EP1326301A1

EP1326301A1 - Antenna housing

Info

Publication number: EP1326301A1
Application number: EP02259032A
Authority: EP
Inventors: Ernest Roger Collinson
Original assignee: E Collinson and Co Ltd
Current assignee: E Collinson and Co Ltd
Priority date: 2002-01-04
Filing date: 2002-12-31
Publication date: 2003-07-09
Also published as: GB0230322D0; GB2384118A; GB0200190D0

Abstract

The invention proposes an antenna housing 3 and in particular apparatus in the form of a storage silo comprising an antenna housing having at least in part low attenuation walls comprising glass reinforced plastics with a foam plastics core.

In its preferred form the antenna housing 3 is mounted on top of an equipment housing 2 which is mounted on top of a storage silo 1. The use od grp for the walls ensures the desired low attenuation, whilst the foam core provides the necessary rigidity. Preferably the antenna housing is made up of a plurality of moulded flanged panels that are bonded or otherwise secured together to form a cylindrical structure having an external appearance of a storage silo or to match that of the storage silo on which it is mounted.

The equipment housing and storage silo may be made of pressed steel sheets or alternatively of grp. The preferred antenna housing panels also have an extension made solely of grp of a length sufficient to accommodate a low attenuation window for an SHF antenna.

Description

The present invention relates to an antenna housing.
A mast to serve as a base site or otherwise for radio communication equipment, for example, as used for mobile/cellular communication systems has to accommodate a number of antenna arrays. Three, six or twelve arrays may need to be accommodated depending on whether the mast is to be used by a single user or by shared users.
Standard lattice type radio masts are generally regarded as unsightly and consequently, their erection is controversial, especially in rural areas. One proposal to overcome this objection and enable the masts to blend into a countryside environment is to accommodate the antennas within a housing which replicates the appearance of a farm storage silo. The farm storage silos that we manufacture are made from a plurality of galvanized steel sheets that are secured together to form a cylindrical storage vessel having a diameter of up to 4.2m. The steel sheets are coated externally with a PVC paint to a desired colour. 4.2m is the maximum diameter that can be easily transported by road in the United Kingdom.
However, antennas can only be successfully located within a housing if the material for the housing wall has low attenuation characteristics. Fibre glass is a suitable material and is readily suitable for forming a housing wall but it has been found that a housing wall made of a chopped strand fibre glass mat would have to have a thickness of 5/6mm to resist wind speeds of up to 26m/s. However, such a wall thickness has attenuation losses of the order of 3.5dB at a frequency of 2 GHz which is deemed unacceptable. Accordingly, simply replacing the steel sheets with fibre glass has proved unsatisfactory. A further problem is that each antenna array has to have an unobstructed field of view so it is not readily possible to introduce additional bracing to solve the problem with strength. This problem is particularly acute when multiple UHF antennas are required to be accommodated within the housing. Increasing the diameter to allow the required field of view aggravates the problem of strength and presents problems with transportation by road if pre-fabricated. Reducing the diameter makes it difficult to accommodate the desired number of antennas.
The present invention aims to solve the above mentioned problems.
Accordingly, one aspect of the present invention provides an antenna housing comprising a plurality of wall sections of low attenuation material and wherein joints between adjacent wall sections provide rigidity to the structure without impeding radio signals.
The housing is preferably cylindrical and its circumference is preferably divided into either six or twelve equal wall sections. The circumferential spacing of the joints is sufficiently large to avoid impinging on the field of view of the radio beam. The construction allows UHF antenna to be arranged to suit individual site requirements. The housing is preferably large enough to house up to 12 UHF antenna of up to 2.5m high on any one level. The antenna may be spaced in groups of either 2, 3, 6 or 12. All or some of the antenna may be located on the same vertical level or on two or more separate levels with a vertical clearance of the order of 0.4m. Each level may be accommodated in a separate antenna housing module. Preferably the housing is also capable of accommodating one tier of SHF link antenna whose size can range from 300mm diameter to 800mm diameter and a minimum vertical separation distance of at least 0.4m between antenna. The SHF antenna may be housed in a separate module.
The material for the wall sections is chosen to give attenuation of no more than 1dB for the UHF antenna at frequencies of up to 2.2 GHz. The SHF antenna require especially low loss housings, typically of the order of 0.5dB, that will not defract the microwave beam. Accordingly, it is proposed to include low loss 'windows' in the wall sections, where the SHF antenna are located or to utilise material and/or thickness for the wall sections that meet the low loss requirements.
