EP1054470B1

EP1054470B1 - Antenna with low visual impact

Info

Publication number: EP1054470B1
Application number: EP99113907A
Authority: EP
Inventors: Carlo Briganti; Giorgio Cortiana; Mario Marabelli; Marco Toni
Original assignee: Siemens SpA
Current assignee: Siemens SpA
Priority date: 1999-05-21
Filing date: 1999-07-16
Publication date: 2006-11-15
Anticipated expiration: 2019-07-16
Also published as: DE69933992D1; ATE345585T1; DE69933992T2; IT1308545B1; EP1054470A3; ITPR990047A1; EP1054470A2

Abstract

The present invention relates to a directional multi-channel antenna of the type comprising a substantially flat reflecting screen (1) and a plurality of dipoles or radiators (2) held up at a certain distance on the reflecting side of the screen (1) and a shell or radome (3) made of a material essentially transparent to the radiation RF transmitted and received by the antenna, containing said reflecting screen, said dipoles or radiators and at least one feeding connector of the antenna. According to the invention said radome (3) is made of an optically transparent material and said screen is, at least for a major part (1a, 1b) set up by a conductor net arc-welded at the crossings. According to an alternative embodiment said conductor net is set up by a first plurality of wires arranged on a first plane and a second plurality of wires arranged on a second plane put at a distance of a predetermined entity from said first plane. <IMAGE>

Description

Field of the Invention

The present invention relates to mobile radio telephone systems and in particular the antennas used to realise a cell coverage of the served territory.
The cellular telephone systems require a complex antenna network with variable shapes and sizes especially in function of the operation frequency spread over the territory.
Such antennas have to be installed at sites distributed over territory, supported by poles or pylons, on existing constructions such as church towers, towers, buildings in a variable number generally from 3 to 12 at each site, in function of the subdivision of the area in cells covered by the respective antennas. Areas densely populate by subscribers as residential areas require a high density of cells and therefore of installations of antennas.

