Note: Descriptions are shown in the official language in which they were submitted.
Z0059~73
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FOCUSSING DEVICE FOR A MICROWAVE ANTENNA
This invention relates to a focussing device for a
microwave antenna.
Microwave antenna are of increasing importance for
the reception of microwave signals from television
broadcast satellites or community television transmission
stations.
The most commonly used antenna at present comprises
a parabolic reflector focussing device which seîves to
focus received signals into a collector, in the form of a
wave guide horn, which passes the signals onwards for
decoding.
Parabolic reflectors are generally relatively
expensive to manufacture and often are objected to as
being unsightly when installed on private dwellings.
To overcome these disadvantages focussing devices
in the form of zone plates have been proposed, a zone
plate comprising a planar, at least in use, substrate of,
for example, glass or plastics material transparent to the
signals to be received, carrying an arrangement of rings
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of a material which either absorbs or reflects the signals
- to be received.
Two known arrangements of rings forming,
respectively; a Fresnel zone plate and a Gabor zone plate,
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can be used as such focussing devices.
It has been proposed to use such zone plate
focussing devices as windows for a building with the
associated collector being located within the building.
It has also been proposed, see International Patent
Publication No. WO88/10521, to provide such a zone plate
focussing device in the form of a flexible member which
can be easily stored when not in use, for example in the
form of a roller blind located behind a window of a
b~lding and maintained in the rolled condition when not
in use but unwou.._ _nto a substantially pianar condition
when in use.
- A disadvantage of known such zone plate focussing
devices is that the rings are formed of a material which
is substantially opaque or reflective to visible light,
and thus when provided as a window or window blind they
substantially reduce the amount of visible light passing
through the window.
According to this invention there is provided a
focussing device for a microwave antenna, comprising a
substrate which is transparent to visible light and to
microwave signals to be received, the substrate carrying
an arrangement of rings of a material which either absorbs
or reflects the microwave signals to be received, the
rings being arranged to focus received microwave signals,
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characterised in that the rings are of a material which is
transparent to visible light.
Thus, the invention provides a focussing device
which can be used, for example, as a window or a window
blind, and which when so used will not unduly adversely
affect the passage of visible light through the window or
blind.
The substrate can be rigid and of either glass or
plastics material, or can be flexible and then of, for
example, plastics material.
The substrate and _ings can together constitute a
Fresnal zone plate or a ~ 30r zone plate.
Various materials are suitable for the rings and
the-e are various methods by wn_ch the rings can be
applied to the substrate, and detailed information on
these matters can be obtained from GB-A-1307642,
US-A-4166876, EP-A-0035906 and EP-A-0104870.
A particularly suitable material is metal coated
glass available from Pilkington Glass Limited of England,
and known as KAPPAFLOAT (trade mark) glass, such metal
coated glass being transparent to visible light and the
rings being easily formed by the removal of unwanted
coating material as by etching. Another suitable material
is Pilkington K Glass (trade mark) available from
Pilkington Glass Limited of England, which is a glass
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substrate with a low emissivity coating of doped tin oxideand a colour supressing underlayer, the rings again being
formed by etching.
A suitable plastics material substrate is stretched
polyester to which rings formed of layers of silver alloy
and oxides of titanium and indium can be applied by
magnetron sputtering in a vacuum chamber to give the
required ring arrangement, such coated plastics material
being available from the Bekaert Group of companies.
This invention will now be described by way of
example with reference to the drawings, in which:-
Figure 1 is a schematic perspective view of anantenna including a focussing device according to the
invention;
Figure 2a illustrates the transmissivity profile of
a Fresnel zone plate;
Figure 2b illustrates the transmissivity profile of
a Gabor zone plate;
Figure 3a, 3b and 3c schematically illustrate a
geometry for use in producing a Gabor zone plate;
Figure 4 is a diagrammatic representation of a
focussing device according to the invention in use
together with a second substrate carrying a coating of a
material which absorbs or reflects the received microwave
signals;
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Figure S illustrates use of a focussing device
according to the invention in the form of a window blind;
and
Figure 6 illustrates a suitable arrangement of
rings forming a Fresnel zone plate for use as a focus~ing
device according to the invention.