Preferably, the wall sections are formed from a plurality of panels that are secured together. Preferably, the height of each wall section is determined by the height of the UHF or SHF antenna to be installed including the necessary vertical separation distance between multiple tiers of antenna. The separation distance is required to prevent interference.
Utilising a plurality of wall sections allows flexibility in construction, enabling individual antenna housings to be constructed to suit special site requirements. The wall sections may be used to build up housing modules. Any particular installation may comprise one or more modules. It is preferred to utilise wall sections that are 3m high for UHF antenna and 1.1m high for SHF antenna. The panels making up the wall sections are preferably the full height of the wall sections, e.g. 3m or 1.1m respectively, and are preferably made in widths equal to 1/6^th or 1/12^th of the antenna housing circumference. The housing can be constructed to accept either a single or double tier of UHF antenna by using either one or two 3m high wall sections.
In a preferred embodiment an opaque removable panel (window) is fitted either above or below (or centrally between) the main 3m UHF panel or panels. The panel is removeable during installation of the antennae thus allowing the UHF antenna within each 30° wall section to be accurately aligned to known landmarks. The panel is manufactured from therrmoformed plastic with a curvature and finish to match the housing, it is conveniently secured with circumferential bolts, and a gasket provides suitable weatherproofing. When an SHF link antenna is required it transmits through the same aperture as the sighting window but the removable panel has a flat rather than curved front face. The front can also be supplied angled from 0° to 15° in increments of 5° to suit the required antenna alignment. The window for the SHF antenna is manufactured specifically from PVC-UE, or any other material that exhibits very low attenuation amongst its properties. Due to PVC-UE having a higher coefficient of thermal expansion compared to reinforce plastic provision has to be made for expansion that would cause the flat face of the 'window' to distort. This is achieved by vacuum forming a "top hat section. This allows the face of the window to expand without deflection. These removable panels are fitted into a separate section of wall 1.1m high thus allowing it to arranged within the vertical height of the housing as required.
An alternative to PVC-UE for the removable panel / window could be clear PTFE. This has the required low loss requirements for the SHF link and would not need to be removed for sighting the UHF antenna. However due to the window being translucent the aesthetics may be impaired. Maintenance lighting within the housing will also be seen when in use. Alternatively the wall sections may comprise of individual panels of either moulded reinforced plastic with or without a foam/honeycomb core, solid PVC-UE or PTFE. In the case of PVC-UE and PTFE the panels are thermoformed to suit the housing and painted to match the main structure, alternatively in the case of other PTFE based materials the panel may be left translucent.
Preferably the wall sections comprise a composite section of moulded reinforced plastics with or without a PVC foam and/or honeycomb core. The reinforced plastics may be sheet or laminate and may be comprised of glass fibre strand, mat, PTFE, or any other low attenuation material. The preferred panels have a flange formed around their edges from, e.g. glass reinforced plastics, to provide both a means of securing the individual panels to each other to form the completed structure and to provide structural integrity to the housing. Preferably the panels are secured together by bonding. Preferably a gutter is formed on the bottom flange of each section to allow the collection of condensation. Preferably the condensate is released to the exterior of the housing through a drain hole. Preferably the outer surface has a leather grain gel coat applied during the moulding process and colour matched to suit the main structure. An advantage of using a reinforced plastic with a foam/honeycomb core is that the material has a much lower U value than solid reinforced plastic, which reduces thermal pickup and minimises condensation. Using two thin glass fibre layers (2x1mm) and very low loss foam core gives low attenuation losses and yet gives high strength and good rigidity. Such a structure is ideal for the walls shielding the UHF antenna. Thicker glass reinforced plastics is conveniently used for wall sections behind which the SHF antenna are situated, with apertures cut therein to accommodate the very low loss windows.
