[0001] This invention relates to a low cost flat array antenna particularly, but not exclusively,
for receiving T.V. transmissions from a geostationary satellite.
[0002] U.S. patent 4,486,758 discloses a flat array triplate antenna for coupling circularly
polarised radiation to a plurality of feed lines by way of a planar array of such
elements each having a pair of orthogonally disposed dipoles, the dipole array being
positioned between conductive sheets each having corresponding arrays of apertures.
The dipole array and associated transmission strip lines are spaced from the conductive
sheets by layers of dielectric material. The dipoles are formed one from each pair
on one surface of a dielectric sheet and the other one of the pair on the opposite
surface of the sheet. The respective strip line networks are also formed on the respective
opposite surfaces of the sheet. In one embodiment of this prior art antenna a third
unapertured conductive sheet is spaced from one of the apertured conductive sheets
to act as a reflective surface.
[0003] The antenna is fabricated in accordance with conventional rigid printed circuit techniques,
which will cause fairly high losses to be encountered, and which care not conducive
with low cost high volume array production.
[0004] European patent application No. 0 252 779A proposes the use of metal plates which
are shaped by pressing and stamping in order to form radiating slots and to emboss
spacing abutments on the plate surfaces facing the dielectric sheet carrying printed
conductors and terminations. Application EP 0 252 779A states that the use of embossed
spacing abutments obviates the need for expensive solid dielectric material to space
the conductive sheets from the dielectric sheet carrying the printed conductors. However,
we have found that embossing of metal plates to form accurate spacing abutments is
a complex and costly procedure to support the stripline. Such a procedure does not
lend itself to low cost array production techniques which require that all manufacturing
operations are rapid and simple as possible whilst providing a product of adequate
quality.
[0005] It is an object of the invention to produce a cheap antenna on a mass production
basis and having a low loss.
[0006] According to the invention there is provided a flat antenna assembly, including a
plastics base support structure, an antenna array mounted on the base support structure,
and a plastics radome cover, wherein the base support structure has a plurality of
first support pillars adapted to receive the antenna and to retain the elements of
the antenna array in a spatial relationship, wherein the base support structure has
a plurality of second pillars engaging corresponding openings in the antenna array
structure whereby to maintain alignment of the array, and wherein the radome cover
and base support have cooperating peripheral sealing means for retaining the cover
on the base support so as to enclose the antenna array.
[0007] In a preferred embodiment of the invention the pairs of orthogonal terminations are
disposed in an orthogonal array and the orientations of each pair of terminations
are at 45
o to the perpendicular axes of the array.
[0008] An embodiment of the invention will now be described with reference to the accompanying
drawings, in which:-
Fig. 1 is a part sectional part schematic view of part of a flat array antenna according
to the embodiment;
Fig. 2 is a perspective exploded view of the flat array antenna;
Fig. 3 is a part sectional view of the flat array antenna showing details of the assembly
fixing arrangements;
Fig. 4 is a plan view of the triplate structure of the antenna;
Fig. 5 is a part sectional view illustrating the manner in which the radome cover
is mounted and sealed to the antenna assembly; and
Fig. 6 is a detail of the plan view showing schematically the orientation of the orthogonal
terminations.
[0009] The various views shown in the drawings are not dimensionally accurate and are for
illustrative purposes only.
[0010] Referring to the drawings, the antenna comprises a suspended dielectric film 11 positioned
between two metal plates 13, 15 by means of foam dielectric layers 17, 19. The dielectric
film 11 has formed thereon, by conventional printed circuit techniques pairs of circuit
terminations or "excitation probes" 21, 23 which are orthogonally positioned with
respect to one another, and this printed film forms the flat stripline feed structure.
[0011] The metal plates 13, 15 are each provided with an array of apertures 25, 27 arranged
in respective orthogonal arrays. The apertures 25 and 27 are fabricated in the flat
metal plate 13, 15 by simple conventional metal stamping out techniques. By way of
example, Applicants have found that it is practical to stamp out an array of holes
in flat aluminium sheet of thickness of only o.5mm. Such flat sheets have sufficient
rigidity when sandwiched with layers of foam dielectric material, such as expanded
polyethylene, to support the suspended stripline formed on a thin polyester film.
[0012] The film 11 and the plates 13, 15 are arranged so that the apertures 25, the pairs
of probes 21, 23 and the apertures 27 are aligned perpendicular to the plane of the
dielectric film 11 and together provide an array of antenna elements. The probes of
each pair are connected to each other by stripline sections (not shown) and all the
stripline sections are connected to a common stripline feed structure (not shown)
in accordance with known techniques to effect reception (or transmission) of circularly
polarised radio frequency signals. Such connection techniques are known from, for
example, U.S. patent No. 4,792,810.
[0013] Additionally the antenna may be provided with a third, generally unapertured metal
plate 29 spaced from the metal plate 15 to act as a reflector plate.