Referring now to Figure 1, a focussing device in
the form of a zone plate 1 in accordance with the
invention, and comprising a glass or plastics material
substrate 2 carrying an arrangement of circular or
elliptical rings A of a material which absorbs or reflects
microwave signals to be received and which is transparent
to visible light, is fixed as a window pane, and sexves to
focus received microwave signals onto a transducer at the
apex of collector in the form of a horn 3. For the ~ake
of clarity the horn 3 is shown mounted over a pane
adjacent that formed by the zone plate 1, but it will be
appreciated that in use the horn 3 will be mounted over
the zone plate 1. The horn 3 will with this arrangement
be made from a transparent plastics material so as not to
unduly adversely affect the passage of visible light
through the window. Otherwise the horn 3 can be mounted
at a position remote from the window (see Figure 5 for
example). The horn 3 feeds the received signals to a
converter 4 which in turn feeds a television or computer S.
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The zone plate 1 is preferably a Fresnel zone plate
the operation and structure of which i5 well known and
will not therefore be described herein. The microwave
absorbing or reflecting rings A are deposited on substrate
2 which is transparent to microwave and visible radiation,
and which may be of glass or plastics material. The rings
A are separated by rings B of the microwave and visible
light transparent substrate 2. The microwave absorbing or
reflecting rings A can comprise a thin layer of indium tin
oxide applied to the substrate 2 by any of a variety of
different techniques as mentioned above, for example
printing, sputtering, vacuum deposition or spraying. Any
of a variety of other materials which absorb or reflect
microwave radiation and yet are transparent to visible
light can otherwise be used for the rings A.
Alternatively, it may be more convenient to coat an
entire substrate 2 with a suitable ring-forming material,
for example indium tin oxide, and then etch away that not
required for the rings A.
In place of the Fresnel zone plate which has a
square wave transmissivity profile as shown in Figure 2a,
a Gabor zone plate having a sine square transmissivity
profile as shown in Figure 2b can be used as the focussing
device 1.
The Fresnel zone plate is well known in the optical
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arts and will only be briefly described herein.
Essentially, a Fresnel zone plate is constructed by
applying an arrangement of rings of material which either
reflects or absorbs incident microwave radiation, onto a
substrate otherwise transparent to such radiation, in
accordance with a simple geometrical relation which
results in a half wavelength difference in the lengths of
the paths from the edges of adjacent rings to a point on
an axis of symmetry which is a focus for radiation
incident on the zone plate.
A well known problem associated with the Fresnel
zone plate, however, is that it produces not only a
principal maximum in irradiance distribution at the
primary focus, but also subsidiary maxima at higher order
focal points. This introduces a focussing loss, which,
coupled with the transmission loss deriving from the
rings, imposes an inconvenient ceiling on the efficiency
of the Fresnel zone plate.
The Gabor zone plate, or sinusoidal zone plate,
focuses only at the primary focus, there being no higher
order focal points. Consequently, there is no focussing
loss of signal.
The Gabor zone plate is of central significance in
the art of holography. Stated simply, the Gabor zone
plate is the interference pattern associated with a point
objéct imaged in a hologram.
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- A further requirement for convenient signal
reception is that the focussing device l of the invention
should be adapted to be mounted on, parallel to or
integral with a window pane or other surface having a line
of sight to a specified received signal source. The ring
arrangement can therefore be in the form of a set of
ellipses whose eccentricity and relative positions along a
common axis are determined by the angle of elevation of
the line-of-sight from the received signal source to the
local horizontal and by the orientation of the mounting
surface to the local meridian.
Referring now to Figures 3a, 3b and 3c, these
illustrate a method of manufacturing a Gabor zone plate
suitable for use as a focussing device according to the
invention. A photo sensitive substrate 31 which provides
microwave absorbing or reflecting material when exposed to
light and developed is arranged so that light from a pair
of monochromatic phase related light sources 32 forms an
interference pattern on the substrate 31. The substrate
31 will, when developed, exhibit a sinusoidal
transmissivity profile (see Figure 2b) and will constitute
a Gabor zone plate.
As noted above, the ring arrangement of a focussing
device is preerably in the form of ellipses whose
eccentricity and relative positions to one another along a
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common axis are in dependence on the angle of elevation of
the line of sight from the received signal source to the
local horizontal and by the orientation of the plane of
the focussing device 1 to the local meridian. In the
illustrated geometry utilised in manufacturing a Gabor
zone plate for use as a focussing device 1, the substrate
31 is positioned with respect to the sources 32 in
dependence upon the required eccentricity and relative
positions of the rings of the zone plate.