In a preferred construction each panel that is used to form the antenna housing has a vertical height sufficient to accommodate one UHF antenna and one SHF antenna. I.e. 4.1m based on the above example. That part of the panel that will shield the UHF antenna is formed from the aforesaid sandwich comprising a core of foam plastics sandwiched between two layers of glass reinforced plastics (grp), whilst the part where the SHF antenna will be located comprises a thicker section of glass reinforced plastics. A material known as Forex may be used for the foam core. Each panel has a peripheral flange of grp extending inwardly from the peripheral surface of the panel. The panel is moulded with a curvature to match that of the outer periphery of the silo. Standard diameters of 3.5m and 4.2m are preferred to facilitate transport to site by road already built up into cylindrical sections. The UHF and SHF sections of the panel are disposed one above the other. The panels can be installed either end up, i.e. with the UHF section uppermost or lowermost. Adjacent panels may be assembled in the same or alternative orientations according to the desired antenna installation pattern.
It will be apparent from the foregoing that the antenna housing is arranged to obscure from view the antenna by virtue of the fact that they are enclosed within the housing, and that the housing is preferably cylindrical. It is further preferred that the antenna housing has the appearance of a storage silo such as used on farms or for other industrial applications. In order that the antenna are presented at the desired height, the antenna housing is mounted on a support structure therefor. Preferably the support structure comprises at least in part a cylindrical structure having the appearance of a storage silo. The cylindrical structure may extend down to ground level, such as is the case with a tower silo, or may be supported on exposed legs. For most practical applications to present the antenna at the desired elevation a section of support structure having a cylindrical configuration resembling a storage silo will be required. Exceptionally the support legs may extend up to the antenna housing module. The aforesaid cylindrical structure on which the antenna module is mounted may comprise a working silo, a dummy silo, an equipment housing or any combination thereof.
Another aspect of the invention provides a storage silo incorporating an antenna housing as aforedescribed.
A further aspect of the invention provides apparatus in the form of a silo comprising an antenna housing having at least in p[art low attenuation walls.
The silo preferably comprises a storage compartment and means for charging and discharging material therefrom. The silo is not necessarily an animal feed silo.
A yet further aspect of the invention provides apparatus in the form of a storage silo comprising an antenna housing having at least in part low attenuation walls comprising glass reinforced plastics with a foam core.
The storage silo is preferably a working silo but may be a dummy silo. It preferably comprises a modular construction. Preferably the silo is constructed and transported in a plurality of pre-fabricated modules. The modules may be made up of one or more of a storage silo module, a base structure, an equipment housing module and one or more antenna housing modules. The antenna housing module preferably comprises one or more UHF antenna housings and at least one sighting or SHF antenna module. A typical construction will comprise at least three pre-fabricated modules comprising: the storage silo support feet; the equipment housing; and the antenna housing with or without an integral roof structure. The storage silo may be formed as part of the base structure or may be transported as sheet and erected on site. The exterior of the antenna housing module or modules is preferably finished in a colour and texture to match that of the silo. The external surface of the antenna housing preferably has a surface finish that minimises retention of water droplets that may reduce the low loss requirements of the housing. Preferably it is treated with a clear silicone type material.
More preferably for a structure comprising a working silo supported by legs, an equipment housing, and an antenna housing mounted one on top of the other, and surmounted by a roof, the silo with its legs and the equipment housing are transported to the intended installation site in one prefabricated unit, and the antenna housing, usually with its roof, is transported to site in another prefabricated unit. Once on site the antenna housing is fitted to the top of the equipment housing before the whole assembly is lifted into the desired position using a crane.
The storage compartment for the silo comprises a hollow cylindrical structure having a double skin roof which also serves as the floor for the equipment housing. The centre of the double skin ceiling/floor structure is provided with a lifting lug by which the unit is lifted into position. The equipment housing preferably has its walls formed from plastic coated galvanised steel sheet secured to a plurality of vertical ribs. The walls may be single or twin skinned , with or without insulation according to design requirements. The equipment housing has a ceiling structure formed by a plurality of radial ribs that extend inwardly from the vertical ribs of the external walls. The ceiling functions as the floor for the antenna housing and is apertured to allow access into the antenna housing. A central aperture with a removable cover allows a lifting line to pass through and connect with the lifting lug.
In a preferred construction an head frame is attached to the floor of the antenna housing. The head frame preferably has a shape that mirrors the number of wall sections of the antenna housing. An hexagonal shape is utilised where the antenna housing comprises six or twelve wall sections. The antennas to be installed in the antenna housing are secured to the head frame by a convenient fixing. The position of the head frame within the antenna housing is preferably angularly adjustable. In the preferred construction the equipment housing affords an unobstructed space for installation of equipment.