Background art

The commonly used antennas present a vertically extending panel configuration. A substantially flat screen of conducting material supports a series of dipoles aligned along the vertical median axes of the panel suitable to radiate and receive electromagnetic signals. The flat screen is essential because it fulfils the function of reflecting the electromagnetic radiation emitted by the dipoles providing for the desired directivity of the antenna.
The document US-A-2 827 628 discloses an antenna, the flat reflecting screen of which is partially made of expanded metal. The reflecting screen has a central part that is made of a solid plate on the opposite edges of which are welded expanded metal screen portions.
The document US-A-4 804 972 discloses a parabolic dish antenna having a radome in the form of a lid made of a radiation transparent plastic that may even be optically transparent for making observable trademark or other signs printed on the concave dish.
The document WO 97/41622 discloses an antenna system comprising an antenna assembly having an open grid reflector structure in a self-supporting wheel ring placed on the top of a trellis; the antenna assembly comprises a number of antenna panels, each including a number of dipole elements mechanically connected to tubular beam transformers spaced to each other along an horizontal axis in order to achieve multiple lobes.
The screen, the dipoles and the connection cables of the dipoles to the antenna connector to the local station are normally enclosed in a shell or radome transparent to RF radiation to protect them from atmospheric agents, dust, guano etc. which if free to accumulate could degrade and compromise the functioning of the antenna. The shell or radome is typically made of fibreglass or other opaque plastic materials.
The assembly has essentially the aspect of an opaque body with a visibly perceptible volume.
The environmental impact of the installation of such antennas especially in urban areas of recognised and undiscussed historical and architectural value quite often cannot be neglected as these antennas have to be installed in positions strategically efficient for an adequate covering of the respective cell.
The installation of antennas - apart from other preoccupations which may be overcome by demonstrable checks of sufficiently low electromagnetic field density below the limits foreseen by the law - often raises objections for the unaestheticisms procured on fronts and/or skylines which creates problems and, in some cases, impedes the installation of radiant systems at points that would provide a most efficient coverage of the territory.
The fibreglass radome, if on one hand it satisfies the requisite of lightness, mechanical resistance and of not being degradable by atmospheric agents and ultraviolet radiation, contributes to confer a bulky appearance to the antenna and to increase its visibility.
By exposure to sunshine, the fibreglass shell causes an increase of the internal temperature which may lead to a temporary degrading of performances.
Moreover, the fibreglass shell has a small (but not negligible) capacity to absorb RF energy which naturally represents a loss and therefore a reduction of the gain of the antenna.
It has been found that a surprising visual aspect of substantial transparency of an installed antenna when watched from a certain distance can be achieved by a combination of an optically transparent radome and of a reflecting screen made of a conductive metallic wire net with meshes having a relatively large empty/solid ratio.
The antenna, when observed at distance and in the context of other architectural objects among which it is installed, becomes little visible and difficult to be noticed by a casual observer. This «invisibility» turns out to become almost total under favourable illumination conditions.
If in back lighting condition a certain visibility of the dipoles, the support structures and the like persists, it turns out that this has a low visual impact. Indeed the observed object appears substantially transparent for a large part of its outline and it is not perceived as an object of visually encumbering volume.
Under optimum grazing light or float light conditions of observation translucency effects of the transparent radome limits the visibility of even the above-mentioned opaque structures.
The use of a wire net with mesh dimensions designed in function of the wavelength spectrum of the RF radiation handled by the antenna instead of a solid metal reflector, provides for performances comparable to those of a solid reflector. Satellite dish antennas are often made with wire net to reduce the resistance offered to the wind and thus reduce the stress on the supporting structures. In the case of directional multi-channel antenna for transmitting as well as for receiving, it has been found that the use of a reflector completely made by a conducting wire net having a considerably large meshes purposely made as large as possible to reduce visual impact and to favour the achievement of a substantially transparent aspect for the whole entire antenna assembly is only possible at a high price in terms of reduced performances.
Even sizing the dimensions of the meshes of the conducting net in such a way to equalise the reflecting behaviour to that of an analogous full conducting surface such behaviour turns out heavily invalidated by the presence of objects, that is of conducting surfaces behind or in the immediately next to the back of the reticulate screen.
In the case of the antennas taken into consideration the presence of a generally metallic supporting pole or the presence of metal brackets for fastening the antenna to the front of a building cause the presence of a so-called evanescent wave, not residual in terms of intensity, which produces perturbations of the correct functioning of the antenna.
When relatively adjacent conductors are present at the back of a screen reflector, the phenomenon of the evanescent wave is due to a prevalence of metallic surfaces present at the back of the screen reflector on the screen itself in gathering part of the RF radiation through the open meshes of the screen reflector. The radiation passing through the screen is scattered by the metallic surfaces at the back of the reticular screen in spurious directions which do not coincide with the desired direction of reflection.

Object of the invention

Its the aim of the present invention to eliminate or reduce considerably the above-mentioned drawbacks by providing an antenna having a remarkably reduced visual impact compared to the known antennas while at the same time neutralizing the phenomenon of the evanescent wave, and assuring comparable or even enhanced performances especially in terms of an increased stability of behaviour within an ample range of environmental conditions.