[0014] The triplate structure and the reflector plate 37 are secured to a rigid base 30,
in this embodiment made of a moulded plastics, by fixings, in this embodiment screws
31, 33 respectively engaging with blind bores in pillars 35, 37 respectively passing
through corresponding holes in the file 11 and in the plates 13, 15 and 29. In addition
there are shouldered pillars or dowels 39 which pass through close fitting holes in
all the layers of the assembled structure to provide accurate lateral alignment of
the apertures 25, 27 with the probes 21, 23. Note that the holes in the metal plates
through which the pillars 35 pass are clearance holes to prevent distortion of the
metal plates when the screws are tightened. Furthermore the screws 31 have shouldered
spacer shank portions 31a of predetermined length so that when properly tightened
they exert no undue compression on the triplate structure consisting of metal plates
13, 15, the suspended dielectric film 11 and supporting foam dielectric layers 17,
19, and determine the maximum spacing between the outer surfaces of the plates at
each fixing location. The length of the shouldered portion 31a is such that when the
plates 13, 15 and foam layers 17, 19 are of maximum thickness allowed by manufacturing
tolerance, there is insignificant compression and distortion of the triplate structure.
Likewise, when the thicknesses of 13, 15, 17 and 19 are the minimum allowable there
will be insignificant slack in the holding of the triplate structure by the screws.
The foam dielectric layers 17, 19 are unapertured except for the holes where the locating
and securing pillars pass through. Thus the screws 31, shoulder 31a and pillars 35,
together with the foamed dielectric layers 17 and 19 accurately control the spacing
between the outer surfaces of the ground planes 13 and 15. The foam dielectric is
soft and gently embraces the feed structure sufficient to prevent any periodic distortions
near the fractional wavelength period mentioned above.
[0015] Applicants have noted that it is advantageous to have a diversity of orientations
for the probe pairs 21, 23. In particular Applicants have determined that the performance
of the antenna array is optimised when the alignments of the individual probes are
at 45
o to the axes of the columns and rows of the orthogonal array of antenna elements.
[0016] The common feed stripline structure referred to above (and not shown in the drawings)
has an output termination which couples into a rectangular waveguide 41 the propagation
axis of which is perpendicular to the plane of the triplate structure.
[0017] In order to reduce leakage losses at the edges of the metal plates the edges may
be formed with lips (not shown) at right angle to the plane of the plates. Likewise
the reflecting plate 29 may be formed with right angled lips 45.
[0018] The antenna assembly is protected against the environment by a plastics radome cover
43 secured to the plastics base support 30 via a peripheral seal. Fig. 5 shows the
detail of a suitable seal between the radome and the base. For clarity the antenna
components, i.e. the triplate structure and back reflector, have been omitted from
Fig. 5. The radome cover 43 has a circumferential flange 51 terminating in an inwardly
directed lip 52. Optionally a further, smaller flange 53 may be mounted on the inner
surface of the radome cover 43 adjacent the flange 51 so as to define a circumferential
channel 54. The base support 30 has a peripheral flange 55 and a further oppositely
directed peripheral flange 56 defining a shoulder 57 therebetween.
[0019] When the structure is assembled, the radome cover 43 is urged against the base support
30 such that the flange 55 of the base support enters the channel 54 and the inwardly
directed lip 52 of the radome cover engages the shoulder 57 whereby to retain the
cover 43 in abutment with the base support 30 and provide a substantially water-tight
seal at the joint therebetween. In some applications an adhesive or sealant may be
applied to the joint between the cover and base support to ensure the integrity of
the seal.
[0020] Advantageously the base support 30 has a breathe opening (not shown) whereby to allow
drainage of any condensed moisture and to prevent a build up of intense pressure e.g.
when the antenna is exposed to the sun.
[0021] The antenna may be assembled by the following sequence.
[0022] The back reflector is positioned in the base support 30 and is secured thereto by
screws 33 engaging pillars 37. Next the triplate assembly is placed over the pillars
39 and the shanked screws 31 are inserted into the pillars 35. Finally the triplate
structure is covered by the radome 43 by pressing the radome against the base support
to engage the seal therebetween. As the pillars 39 and the shanked screws 31 provide
accurate linearised positioning of the antenna components, the entire assembly can
be effected rapidly and by relatively unskilled staff. This allows the antenna to
be produced by low cost mass production techniques.
1. A flat antenna assembly, including a plastics base support structure (30), an antenna
array (13, 17, 11, 19, 15) mounted on the base support structure, and a plastics radome
cover (43), characterised in that the base support structure has a plurality of first
support pillars (35) adapted to receive the antenna array and to retain the elements
of the antenna array in a spatial relationship, that the base support structure has
a plurality of second pillars (39) engaging corresponding openings in the antenna
array structure whereby to maintain alignment of the array, and that the radome cover
and base support have cooperating peripheral sealing (52, 55) means for retaining
the cover on the base support so as to enclose the antenna array.
2. An antenna assembly as claimed in claim 1, characterised in that the antenna array
is a triplate structure supported between a pair of ground planes (13, 15).
3. An antenna assembly as claimed in claim 2, characterised in that the triplate feed
is disposed on a thin dieletric substrate (11) which is maintained flat and parallel
to the ground planes by a foam dielectric material (12, 19).
4. An antenna as claimed in claim 3, characterised in that the foam dielectric comprises
on each side of said substrate a respective continuous foam dielectric sheet which
is soft and gently embraces both the substrate and the respective ground plane.
5. An antenna assembly as claimed in any one of the preceding claims, characterised in
that a plurality of third pillars (37) are provided on the base support structure
whereby to support a back reflector (29) at a predetermined distance from the antenna
array.
6. An antenna assembly as claimed in claim 5, characterised in that said back reflector
is further supported on shoulders provided on said first and second pillars.
7. An antenna assembly as claimed in any one of the preceding claims, characterised in
that said peripheral sealing means comprises inwardly directed lip adapted to engage
a corresponding peripheral shoulder on the base support.