In one alternative the antenna housing has a central steel structure or core that provides support for both the lightweight panels forming the wall sections and for flexible antenna mounting arrangements. In addition, intermediate access platforms can be arranged to provide access for riggers during installation and maintenance. The access decking is either six sided or more usually twelve sided. Preferably it is designed to correspond to the number of side wall sections used in the particular design. The access decking has mounting plates arranged around its periphery to provide support for the antenna mounting poles. Brackets provide a flexible method of mounting the antenna. Preferably the mounting arrangement allows the UHF antenna to be arranged towards the outside of the housing and for the SHF antenna to be arranged radially inwardly thereof. Advantageously the wall sections are mountable at any desired circumferential position around the central vertical core of the structure (and likewise the antenna are mountable at any desired circumferential position) in increments of 1° enable individual antennas disposed within the housing to be accurately aligned on a desired bearing. Preferably each antenna mounted within the housing has provision for a removable viewing panel to allow each individual UHF antenna to be accurately aligned during installation to known landmarks for example via an optical device fitted to each individual antenna.
The housing is surmounted by a roof structure. Conveniently, it is formed by GRP mouldings. It is preferred that interior of the housing is maintained within a temperature range of 0° C to 35° C. To that end the housing incorporates an automatic thermostatically controlled ventilation system. Preferably the roof of the housing incorporates a ventilation duct, preferably centrally positioned in the roof. Means may be provided to collect and remove any condensate that may gather on the internal surface of the housing. An air conditioning unit and/or de-humidifier may be provided to maintain the desired temperature and/or humidity within the housing. Preferably it is incorporated in a separate equipment housing. The equipment housing also incorporates the necessary electronics for the antenna system. Because of the proximity of the telecommunications equipment to the antenna the cable runs are shorter and this has the advantage of reducing transmission losses by approximately 1dB. Heat from the equipment may be used to fight the build up of ice on the antenna housing by the provision of heat transmission ducts or other heat sink provisions. A lightening conductor is provided for the structure.
In an alternative the wall sections of the antenna housing are mounted to a support structure comprising a plurality of substantially vertical ribs extending between a support structure therefor. The wall sections comprise a plurality of vertical panel members made from low attenuation material and located along their opposite vertical edges by the ribs. The vertical ribs may be manufactured from either extruded, moulded or otherwise fabricated aluminium, PVC or reinforced plastic.
Preferably, the support structure further comprises two spaced circumferential hoops and the ribs extend between the hoops.
More particularly there are either six or twelve ribs and it is preferred that they are constructed in the manner of glazing bars that are preferably adapted to receive the panels in a snap fit manner, using glazing beads. The preferred material for the panels is foamed PVC. A panel thickness of 10mm when made of foamed PVC has been found to provide a suitably low attenuation. Typically of the order of 0.5-1dB at frequencies up to 2.2GHz.
With a housing diameter of 4.2m, the desired propagation angle of the UHF antenna arrays can be accommodated with up to 12 ribs.
The preferred housing also incorporates the necessary telecommunications equipment in an equipment module that can be disposed immediately below the antenna housing in an equipment housing. It is particularly advantageous if the antenna housing and the equipment housing are incorporated in a working storage silo. Usually the antenna housing will form the upper part of the structure and the equipment housing a portion of the structure immediately below the antenna housing. The construction of the storage silo can follow that which is common for storage silos. Advantageously, a modular construction is used comprising a silo base with a storage silo compartment, an equipment housing and the antenna housing. Each module is preferably pre-fabricated off site and the modules assembled on site.
The present invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:-
Figure 1 is a schematic side view of a storage silo accommodating an antenna housing according to one embodiment of the present invention,
Figure 2 is an enlarged cross-sectional view of the antenna housing shown in the embodiment of figure 1
Figure 3 is a section on B-B of figure 2
Figure 4 is a section on A-A of figure 3 or 5
Figure 5 is section on C-C of figures 2 or 7
Figure 6 is a simplified diagrammatic section through the housing of figure 1 illustrating the antenna radiation angle for an embodiment having two UHF antennas, and
Figure 7 is a cross-sectional view of an antenna housing according to a second embodiment.