Summary of the invention

It has been found out that a sharp reduction of the visual impact of the complete antenna structure obtainable from said association of an optically transparent radome and of the use of a reticular screen made of sufficiently large meshes and with the highest possible of empty/full ratio to promote the «transparency» of the whole can be obtained without experiencing the phenomenon of evanescent wave by making a relatively narrow central part of the screen with a conductor net, or realising said minor central part of the screen by a conductor net with relatively denser meshes and with a reduced empty/full ratio compared to the conductor net, the major part of the reflecting screen is made of.
In practice the presence of a relatively small central portion (made of net with a differentiated structure with respect to the net making up the major part of the screen) reflector turns out to be appropriate particularly if the dimensions of this central portion are equal or longer than the geometric projection of said objects and support structures present behind the screen reflector.
In addition, has been found that the result is even better if said central portion of a net of different structure with respect to the net constituting the largest portion of the screen reflector is made of a metallic material with high conductivity properties. To this end, the cross section of the wire may be augmented with respect to the wire used for the other portion of the screen reflector.
This central part of the reflecting screen with such a differentiated structure compared to the surrounding major part of the screen is subject to a concentration of induced currents that may produce relatively large current density levels. The presence of at conductor section sufficiently large in this area of the screen improves the performances of the antenna (both during reception and transmission phases).
According to a preferred embodiment of the antenna of the present invention, such minor central portion of the screen reflector is the planer face of a tubular metallic structure extending for the whole lengthwise dimension of the screen. The tubular structure can be conveniently made of two channels coupled to each other. The tubular shape of said central element houses the connecting cables of the dipoles according to a defined configuration departing from one or more connectors installed at one end of the antenna, usually in an end cover or bottom of the radome made of optically transparent material.
The metallic wires forming the conductive net that constitutes the major portion of the screen reflector are preferably arc welded at crossings in order to prevent the constitution of spurious dipoles because of insufficient electric continuity in the reticulated plane reflector structure.
Besides this, it has been found that by favouring during the arc-welding the constitution of globules of metallic material at the welding spots at the crossings of orthogonal metallic wires, the behaviour of the antenna improves. This can be explained by the fact that the globule with an increased conducting cross section that is formed at a crossing of the so arc welded wires offer a conductor cross section for the induced currents on the screen reflector just in coincidence of junction points where the highest current densities are experienced.
Such improving effect can be further emphasised by submitting the entire composite screen reflector to a immersion tin-plating process using an eutectic alloy of tin and silver, of enhanced conductivity or alternatively to hot galvanisation or to silver-plating.

Brief description of the drawings

These and other characteristics will became even more evident through the following description of a preferred embodiment which will now be illustrated just by way of example and by referring to the attached drawings, wherein:

Fig. 1 is a front view of a screen reflector of an antenna implemented according to a first embodiment of the present invention;
Fig. 2 is a plan view of the screen reflector of Fig. 1;
Fig. 3 is a front view of a screen reflector of an antenna made according to an alternative embodiment;
Fig. 4 is a plan view screen reflector of Fig. 3;
Fig. 5 is a front view of a complete double polarisation antenna structure comprising a screen reflector of Figures 1 and 2;
Fig. 6 is a front view of a different type of antenna with single polarisation;
Fig. 7 is a section of a transparent radome at the antenna;
Figures 8 and 9 are photos of an installed antenna made according to this invention made taken under different light conditions.

