Rotatable antenna apparatus

1. A rotatable antenna apparatus comprising:

a fixed unit for attachment of the apparatus to an external structure;

a rotatable unit mounted on the fixed unit and comprising an antenna assembly and processing circuitry coupled to the antenna assembly for performing signal processing operations;

an interface unit coupled to both the fixed unit and the rotatable unit, and through which a cable is routed to provide a wired connection from the fixed unit to the processing circuitry, the interface unit providing a cable housing within which a length of the cable is enclosed in a coiled arrangement; and

a control mechanism coupled to said interface unit and arranged to constrain the amount to which the length of cable is wound and unwound within said cable housing so as to inhibit application of a stretching stress on said cable during rotation of said rotatable unit.

2. A rotatable antenna apparatus as claimed in claim 1 , further comprising:

a shaft connected to the rotatable unit and located for rotation within the fixed unit;

the cable housing being located around said shaft.

3. A rotatable antenna apparatus as claimed in claim 1 , wherein the length of cable is arranged to move between an inner coiled configuration and an outer coiled configuration within the cable housing during rotation of said rotatable unit.

4. A rotatable antenna apparatus as claimed in claim 1 , wherein:

the cable housing comprises a fixed portion and a rotatable portion, the length of cable being connected to both the fixed portion and the rotatable portion; and

the control mechanism is arranged to cause the rotatable portion to rotate during rotation of the rotatable unit.

5. A rotatable antenna apparatus as claimed in claim 4 , wherein:

the rotatable unit comprises a support for the antenna assembly and the processing circuitry; and

the control mechanism comprises interengaging formations on the support and on the rotatable portion of the cable housing to cause the rotatable portion to rotate during rotation of the support.

6. A rotatable antenna apparatus as claimed in claim 5 , wherein the interengaging formations comprise a protrusion extending from the rotatable portion and a recess in the support configured to receive the protrusion.

7. A rotatable antenna apparatus as claimed in claim 6 , wherein said recess is a slot that allows the support to be rotated by a predetermined amount before the rotatable portion begins to rotate.

8. A rotatable antenna apparatus as claimed in claim 5 , wherein:

the control mechanism further comprises a calibration mechanism to detect a zero reference point at which the length of cable adopts a first coiled configuration;

the support then being rotatable in a predetermined direction up to a specified maximum angle of rotation during which time rotation of the rotatable portion causes the length of cable to transition from the first coiled configuration to a second coiled configuration, both the first and second coiled configurations being configurations in which the length of cable is not subjected to said stretching stress.

9. A rotatable antenna apparatus as claimed in claim 8 , wherein:

the calibration mechanism comprises an actuator freely rotatable between first and second end stops, and a protruding element protruding from the support;

during calibration the support being rotated in a direction opposite to said predetermined direction until the protruding element engages with the actuator and moves the actuator into contact with the first end stop.

10. A rotatable antenna apparatus as claimed in claim 9 , wherein the protruding element incorporates a switch mechanism that is activated when the first end stop prevents further movement of the actuator, to indicate that the zero reference point has been reached.

11. A rotatable antenna apparatus as claimed in claim 9 , wherein the length of cable is mounted in said cable housing with sufficient slack to allow the length of cable to adopt the first coiled configuration during said calibration without imparting the stretching stress on the cable.

12. A rotatable antenna apparatus as claimed in claim 9 , wherein the second end stop provides a physical limit on the amount of rotation that can be applied to the rotating portion of the cable housing in said predetermined direction starting from said zero reference point.

13. A rotatable antenna apparatus as claimed in claim 8 , further comprising:

a motor to rotate said support;

the motor being arranged to limit its output so as to ensure that the support is not rotated in said predetermined direction by more than said specified maximum angle of rotation from said zero reference point.

14. A rotatable antenna apparatus as claimed in claim 8 , wherein said specified maximum angle of rotation exceeds 360 degrees.

15. A rotatable antenna apparatus as claimed in claim 1 , wherein the cable housing is configured to constrain the cable in a predetermined orientation whilst the cable is wound and unwound within said cable housing.

16. A rotatable antenna apparatus as claimed in claim 1 , wherein said cable has a rectangular cross-section, and is orientated within the cable housing such that wider sides of the cable face each other in adjacent coils.

17. A rotatable antenna apparatus as claimed in claim 16 , wherein an additional cable is interleaved with said cable in the cable housing.

18. A rotatable antenna apparatus as claimed in claim 17 , wherein said additional cable has a low surface friction.

19. A rotatable antenna apparatus as claimed in claim 1 , wherein said cable is an Ethernet cable.

20. A rotatable antenna apparatus comprising:

a fixed means for attachment of the apparatus to an external structure;

a rotatable means for mounting on the fixed means and comprising an antenna means and processing means coupled to the antenna means for performing signal processing operations;

an interface means for coupling to both the fixed means and the rotatable means, and through which a cable is routed to provide a wired connection from the fixed means to the processing means, the interface means for providing a cable housing within which a length of the cable is enclosed in a coiled arrangement; and

a control means for coupling to said interface means and for constraining the amount to which the length of cable is wound and unwound within said cable housing so as to inhibit application of a stretching stress on said cable during rotation of said rotatable means.