The present invention relates to an antenna housing designed to suit the installation of one or both of UHF and SHF antenna and configured to resemble a storage silo. The storage silo may be a working silo, a dummy silo, a tower or similar structure. The present invention is described hereinafter, by way of example, in relation to its incorporation in to a working silo. Referring firstly to the drawings of figures 1 to 6, a first embodiment of storage silo 1 is illustrated and comprises a base framework comprising six circumferentially spaced uprights 31 connected by horizontal stringers 33 with diagonal braces 35. The uprights connect with a hip ring 36. The uprights are provided with feet for mounting on a concrete plinth 37. The base framework is superposed by a cylindrical storage silo 19 formed by a plurality of pressed galvanised steel sheets 20 that are bolted or riveted together to form a load bearing structure. The sheets are connected to the hip ring 36. The storage silo has a 60° or 67° cone 16 to its base and is provided with the usual equipment to allow it to function as a storage silo including an access panel 14, filler pipe 17, ventilation pipe 10 and outlet valve control, etc. which are common in the art and not described in any further detail.
An equipment housing 2 is disposed above the storage silo 19 and is defined between hoops 41, 43 which define floors for the equipment housing and for an antenna housing 3 thereabove. The construction of the equipment housing follows similar lines to the silo construction. Its outer wall comprises a plurality of pressed galvanised steel sheets secured to vertical ribs. The floor is of double skin construction and comprises a plurality of radial ribs extending from the hoop 41 and in a preferred embodiment these connect to a central ring (not illustrated). The floor is of galvanised steel sheets. A removable cover plate affords access to the lifting lug when required for on site erection of the silo. The equipment housing has an access door 57 in a side wall thereof and typically incorporates electronics for the telecommunications equipment as well as air conditioning plant as described further hereinafter.
Access to the equipment housing is by way of an external access ladder 11. The floor of the equipment housing is supported by the walls of the storage silo and also forms the roof for the storage silo. Upper hoop 43 supports a plurality of radial ribs that provide the floor for the antenna housing3 that is surmounted on top of the equipment housing. The floor is apertured to allow access to the antenna housing and to provide for air circulation between the antenna housing and the equipment housing. Where multiple antenna housing modules are provide, as illustrated in the particular embodiments of figures 2 and 7, intermediate access decking platforms 46 may be provided as described and illustrated in further detail with reference to figures 3 and 5. The antenna housing 3 is described in further detail hereinafter.
The antenna housing 3 is formed by a plurality of wall sections 41, twelve in the illustrated embodiment, which are disposed on equal circumferential spacings. The wall sections comprise a composite section of moulded reinforced plastic with a PVC foam and/or honeycomb core. A preferred panel construction comprises 10mm thick PVC foam sandwiched between two layers of 1mm thick GRP. Alternatively, the outer layers may made of glass fibre strand, mat, PTFE or any other low attenuation material. The panels are moulded and have a flange 43 formed around their edges to provide both a means of bonding or bolting individual panels to each other to form the completed structure and to provide structural integrity to the housing. Preferably the adjacent upright edges of the panels are secured directly to one another. Exceptionally a reinforcing bar may be disposed between the adjacent panels. The bar may be made of plastics, steel or aluminium. A gutter 51 is formed on the bottom flange of each section to allow the collection of condensation. The condensate is released to the exterior of the housing through a drain hole. The outer surface has a leather grain gel coat applied during the moulding process and colour matched to suit the main structure of the silo. A further benefit of using a reinforced plastic with a foam/honeycomb core is that the material has a much lower U value than solid reinforced plastic, which reduces thermal pickup and minimises condensation.
The intermediate access decking 46 comprises a plurality of five sided deck elements 48 (six in the illustrated embodiment) - see figures 3 and 5 - which depend from the internal steel lattice. Brackets 50 extend radially outwardly and provide mounting points to which the wall sections 41 are secured -see figures 2 and 3. The deck elements define a central aperture 52 that accommodates an access ladder 54. The ladder is attached to a mounting ring 56 at its upper end which in turn attaches to an upper antenna ring support channel 58. The decking elements and/or the brackets can be mounted at any desired circumferential position with respect to the internal structure.
In the embodiment illustrated in figure 1 and 2 the antenna housing comprises a first module that is configured to accommodate a single tier of UHF antenna. Two UHF antenna are shown at 61. The antenna 61 are attached to a respective antenna mounting pole 67 which is itself attached to the structure by a circular mounting ring 62. The ring allows the poles/antenna to be mounted at any desired cicumferential angular position. Each wall section is 3m high and in the illustrated embodiment has a periphery equal to 1/12 of the circumference of the housing. However, the housing can be constructed to accept a double tier of UHF antenna by using two 3m high wall sections secured on top of one another as illustrated in the embodiment shown in figure 7.