Description of preferred embodiments of the invention

Referring to the Figures 1, 2, the screen reflector 1 is a substantially flat panel of a common rectangular shape, the length of which is commensured to the number of dipoles or radiators 2, mounted on short spacer columns aligned along the central longitudinal axes of the rectangular panel, as better illustrated in Figures 5 and 6 (in Fig.1, 4 dipoles or radiators 2 are depicted). Theoretically the number of dipoles that can be disposed on a same antenna, and therefore the high of the reflecting panel 1 can be even very large, depending from the number of channels that must be managed by the antenna «illuminating» a certain cell of those in which a certain territory is divided, and on the RF power requirements, that can be satisfied by multiplying the number of dipoles to be fed in phase with the same signal.
According to an essential aspect of the antenna of the invention, the reflecting panel 1 has, at least a major part in terms of area, made by a conducting wire net 1a and 1b and a minor central part 1c made of a conducting wire net of different structure from the structure of the wire nets 1a and 1b, and more precisely having smaller meshes and a reduced empty/full ratio than the two side nets 1a and 1b constituting the major part of the panel (Figures 3 and 4).
It is essential to assure a perfect electric continuity with the two side nets 1a and 1b forming the major part of the area of the reflecting panel. Preferably such an electric continuity between the nets 1a and 1b and the central net 1c is obtained by soldering.
The particular P' Fig. 1 shows a peculiar aspect of the present invention, that is the fact that the metallic wires that form the conducting nets are oriented parallel to the axes of symmetry of the antenna and they are arc-welded at the crossings. Upon arc-welding globules of the metallic material are formed at the welding spots and this is preferably favoured in order to improve the performance of the antenna, as explained above.
From studies conducted by the applicant it has also been observed that a further reduction of losses is obtained, if the metallic wires of the nets are oriented at 45° with respect to the main axes of the antenna itself, as shown in the particular P" of Fig. 1. Even in this case, the forming of metallic globules at the welding spots at the crossings between two wires improves the performance of the antenna.
The aforesaid orientation at 45° of the metallic wires has proved to be particularly useful, when the antenna uses a polarisation comprised between +45° and -45°, because in this way the wires turn out to be parallel to the polarisation plane of the radiated signal.
When instead a vertical and horizontal polarisation is used, the arrangement of the wires shown in the particular P' of Fig. 1 is preferred.
Always from the studies conducted by the applicant it has been found, that a better polarisation separation of the radiated wave is obtained if the vertically oriented wires are not placed in electrical contact with the horizontal wires. Such advantage is obtained, when the vertically oriented wires are disposed on a plane that is offset by a separator distance function of λ, where λ is the wavelength of the signal, from the plane on which are disposed the horizontally oriented wires.
Without departing from the scope of the present invention it is possible to dispose the mutually orthogonal wires of the net on two distinct levels even when they are inclined by 45° in respect to the main axes of the antenna, when a polarisation +45° - 45° is used. Such an arrangement is illustrated in the particular P' of Fig. 3. In the particular P" of Fig. 3, is instead represented a dual condition to that shown in the particular P" of Fig. 1 in which the mutually orthogonal wires (this time not in contact) are disposed at 45° with respect to the main axes of symmetry of the antenna.
The dimensions of the mesh of the two lateral nets 1a and 1b and the relative empty/full ratios are defined in function of the minimum wavelength to be handled by the antenna in order to guarantee a behaviour similar to that of a solid plate reflector.
In case of a typical application in a mobile radio telephone system in the band 900MHz and/or 1800MHz the dimensions of the meshes of the two nets 1a and 1b can be of 5 + 10 mm and, if it is used an arc welded net of drawn steel wire of a diameter comprised between 0,5 + 1,5 mm, there will be a empty/full ratio comprised between 1/10 and 1/20.
By employing a material of a higher conductivity than iron it is possible to further reduce the wire diameter in proportion to the increased conductivity.
The central part 1c, along which the dipoles 2 are disposed, mounted on short columns 4, typically aligned along the central axes of the panel, can be, according to the embodiment of Figures 1 and 2, the planar face of a rectangular section channel 5 made of a metallic grid, of high electric conductivity which can be conveniently closed on the back side by a cover or by a similar channel of a rectangular section 6, joined permanently to the channel 5 such to form a tubular element inside which the connection cables of the dipoles (not shown in the illustration) can be tidily arranged.
Channel 5 or at least the flat side coplanar and electrically joined with the side nets 1a and 1b has a thickness commensurate to the electric conductivity of the metallic material with which it is made and generally comprised between 0.5 and 1.5 or more millimetres.
Substantially the central part 1c of the reflecting panel is provided with a enhanced electric conductivity in order to minimise the resistance met by the currents that are induced on the screen reflector by the fields produced by the dipoles.
Figures 3 and 4 show another embodiment of the reflecting panel, according to which the central part 1c is made of a wire net 7 with smaller meshes and reduced empty/full ratio compared to the wire net of the side portions 1a and 1b of the panel.
Also the equivalent conducting cross section referred to currents induced on the plane of the screen reflector is sensibly increased in the central part 1c, by making such a net with metallic wire of large diameter than that of the wire with which the side nets 1a and 1b are made.