The geometry of the manufacturing set up is
determined according to the equation
; 1 = ~ (Dl-l_ D^-l)
~ f ~1
where f = focal distance of the resultant zone plate
= wavelength of the received microwave radiation
~ = wavelength of light used to make the zone
plate (i.e. corresponding to peak response of
photosensitive emulsion)
Dl, D2 = distances of the phase related sources
- from the substrate.
For a received microwave signal of 2cm wavelength
and a substrate 31 with an emulsion having a peak response
at 4 x 105mm then, for a focal length of lm, Dl and
D2 should be 5m and 5.55m respectively from the centre
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of the resulting interference pattern on the substrate
31. This is illustrated in Figure 3A.
In particular, the substrate 31 may be arranged so
that, for Cartesian axes centred on the centre of the
interference pattern with the light sources 32 positioned
on the x-axis, when an angle A is defined as the angle
from the x-axis to the line defined by the intersection of
the substrate 31 and the xy plane; and an angle B is
defined as the angle from the x-axis to the line defined
by the intersection of the substrate 31 and the zx plane,
then angle A aljo corresponds to the angle of orientation
of the substrate 31 in use as a focussing device 1 in an
antenna, to the local meridian, and angle B also
corresponds to the angle of elevation of the line of sight
between the received signal source and the local
horizontal.
Figure 3b illustrates the substrate 31 turned
through angle A whereas Figure 3c illustrates the
substrate 31 turned through angle B. In practice, the
substrate 31 will be turned through both angles.
Referring now to Figure 4, as is known, the
effectiveness of focussing devices in microwave antenna
can be improved by the use of a microwave reflective
member on the side of the focussing device remote from the
received signal source, this resulting in a stronger
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signal on the same side of the focussing de~ice as the
received signal source. Thus, as shown in Figure 4, a
zoneplate constituting a focussing device 1 in accordance
with the invention can be utilised together with a member
6 carrying a microwave reflective coating to obtain
improved operation of the antenna of which the focussing
device 1 forms part. Such an arrangement as shown in
Figure 4 is ideally suited to the provision of a double
glazed window serving as a focussing device, with the
rings of the zone plate 1 being on either side A or B of
the substrate and the microwave reflective coating of
material transparent to visible light on either side C or
D of member 6. The member 6 can comprise the same
combination of substrate and coating material as used for
the zone plate 1. The thickness of the substrates of the
zone plate 1 and member 6, which substrates can be of
glass or plastics material, and the spacing between the
substrates, will be set to obtain the required operation,
and the spacing can be zero provided that the rings of the
zone plate 1 are not on the side B thereof when the
coating of the member 6 is on the side C thereof.
Preferably the spacing between the rings of the zone plate
1 and the coating of the member 6 is one quarter the
wavelength of the received microwave signal.
ReferrLng now to Figure 5, this shows an antenna
arrangement including a focussing device 1 in accordance
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with the invention, in which the focussing device 1 is in
the form of a window roller blind. For such anarrangement
the substrate for the zone plate 1 would be of flexible
plastics material carried by a roller 7 mounted by means
of brackets 8 to the frame 9 of a window 2 such that for
use the focussing device 1 can be unwound from the roller
7 to cover the window 2, as shown in Figure 5. When not
in use the focussing device 1 will be wound on the roller
7, leaving the window 2 clear. Also as shown, in this
arrangement the collector horn 3 is arranged remote from
the window 2, together with the converter 4 and the
television 5.
Referring now to Figure 6, if a Fresnel zone plate
is used as a focussing device 1 according to the
invention, then a circular arrangement of rings will focus
received signals at a point on the perpendicular axis
intersecting the plane of the zone plate, provided that
the plane is at 90 to the line of sight of the received
signal source. Since this requirement would not be met
by, for instance, direct broadcasting satellites radiating
onto a vertical focussing device 1, modification of the
circular system of zones is desirable. The modification
is such that the ring arrangement becomes a set of
ellipses not having a common centre but instead being
displaced along their common major axis. Such a zone
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plate is shown in Figure 6. In common with conventional
circular ring zone plates, the radii of the substrate
rings B and the microwave reflecting or absorbing rings A
are such that the distance from the ring edges to the
focal point (i.e. the collector horn 3) increa~es by half
a wavelength of the incident radiation between adjacent
rings, the distance of the focal point to the centre of
the first, i.e. central, ring being approximately 1.5
times the width of the largest ring.
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