Each wall section can be removed individually if so required for access to the antenna or for sighting purposes on installation. However, it is preferred to provide a separate viewing window in an additional circumferential housing module as seen at 4 in figures 1, 2 and 7. The housing module 4 may be fitted either above or below (or centrally between) the main antenna module comprising the 3m UHF panel or panels. In the illustrations it is above the UHF panel sections. The separate section of wall is 1.1m high. It is formed from panels 41' that are made from 5-6mm GRP and each panel accommodates a window 67. The window is conveniently formed as a removable element in a panel forming the separate housing part. Each window is preferably opaque. The window is removed during installation of the antennae thus allowing the UHF antenna within each 30° wall section to be accurately aligned to known landmarks. The window is manufactured from themoformed plastic with a curvature and finish to match the housing. It is secured with circumferential bolts, and a gasket provides suitable weatherproofing.
When an SHF link antenna 61 is required (as illustrated in figures 2 and 7) it transmits through the same aperture as the sighting window but the removable panel has a flat face (see 67' in figure 3) rather than a curved front face. The front can also be supplied angled from 0° to 15° in increments of 5° to suit the required antenna alignment - see for example 67" in figure 3. The window for the SHF antenna is manufactured specifically from PVC-UE, or any other material that exhibits very low attenuation amongst its properties. Due to PVC-UE having a higher coefficient of thermal expansion compared to reinforce plastic provision has to be made for expansion that would cause the flat face of the 'window' to distort. This is achieved by vacuum forming a "top hat section. This allows the face of the window to expand without deflection. PVC-UE is extruded UV stabilized PVC sheet that has been foamed or expanded to increase air content and reduce the PVC content. Alternatively PTFE may be used.
The SHF antenna are located in a corresponding manner to the UHF antenna. Respective upper and lower mounting rings 62' are provided. A mounting pole 63' is secured between the rings and the SHF antenna 66 is attached to the pole. The mounting arrangement allows the SHF antenna to be mounted in any desired angular position. Two possibilities are shown in Figure 3.
A lifting point is provided at 90 attached to the internal structure for lifting the modules into position. Also shown in figure 2 is the moulded roof structure 92 comprising, for example, three Moulded GRP panels that are bolted together and secured to the internal structure. A roof ventilator is shown at 95 and a roof ventilator actuator at 94. An upper antenna ring coupling plate is shown at 95.
Figure 6 illustrates how the field of view of the UHF antenna 61 passes unimpeded between the joints of the wall panels 41. Two UHF antenna are shown in the illustrated embodiment.
Figure 7 illustrates another embodiment in which two UHF antenna modules 3 are provided to accommodate two tiers of UHF antenna 61. The two UHF modules 3 are superposed by the module 4 accommodating the viewing windows and any SHF antenna. Corresponding numbers have been used to denote identical parts described heretofore and are not described further. The internal structure for each module is bolted to the next to form the desired size of antenna module. The embodiment of figure 7 includes an equipment housing 2 and a working storage silo as described in relation to the embodiment of figure 1.
Lightening protection is provided for the structure and a lightening conductor is shown at 100 in figures 1, 2 and 7. For a typical construction, such as that illustrated in figures 1 and 2, the base structure up to the hip ring 36 is formed as one pre-fabricated module and the antenna housing parts 3,4 and roof 5 are formed as a second pre-fabricated module. It is preferred to form the equipment housing as a further pre-fabricated module. The panels forming the walls of the storage silo may be pre-fabricated in conjunction with either the base module or the equipment housing, but more preferably they are transported as flat sheet and erected on site. The detailed construction of the silo will depend on whether it is a working or dummy silo. If the former, explosion protection provisions will usually be incorporated. Usually this will dictate a separate roof structure for the silo or reinforcement of the equipment housing floor as well as the provision of side explosion panels in the silo.
In an alternative construction the base and storage silo are constructed as aforedescribed and likewise the equipment housing and its floor. The floor of the antenna housing comprises a plurality of radial ribs extending from hoop 43 and connected to a central ring. The floor is apertured as aforesaid to permit access from the equipment housing and for air circulation purposes. However, the floor also provides a mounting surface for a head frame to which the antennas are mounted, directly or indirectly. A preferred head frame is hexagonal and its angular position is adjustable with respect to the floor. To that end the floor has a plurality of mounting holes disposed on a common pitch circle diameter that can be aligned with holes in the base of the head frame to allow angular adjustment in say 1⁰ increments. Preferably one of the six faces of the head frame is set to be normal to grid North. In practice the silo may be positioned to achieve the desired alignment without having to adjust the head frame separately.