Always referred to the case of a typical application for mobile radio telephone systems in the 900MHz and/or 1800MHz bands, the dimensions of the meshes of the net constituting the central part 1c of the reflecting panel can be of 1 to 5 millimetres, and in case a steel wire net, arc-welded at the crossings, with a wire diameter of 0,5 to 1,5 millimetres is used, the empty/full ratio may range from about 1/10 to about 1/20.
As shown in Fig. 4, behind the central portion 1c of the reflecting panel 1 there may be a channel 8 made of optically opaque material fastened to the backside of the panel in order to define a channel through which the connecting cables of the dipoles may be arranged in a tidy way.
Both the nets 1a and 1b with a large empty/full ratio and the net with denser meshes 7 constituting the central part of area of the reflecting panel, are made of metallic wire, arc-welded at the crossings, in order to assure a homogenous conductivity over the reflecting plane and the absence of spurious dipoles which could be created in case of interruptions of the electric continuity at wire crossings.
Similarly to what previously illustrated with reference to Fig. 1, the wires can be oriented at 45° with respect to the main axes of the antenna. Similarly to what previously illustrated with reference to Fig. 1, the vertically oriented wires can lay on a plane spaced by a certain distance from the parallel plane on which the horizontally oriented wires lay.
Figures 5 and 6 are complete views of two directional multi-channel with high transparency according to the present invention.
Fig. 5 shows a so-called double polarisation antenna, that is, through which the electromagnetic radiation with a polarisation of +45° and with a polarisation of -45° is radiated. Fig. 6 represents an antenna with a single polarisation.
The radome 3 is entirely made by an optically transparent material. Preferably the material employed is a polymethyl methacrylate, even if others transparent plastic materials such as polycarbonate can be employed.
From the studies conducted by the applicant it has been found out, that the polymethyl methacrylate has a good resistance to the effects of the exposition to ultraviolet rays, and it has low dielectric losses which make it exceptionally transparent to the RF electromagnetic radiation thus minimising absorption.
The radome 3 envelopes completely the components of the antenna inside a completely sealed space.
The tubular body of the radome 3 can be extruded in a single piece or made of two suitably shaped channels joined together.
The two ends are dosed respectively by a lid 9 and by a bottom 10 made of the same transparent material constituting the tubular body of the radome.
In the case of the antenna with double polarisation of the Fig. 5, two connectors 11 and 12 are installed through the bottom 10 for the connection of coaxial feeding cables, while in the single polarization example of Fig. 6 there is a single connector 11.
Fig. 7 shows a preferred embodiment of the transparent radome 3 of the antenna of low visual impact according to this invention.
In the illustrated example the radome 3 is constituted by two channel shaped half- shells 3a and 3b, constituting the front side and the backside of the tubular radome, respectively. The two half-shells are permanently joined together by gluing them along the overlapping borders of the edges of the two channels 3a and 3b.
According to a highly preferred embodiment the cross section of the channels is such to form a polygonal cross section of the radome, having substantially planar sides connected to each other.
This particular shape, contrary to a commonly curved cross section of the known radomes, eliminates the persistence of a generation reflecting incident sun light or artificial light toward an observer.
On the contrary, a polygonal section will hardly present such a subtly persistent optical reflecting line.
Figures 5 and 6 show the presence of a support pole 13 which can commonly be of galvanised steel or of stainless steel, titanium or even more preferably a pole of fibreglass or another non conducting composite of high mechanical strength.
As can be observed, the central part 1c of the planar screen reflector of the antenna has a width sufficient to cover completely the shape of the supporting pole 13.
In the two examples shown in the Figures 5 and 6, the area of the nets 1a and 1b of the reflecting panel of the antenna constitutes about 80% of the total area of the screen reflector.
It is this large proportion of the area of the reflecting panel of the antenna (about 80%) that, in association with the optically transparent radome 3 gives to the whole antenna assembly a high transparency and under favourable light conditions a substantial «invisibility».
Moreover, the transparency of the radome reduces considerably the overheating effect of the inner space, thus enhancing constancy of the electrical behaviour of the antenna even under strong sunshine.
The transparency of the assembly can be clearly perceived by watching the two photos of an antenna of the type illustrated in Fig. 5 and reproduced in the Figures 8 and 9 in two different conditions of illumination.
Though, when observed facing against the light source (worst case) a certain visibility of the opaque elements of the assembly remains, under more favourable light conditions (Fig. 10), a translucency of the optically transparent radome effectively hides from sight even the opaque parts of the antenna.
In any case even, under the worst conditions, the observed antenna is perceived as an object of small and insignificant volume and it becomes even less visible if it is placed in a context rich of architectural features such as a spire, a facade of a building, a bell-tower, and alike environments.
In practice antennas made according to the present invention, of the type described in Figures 5 and 6 have been tested and their electrical characteristics and performances have been found to match and, under certain aspects, surpass those of comparable antennas in terms of shape and dimensions of the prior art commonly using a solid metal reflector and an optically opaque fibreglass radome.

