Abstract:
An assembly of mechanically steerable directional radio antennas, comprising a primary antenna and at least one secondary antenna, arranged such that the or each secondary antenna is capable of being physically steered over a limited azimuthal arc relative to the primary antenna, and is at least partially within the swept volume of the primary antenna. By allowing the swept volumes of the antennas to overlap, a compact assembly can be provided, while by limiting the azimuthal movement of the secondary antennas relative to the primary, it can be arranged that the antennas do not foul each other.

Description:
This application is the US national phase of international application PCT/GB01/03983 filed 5 Sep. 2001 which designated the U.S. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radio antennas, and in particular to directional antennas arranged for point to point communication. Various proposals have been made for local microwave distribution systems, in which generally a central node is connected by a fixed cable (optical fibre or conventionally wired), or by other means, to other switched or packet systems, the central node acting as a distribution point from which a large number of end users can be served by microwave links. 
     2. Related Art 
     Such systems have been proposed for many years: see for example an article 29  GHz Point to Point Radio Systems for Local Distribution  by S Mohamed and M Pilgrim in the  British Telecommunications Technology Journal  Vol2, No 1 (January 1984). Generally, each end user employs a directional antenna aimed at a corresponding antenna at the central node. In certain cases one end user&#39;s installation may act as a relay station to allow communication between the central node and a second end user which is out of range of the central node (typical range for a 40 GHz transmitter is of the order of 2 km), or does not have an unobstructed line of sight to the central node. 
     More recent proposals have extended this principle to develop a “mesh” system, in which only a few base stations are required and the user stations are connected to their nearest base station through one or more such relays. Such a system is illustrated in International Patent Specification WO98/27694. To provide multiple routing for packet data systems, and for sufficient robustness to the system in the event of a user station ceasing to operate, either temporarily as the result of a system failure or permanently (for example should the user no longer wish to use the service), each user station is provided with several antennas for provision of links with several neighbouring user stations. The mesh may be served by more than one base station, as shown in FIG.  1 . 
     When a new user station is to be connected to the network, the connectivity of the mesh has to be changed to accommodate it. This requires re-alignment of the the directional antennas of some of the neighbouring stations, so that the new station can be connected into the mesh. Similarly, if a station is taken out of service, antennas on neighbouring stations may have to be redirected. It is envisaged that such redirection be carried out by the network operator remotely, rather than requiring a site visit. 
     One way to achieve this is disclosed in International Patent Application WO 99/65162, in which an fixed array of thirty-two directional antennas is provided. Each antenna is aligned in a different azimuthal direction. The antennas are switched on or off according to the current requirements of the mesh network. Several adjacent antennas can be used together as a phased array. This system is somewhat cumbersome as it requires space for a large number of antennas, only a few of which are in use at any one time. An alternative arrangement shown in International Patent Application WO 99/65105 uses a remotely controlled mechanically steerable antenna. This reduces the volume of the installation. However, in order to act as a relay the station must have more than one such antenna, each independently controlled. To avoid fouling each other, each antenna would have to be mounted in a volume clear of the other antennas&#39; swept volumes. The simplest arrangement is a vertical stack of such antennas, each rotatable about a common vertical axis. However, such an arrangement is cumbersome, and its size and weight makes rooftop installation difficult. It is desirable to minimise the size of such equipment for reasons of materials costs, wind loading, simplicity of installation, and aesthetics. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, there is provided an assembly of mechanically steerable directional radio antennas, comprising a primary antenna and at least one secondary antenna, arranged such that the or each secondary antenna is capable of being physically steered over a limited azimuthal arc relative to the primary antenna, and is at least partially within the volume swept by the primary antenna. By allowing the swept volumes of the antennas to overlap, a compact assembly can be provided, whilst by limiting the azimuthal movement of the secondary antennas relative to the primary, it can be arranged that the antennas do not foul each other. 
     In a preferred arrangement, some of the secondary antennas may be vertically offset from each other. In some of the embodiments to be described, the secondary antennas rotate about the same vertical axis as the primary antenna, whilst in the other their axes of rotation are parallel. 