A preferred antenna housing comprises 6 or 12 wall sections that are of a height to accommodate both a UHF antenna and a SHF antenna and for the head frame to have a corresponding height. This avoids the need for a separate SHF antenna module and also means that the intermediate access deck shown in the embodiment of figure 2 can be omitted. Each panel of the wall section thus comprises a low attenuation part (say approximately 3m thereof) formed by the foresaid two layers of grp sandwiching a foam core, whilst the other part (say 1.1m thereof) comprises a thicker grp section of the order of say 5m thick without the foam core. Where viewing windows or SHF antenna are required, apertures are cut into this part to accommodate the viewing window or the low attenuation window as the case may be in a manner substantially as previously described.
The panels can be orientated to have the UHF antennas all positioned on the same level, or to have them positioned at different levels so that the SHF antenna are positioned either uppermost or lowermost.
The silo construction described above has centred around the formation of the walls using pressed steel sheets which in themselves do not have a sufficiently low attenuation to accommodate either UHF or SHF antennas. Hence the need to introduce the low attenuation section made of the aforesaid grp with foam core.
Another design of silo that is in use utilises a one piece cylindrical tower structure with the walls formed of glass reinforced plastics. Often these are formed using a rotational moulding technique. However wall thicknesses are such that they have a high attenuation factor and therefore the existing designs are not suitable to accommodate antennas. However, another aspect of the invention proposes reducing the wall thickness in specific regions to reduce the attenuation to an acceptable level.
The strength requirement could be met by incorporating the aforementioned foam sandwich between two layers of grp. Accordingly the present invention also contemplates a storage silo made of grp in which at least a part of the wall thereof, more particularly a circumferential band thereof, has a reduced thickness of grp to provide a low attenuation section with or without the introduction of a reinforced foam component. Conveniently an internal steel skeleton provides mounting points for the antennas.

Claims

Apparatus in the form of a silo and characterized in that it comprises an antenna housing (3) having at least in part low attenuation walls.
Apparatus as claimed in claim 1 and comprising a storage compartment (1) and means for charging and discharging material therefrom.
Apparatus comprising an antenna housing (3) and charactersied in that the antenna housing comprises a plurality of wall sections of low attenuation material and wherein joints between adjacent wall sections provide rigidity to the structure without impeding radio signals.
Apparatus as claimed in claim 3 in which the periphery of the antenna housing is divided into a plurality of equal wall sections and in which the circumferential spacing of the joint between each wall section is sufficiently large to avoid impinging on the field of view of the radio beam of any antenna mounted therein.
Apparatus as claimed in claims 1, 2, 3 or 4 in which the low attenuation walls comprise glass reinforced plastics with a foam plastics core.
Apparatus comprising an antenna housing and characterized in that the housing has at least in part low attenuation walls comprising glass reinforced plastics with a foam plastics core.
Apparatus as claimed in any one of claims 1 to 6 in which the antenna housing comprises a plurality of panels secured together in edge relation to form a continuous peripheral wall section and wherein at least opposite sides of the panels that are upright in use are formed with flanges by which the panels are secured together and wherein the flanges are formed of glass reinforced plastics.
Apparatus as claimed in any one of claims 5, 6 or 7 in which each panel has in addition to a first part formed by sandwiching a core of foam plastics between two layers of glass reinforced plastics, a second part comprised of glass reinforced plastics without the foam core.
Apparatus as claimed in any one of claims 3 to 8 in which the antenna housing is mounted on top of a storage compartment.
Apparatus as claimed in claim 9 and further comprising an equipment housing.
Apparatus as claimed in any one of the preceding claims and comprising at least a further antenna housing.
Apparatus as claimed in any one of the preceding claims and wherein the at least a further antenna housing accommodates at least one UHF antenna.
Apparatus as claimed in any one of the preceding claims and wherein the at least a further antenna housing accommodates at least one SHF antenna.
Apparatus as claimed in claim 21 in which the antenna housing is provided with a low attenuation window for each SHF antenna.