Claims

Directional multi-channel antenna for a mobile radio telephone system, comprising:
- a substantially flat screen reflector (1) including a conductive metallic wire net (1a, 1b) with meshes having a relatively large empty/solid ratio in order to reduce the visibility of the antenna;

- a plurality of dipoles or radiators (2) mechanically supported at a certain distance from the reflecting surface of the screen reflector (1);

- a shell or radome (3) of a material optically transparent and transparent to RF radiation transmitted and received by the antenna enclosing the screen reflector, the dipoles and connecting wirings of the dipoles, and at least a connector for an antenna connection cable.
characterized in that a central part (1c) of said screen reflector (1) of relatively limited area compared to the whole area of the reflector and projectively coinciding with the zone of fastening of the antenna to a supporting pole, or alike structure, includes a wire net (1c) having a reduced empty/full ratio and reduced mesh size compared to the empty/full ratio and mesh size of the wire net of the major portion (1a, 1b) of the area of the screen reflector (1) for suppressing the evanescent wave generated therein.
Directional multi-channel antenna for a mobile radio telephone system, comprising:
- a substantially flat screen reflector (1) including a conductive metallic wire net (1a, 1b)with meshes having a relatively large empty/solid ratio in order to reduce the visibility of the antenna;

- a plurality of dipoles or radiators (2) mechanically supported at a certain distance from the reflecting surface of the screen reflector (1);

- a shell or radome (3) of a material optically transparent and transparent to RF radiation transmitted and received by the antenna enclosing the screen reflector, the dipoles and connecting wirings of the dipoles, and at least a connector for an antenna connection cable.
characterized in that a central part (1c) of said screen reflector (1) of relatively limited area compared to the whole area of the reflector and projectively coinciding with the zone of fastening of the antenna to a supporting pole, or alike structure, includes a coplanar flat metallic body (1c) soldered to the wire net (1a, 1b) for suppressing the evanescent wave generated therein.
Antenna according to one of the preceding claims, characterized in that the wires of said wire net are oriented parallel to the structural or symmetry axes of the antenna.
Antenna according to one of the preceding claims, characterized in that the wires of said wire net are oriented at 45° in respect to the structural or symmetry axes of the antenna.
Antenna according to one of the preceding claims, characterized in that the wires of said wire net are arc-welded or soldered at cross points.
Antenna according to one of the preceding claims, characterized in that said wire net are composed of a first plurality of parallel wires arranged on a first plane and by a second plurality of parallel wires orthogonal to and electrically isolated from the wires of said first plurality arranged on a second plane parallel to the first plane and spaced therefrom by a predetermined distance.
Antenna according to claim 1 or claim 2, characterized in that said radome (3) of optically transparent material is of a thermoplastic resin belonging to the group composed of polymethyl methacrylates and polycarbonates.
Antenna according to claim 7, characterized in that said optically transparent radome (3) is of polymethyl methacrylate.
Antenna according to one of the preceding claims, characterized in that said radome (3) of optically transparent material has a polygonal cross-section composed of rectilinear sides.
Antenna according to claim 2, characterized in that said coplanar flat metallic body (1c) is a flat side of a rectangular box extending for the entire height of the screen reflector (1) inside of which run said connecting wirings of the dipoles (2).
Antenna according to claim 1, characterized in that said wire net having reduced mesh size (1c) is a flat side of a tubular wire net body extending for the entire height of the screen reflector (1) inside of which run said connecting wirings of the dipoles (2).
Antenna according to one of the preceding claims, characterized in that said screen reflector (1) has on at least the reflecting side a coating of a conducting material belonging to the group composed of zinc, eutectic alloy of tin and silver, and silver.