     The secondary antennas may be plate antennas, such as flat plate array antennas, which carry printed, etched, machined or other radiative elements, each arranged to move over part of the circumference of the swept volume of the primary antenna. The primary antenna may also be a plate antenna, or a horn antenna. The swept volumes of two or more of the secondary antennas may overlap. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Four embodiments of the invention will now be described, by way of example only, with reference to the drawings in which: 
         FIG. 1  is a schematic illustration of a microwave distribution mesh system of the kind for which this invention is intended for use: 
         FIGS. 2 and 3  are respectively a schematic sectional elevation and plan view of an antenna assembly according to a first embodiment of the invention: 
         FIG. 3  is a schematic sectional plan view of the antenna assembly of FIG.  2 : 
         FIG. 4  is a schematic sectional elevation of an antenna assembly according to a second embodiment of the invention: 
         FIG. 5  is a schematic sectional elevation of an antenna assembly according to a third embodiment of the invention 
         FIGS. 6 and 7  are respectively a schematic sectional elevation and plan view of an antenna assembly according to a fourth embodiment of the invention: 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1 , which is a reproduction of a Figure from International Patent Specification WO98/27694, shows a simple example of a network of the kind for which the present invention is intended for use. In the example shown, there are sixteen subscribers or users, each of which is associated with a network node  2 . Each node  2  has a radio transceiver unit which is able to transmit and receive high frequency radio signals, for example between 1 GHz to 40 GHz or more. The transceiver unit of each node  2  is in direct line-of-sight contact with several other similar units at other respective nodes  2  by direct line-of-sight wireless links  3 . It can be seen from  FIG. 1  that the nodes  2  of the network  1  can communicate with each other either directly, or by way of other nodes if necessary to avoid buildings  6  or other obstructions which otherwise block direct line-of-sight connection between particular nodes  2 , or to overcome the limited range of transmitters working at these frequencies. A message from any one node  2  to any other node  2  will typically traverse several links  3  in a series of “hops” across the system  1 . Interconnect trunks  4  connect specified nodes  2  to a trunk network  5 . 
     Each node  2  is provided with at least the same number of antennas as there are links  3  associated with that node  2 . To allow reconfiguration of the network as nodes  2  or obstructions  6  are added or removed from the system  1  the nodes are provided with the capability to adjust the directions of their associated links. In one arrangement discussed in the prior art reference WO98/27694, an array of fixed antennas is provided, the appropriate antenna for each link  3  required being switched in as required. Such an arrangement requires a much larger number of antennas to be provided at each node than are actually needed at any one time, significantly increasing the bulk and capital cost of the node installation. In alternative arrangements a smaller number of independently steerable antennas are provided. The steering may be electrical (that is, by controlling the electrical characteristics of the antenna to control the effective boresight direction) or by physical movement of the antenna. It is of course possible for different nodes  2  to use different types of antenna assembly. 
     To obtain optimum use of the radio spectrum and minimise the amount of equipment required at each node, the antennas at a given node  2  may share a single transceiver, using any known multiplexing technique to serve all the links  3  from the one node  2 . 
       FIGS. 2 and 3  show schematically an antenna assembly  7  according to the invention, for use at one or more of the nodes  2  of such a network.  FIG. 2  is an elevation, and  FIG. 3  is a plan view. Both Figures show part of the outer housing removed, and  FIG. 3  also has one of the motor assemblies removed. Electrical connections are also omitted from both Figures for clarity. 
     The antenna assembly  7  has an outer housing  8 , transparent to radio waves, provided to protect the components within from the weather, and to provide an aesthetically unobtrusive appearance. In this embodiment the housing is spherical, but other shapes may also be employed. It may be secured to a building or other structure by any suitable means, from which it may also obtain its power supply. 
     Mounted within the upper part of the housing  8  are two concentric spindles  9 ,  10  extending vertically downwards, whilst in the lower part of the assembly two further concentric spindles  11 ,  12  extend vertically upwards. 
     The inner spindle  9  of the upper pair is connected to the horn  13  of a directional antenna, such that the horn  13  can be turned to any selected azimuthal orientation, to establish radio contact with a directional antenna at another node  2 . The rotational freedom of the horn  13  defines a cylindrical swept volume, having a diameter equal to the length of the horn antenna, (including the associated waveguide), a height equal to the height of the horn, and a vertical axis defined by the spindle  9 . The inner spindle  11  of the lower pair ends in a bearing  14  supporting the horn  13 . The dimensions of the housing  8  are largely constrained by the size of the antenna horn. 
     The other spindles  10 ,  11 ,  12  are each connected by a respective spacer arm  15 ,  16 ,  17  to a respective flat plate antenna  18 ,  19 ,  20 . These antennas are mounted at least partially within the swept volume of the horn  13 , but their movements are limited such that they do not foul the horn  13  itself. The flat plate antennas  18 ,  19 ,  20  can all move in azimuth through approximately 270°, relative to the position of the horn  13 , being prevented by the horn  13  itself from occupying a position less than 45° either side of the boresight of the horn. In the embodiment depicted the two flat plate antennas  19 ,  20  connected to the lower spindles  11 ,  12  both have the same vertical extent, and therefore are further constrained not to occupy positions within 45° of each other. 
     Electrical connections (not shown) are provided between each antenna  13 ,  18 ,  19 ,  20  and a transceiver  21 , which may be located within the housing  8  as shown or elsewhere. The transceiver  21  relays signals between the antennas  13 ,  18 ,  19 ,  20  in its function as a node  2  of the network  1 , and also has a feed to and from the user terminal associated with the node  2 . The user terminal will typically be within the building upon which the antenna assembly  7  is mounted. The assembly  7  may also obtain its power supply from the building, or from a self contained system such as solar panels mounted on the upper part of the housing  8  where they will not obstruct the passage of radio signals to and from the antennas  13 ,  18 ,  19 ,  20 . 
