recon-
stituted as an axially symmetric beam focussed on focal point F2'.
Since the focal point F2' is made to coincide with that of the dual-reflector
aerial 6a, 6b, the beam is then radiated into space as an axially symmetric,
directed plane wave along the axis of the main reflector 6a. Thus for the
first time is produced a four-reflector beam-waveguide feeder system for
a satellite communications aerial system wherein, by application of the
principles of electromagnetic wave theory, spillover of microwave energy
from the reflectors used within the system iB prevented while permitting
efficient illumination of the reflectors, and by the use of a new invention
giving positive control over the cross-polarisation distortion arising from
the use of oblique curved reflectors, true axial symmetry and therefore
mode purity of the transmitted microwave beam is obtained, along with
complete suppression of cross-polarised waves generated within the
beam-waveguide feeder system.
Although the operation of the present invention is explained
primarily in terms of a transmitting aerial system, an illustration of its
application to a receiving aerial system is also in order, As explained
above and illustrated in Figure 6, due to the finite size of the reflectors
relative to a wavelength, the rays within the beam waveguide system are
not straight but curved. Therefore at focal point Fz' the transmitted
wave is not in fact brought to a true point focus but instead is distributed
over a closely defined area within a plane normal to the beam axis and
containing point F2', within which area there is a fixed amplitude and
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phase distribution of the energy within the wave. In order to make
efficient use of this energy and to produce the desired narrow-beam
directed plane wave along its axis, the dual-reflector aerial comprising
main reflector 6a and subreflector 6b is designed according to the princi-
ples of wave theory and therefore deviates in shape from the geometrical
designs of Gregory and Cassegrain. Such an aerial will, upon being
irradiated by a plane wave indicent upon its axis, focus that wave into
an area in the plane of focal point F2' such that the spatial distribution
of the energy within that area is identical to that of the wave fed to the
aerial during transmission. Feeding the beam waveguide from the region
of focal point F2' with this distributed wave as distinct from a spherical
wave emanating from F2' has the eff~ct that the amplitude and phase
distributions of the energy within the wave are such that the process
occurring during the transmission mode is exactly reversed and the
wave is brought to a point focus at Fl ' with its axial symmetry and
mode purity unchanged. This is in full accordance with the principle of
reciprocity of a lossless network and an aerial system designed according
to the principles of wave theory and incorporating a beam-waveguide
feeder system constructed according to the present invention is said to
be "reciprocal" and will both transmit and receive microwave radio
signals without loss or distortion
The present invention is found to confer a further advantage in
addition to the original aims of the invention in that, although for the
purpose of controlling cross-polarisation distortion it is possible to use
a convex hyperbolic reflector for reflector 8 in Figure 5, if in fact this
reflector is made concave as is shown in Figure 5, it provides a degree
of focussing of the beam in addition to its prime function of correcting
axial asymmetry, As a result the distance from focal point Fl ' along
the axis of the beam to the first ellipsoid reflector 9 is reduced relative
to the case where the first reflector in the system is plane. Thus for a
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given set of reflectors 8, 9, 10, 5 the necessary feed horn 1 is closer
to the first reflector 8 and has a smaller aperture dimension la than
would be the case if a plane first reflector were to be used, making for
a smaller, lighter and more convenient horn assembly, Alternatively,
for a given size of horn it is possible for the two ellipsoid reflectors
9 and 10 and plane reflector 5 to be reduced in size relative to the sizes
necessary if the first reflector were plane. It is also possible to take
partial advantage of both of these benefits in order to arrive at the system
providing the best possible configuration for a given application,
Obviously, numerous additional modifications and variations
of the present invention are possible in the light of the above teachings.
It is therefore to be understood that within the scope of the appended
claims, the invention may be practised otherwise than as specifically
described herein.
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