[0001] inis invention relates to antenna systems, more particularly but not exclusively,
antenna systems for use on board spacecraft such as communications satellites.
[0002] It is desirable to be able to vary or 'scan' the pointing direction of an antenna
on a spacecraft with respect to the spacecraft, ie to vary the antenna pointing direction
without varying the position of the spacecraft as a whole. A known, advantageous way
of doing this is to use an antenna system comprising a movable reflector dish but
a fixed antenna feed (the feed being the horn, waveguide aperture or the like from
which the communications signal is emitted to the dish or which receives the signal
from the dish). The antenna feed should be positioned at the focus of the reflector
dish. However, for practical reasons, the movement of the dish to scan the antenna
pointing direction will not be about this focal point. Hence, during such scanning,
the fixed position of the feed will not remain at the focal point and there will be
a loss of gain and performance.
[0003] An object of this invention is to substantially reduce or even eliminate the separation
of the focus from the feed during movement of the reflector.
[0004] According to the invention there is provided an antenna system comprising support
means, a main radio-frequency reflector dish of which the reflector surface is its
concave surface, an auxiliary radio-frequency reflector dish of which the reflector
surface is its convex surface, the two dishes being fixed with respect to one another
with the reflector surfaces thereof facing one another, the two dishes being supported
by said support means for together turning with respect to the support means about
a point lying between the reflector surfaces, antenna pointing means connected to
the reflector dishes and operable for controlling said turning of the reflector dishes
to point the main reflector dish in a desired direction, and antenna feed means which
is fixed with respect to the support means and which has a portion at least near to
said point for emitting radio-frequency energy to or receiving such energy from said
main reflector dish by way of said auxiliary reflector dish.
[0005] The antenna system is particularly suitable for use in space but may be useful for
other applications. Cassegrain, Gregorian and Newtonian reflector dishes may be used
as well as other forms shaped for a particular application.
[0006] By way of example, one embodiment of the invention will now be described with reference
to the accompanying figures in which:-
Figure 1 is fa diagrammatic view of an antenna system on board a spacecraft,
Figure 2 corresponds to figure 1 but shows the reflectors of the antenna system turned
to point the antenna beam in a different direction with respect to the spacecraft,
and
Figure 3 shows typical sum and difference signal strengths.
[0007] Figure 1 shows part of a spacecraft 1 to which there is mounted a Cassegrain antenna
system comprising a main parabolic reflector dish 2 and an auxiliary reflector dish
3 which is fixed, for example by way of support arms (not shown), to the main dish
2 such that the convex reflector surface of the dish 3 faces the concave reflector
surface of dish 2. The dish 2 is fixed to the movable part 4 of a swash-plate antenna
pointing mechanism of which a fixed part 5 is supported via a hollow cone-shaped support
pillar 6 on the surface of the spacecraft. The antenna pointing mechanism is shown
only diagrammatically but its construction which includes a suitable drive motor (not
shown) and its operation are well known to those skilled in the art. Its function
is to permit the antenna system to be turned so that the direction of the transmission/reception
beam of the antenna dish 2 can be varied or scanned with respect to the spacecraft
as shown in figure 2. A fixed antenna feed horn 7, communicating via a waveguide 8
with radio communications apparatus (not shown) on board the spacecraft, is positioned
within the pointing mechanism so as to feed the antenna dish 2 by reflection from
the auxiliary dish 3. The effect of dish 3 is to produce a virtual image of the horn
aperture at point 9, which point is at least near the focal point of the dish 2. Meanwhile,
the horn aperture is in fact positioned at least near the point about which the antenna
dish 3 is turned by the antenna pointing mechanism. Thus, despite such turning, the
apparent position of the horn aperture (as far as dish 3 is concerned) remains at
the proper focal point.
[0008] Because the main and auxiliary reflector dishes move together and rotate as a unit
there are no distortions introduced when the beam scans. Some loss of gain occurs
through spillover at the edges of the auxiliary reflector as illustrated at 10 in
Figure 2 but these losses may be controlled by increasing the subtended angle of the
auxiliary reflector from the focus. With proper design, the illustrated system may
be able to achieve comparatively large scans with low loss (eg 35 beamwidths with
an approximate 1 dB loss).
[0009] The centre of rotation of the reflector dish assembly may be displaced somewhat from
the feed horn aperture, say to accommodate other system design features, but increased
distortion will occur.
[0010] It is desirable for many uses to be able to control the antenna pointing direction
by receiving the signal from a beacon sited upon the earth via a special r.f. sensing
part of the antenna. For this r.f. sensing components (not shown) may be attached
to the feed horn. The scanning process introduces asymmetry into the main reflector
aperture illumination which has more serious effects upon the shape of a difference
pattern (see Figure 3) than a sum pattern. To correct this asymmetry a combination
of sum and difference patterns may be used to squint the reception beam pattern of
the beacon signal receiving components so as to follow the rotation of the antenna
system and hence correct the asymmetry. The sum and difference patterns may be generated
by either multiple horn systems or mode extraction systems in which the signals are
obtained from the various modes of waveguide. For circular waveguides, the modes TEll
or HEll are normally used for sum signal and the combinations TM01/TE21, dual TE21
and TM01"TE01 as the difference signal. To maintain the null depth whilst scanning,
the antenna sum and difference modes are added which squint the feed illumination
to follow the auxiliary reflector. This is also possible with the communications signal
but it is not usually necessary. The addition of sum and difference modes may be made
in several ways. The modes can be added in an r.f. comparator, in a multiplexer modulator
or at low frequency or baseband within the tracking receiver. The sum pattern is usually
added at a low level (eg 20dB) relative to the difference pattern.
1. An antenna system comprising support means, a main radio-frequency reflector dish
of which the reflector surface is its concave surface, an auxiliary radio-frequency
reflector dish of which the reflector surface is its convex surface, the two dishes
being fixed with respect to one another with the reflector surfaces thereof facing
one another, the two dishes being supported by said support means for together turning
with respect to the support means about a point lying between the reflector surfaces,
antenna pointing means connected to the reflector dishes and operable for controlling
said turning of the reflector dishes to point the main reflector dish in a desired
direction, and antenna feed means which is fixed with respect to the support means
and which has a portion at least near to said point for emitting radio-frequency energy
to or receiving such energy from said main reflector dish by way of said auxiliary
reflector dish.
2. An antenna system according to claim 1, including radio-frequency signal receiving
means fixed to said antenna feed means for receiving via said main and auxiliary reflector
dishes a beacon signal for assisting in the correct pointing of said main reflector
dish.
3. An antenna system according to claim 1, wherein the axial direction of the reception
pattern of the radio-frequency signal receiving means is variable to follow the turning
of said auxiliary dish reflector.