     An assembly of antennas of this kind could be aligned by hand. However, antenna assemblies are typically located in elevated locations which are difficult of access. Moreover, to establish a new link  3  requires simultaneous alignment of antennas at two separate nodes  2 . To avoid the need for site visits, it is therefore preferred to align the antennas by remote control. A control system  23  (shown in  FIG. 4 ) is therefore provided for controlling the positions of the directional antennas  13 ,  18 ,  19 ,  20 , by means of motors  24 ,  25  mounted in the housing  8  and capable of driving the spindles  9 ,  10 ,  11 ,  12  to move the antennas  13 ,  18 ,  19 ,  20  relative to the housing  8 . Each spindle  9 ,  10 ,  11 ,  12  can be driven independently of the others. As shown in  FIG. 2 , the upper spindles  9 ,  10  can be driven by an upper motor assembly  24 , and the lower spindles  11 ,  12  by a lower motor assembly  25 . The upper motor assembly  24  may comprise a separate electric motor for each spindle  9 ,  10 , or a single motor may be provided whose output spindle can be selectively connected to either spindle  9 ,  10 . The connections between the lower motor assembly  25  and the lower spindles  11 ,  12  are similar. It will be appreciated that suitable mechanical connections may be used to allow a single motor to selectively drive any of the spindles  9 ,  10 ,  11 ,  12 . 
     Control may be achieved by radio signals received from the network controller through one or more of the directional antennas  13 ,  18 ,  19 ,  20 . However, before initial installation or reconfiguration is performed, it is likely that none of the directional antennas will be aimed towards a transmitter from which such control signals can be received, so it is preferred that the control signals are transmitted to the user terminal by an alternative telephone system, such as the public switched telephone network (PSTN), and then to the antenna control system  23  by means of the user connection. If a fixed PSTN connection is not available, an omnidirectional antenna may be provided to receive control radio signals, for example to a cellular telephone integrated in the control system  23 . When the network  1  is to be reconfigured, either on installation of the node  2  or subsequently on changes to other nodes, the network operator transmits coarse control signals to the control system  23  of the antenna assembly, causing the motors in the motor assemblies to move the antennas  13 ,  18 ,  19 ,  20  into the required positions. The angular constraints on the movement of the antennas may be programmed into the control systems of the network operator, to prevent the network operator commanding an incompatible set of orientations. Alternatively, the required directions may be specified by the network operator, the control system  23  selecting which antenna to aim in each specified direction according to constraints programmed into the control system  23  itself. Automated techniques for acquisition of neighbouring nodes are also possible. 
     Fine control of the antennas&#39; positions can be carried out by any suitable means, such as by transmitting a signal from the antenna at one end of a link  3  to the antenna at the other end, and moving both antennas co-operatively to optimise the received signal. 
     The performance of the antennas  13 ,  18 ,  19 ,  20  may differ because of their different designs. The choice of which antenna to use for each link  3  can be made to optimise the overall quality of the network  1 , for example by using the most powerful antenna at a given node  2  for the link  3  with most attenuation. 
     In the embodiment depicted in  FIGS. 2 and 3 , the assembly comprises one horn antenna  13  and three flat plate antennas  18 ,  19 ,  20 . However, this is not to be taken as limitative. Alternative configurations with more or fewer antennas, or with different types of antennas, fall within the scope of the claims. For example, the horn antenna  13  may be replaced by a further flat plate antenna  22  as shown in FIG.  4 . This embodiment is similar to that of  FIGS. 2 and 3  in other respects, and corresponding elements are given the same reference numerals. In this embodiment all the antennas  22 ,  18 ,  19 ,  20 , are driven from a single motor assembly  25  through respective concentric spindles  9 ,  10 ,  11 ,  12  to which they are connected by respective spacers  31 ,  15 ,  16 ,  17 . 
     The sizes of the antennas may be varied to improve gain, but because their swept areas overlap any increase in size will limit the angle through which they can move relative to each other without fouling. 
     In an alternative configuration shown in  FIG. 5 , in which components equivalent to those in  FIGS. 2 and 3  again have the same reference numerals, first and second horn antennas  27 ,  28  are mounted on the main horn antenna  13 , arranged for relative rotational movement of the first and second antennas  27 ,  28  at least partially within the swept volume of the main antenna  13 . This simplifies the control system, as the mountings can be designed to prevent fouling movements, but makes electrical connection more complex, and requires a complex drive train if more than one antenna is to be driven by the same motor. In this embodiment, each antenna  13 ,  27 ,  28  has its own motor  24 ,  29 ,  30 . 
     In a further configuration shown in  FIGS. 6 and 7 , the arrangement of  FIGS. 2 and 3  is modified by arranging that the flat antennas  18 ,  19  (for clarity, only two are shown) are driven by respective gear wheels  32 ,  33  along a curved toothed track  34  mounted on the horn antenna  13 . The gear wheels  32 ,  33  can be selectively driven from a gearbox  35  through respective drive trains  36 ,  37 , for relative movement between the flat antennas  18 ,  19  and the track  34  and hence the horn antenna  13 . The control unit  23  controls the gearbox  35  to select which drive train is to be driven from the motor  24 . The drive trains  36 ,  37  may be replaced by separate electric motions, each driving a respective wheel  32 ,  33 .