The invention relates to an antenna suitable for satellite communications . Background

In many circumstances, it is necessary to be able to operate at different frequency bands on different

satellites thus requiring a large number of antennas .

Summary of the Invention

The invention provides an antenna comprising:

a primary reflector;

a prime focus feed;

a dual-reflector feed; and

a sub-reflector,

the prime focus feed and the sub-reflector mounted for movement relative to the primary reflector between a first configuration in which the prime focus feed and the primary reflector define a first signal path and a second configuration in which the primary reflector, the sub- reflector and the dual-reflector feed define a second signal path.

In an embodiment, the prime focus feed and sub-reflector are mounted together for rotational movement around a pivot point between the first and second configurations . In an embodiment, the prime focus feed and sub-reflector are mounted to the primary reflector. In an embodiment, the dual-reflector feed is co-located with the primary reflector.

In an embodiment, the dual-reflector feed is off-set from the primary reflector.

In an embodiment, the sub-reflector is mounted for independent movement relative to the prime focus feed. In an embodiment, the prime focus feed and sub-reflector are mounted for rotational movement around a pivot point between the first and second configurations .

In an embodiment, the sub-reflector is mounted for translational movement relative to the prime focus feed.

Brief Description of the Drawings

Figures IA and IB show a Gregorian-type antenna in first and second configurations;

Figures 2A and 2B show a Cassegrain-type antenna in first and second configurations; Figures 3A and 3B show an offset Cassegrain-type antenna in first and second configurations ;

Figures 4A and 4B show an offset Gregorian-type antenna in first and second configurations; and

Figure 5 , shows a detail of the Cassegrain-type antenna of Figure 2.

Detailed Description

The embodiment relates to a reconfigurable multi-band antenna that can operate in one set of frequency bands with a prime focus feed and non-simultaneously at another set of frequency bands with a dual reflector arrangement incorporating a sub-reflector and dual-reflector feed. In the embodiment, the feed is a feed horn but the feed could also be a patch, crossed dipole, phase array etc and could be different for each band. For example, a patch for L- band and a horn for X-band. This antenna arrangement may provide simultaneous coverage at multiple frequency bands but not at all band sets at once . This arrangement is based on the realisation that while it is desirable to be able to operate at different frequency bands on different satellites , simultaneous operation may not be required for all bands. Further, antennas of the embodiment have the advantage of reducing the space required to mount the relatively large antennas required for satellite

communications while allowing operation in a plurality of different frequency bands on different satellites . This advantage is particularly important in maritime

applications where deck space is at a premium.

The embodiment provides an antenna system that provides a prime focus feed, which will generally be used for lower frequency satellite systems such as Inmarsat and a sub- reflector that can be mechanically switched in place of the prime focus feed to provide a dual reflector feed configuration which will be used, for example, for the higher frequency commercial and military satellites . Both feeds may operate in either simplex mode (receive Only) or full duplex mode (transmit and receive) .

In one mode of operation, the dual-reflector feed

illuminates the sub-reflector which in turn illuminates the primary reflector. In this mode, the prime focus feed is behind the sub-reflector and is not illuminated by the signal from the dual-reflector feed. In each of the following embodiments the prime focus feed is a low frequency feed, for example for L-band or C-band whereas the dual-reflector feed is a high frequency feed such as for X-band, Ku-band, K-band, Ka-band etc. A person skilled in the art will appreciate that while this arrangement has certain advantages , other configurations are possible.

Referring to Figures 1 to 4 , it is noted that in each of these figures a dashed line marked Y with a subscript corresponding to the figure number (e.g. Y _IA for Figure IA) shows the outermost possible signal path for the

arrangement shown in the relevant figure whereas a line marked Z with a subscript corresponding to the figure number (e.g. Z _iA ) shows an exemplary single signal path. A person skilled in the art will appreciate that in all of Figures 1 to 4 , the signal path can be reversed for the feed to receive a signal rather than transmit a signal . Referring to Figure IA, a Gregorian-type antenna 100 is shown in a first configuration which corresponds to a high frequency mode of operation where a signal is emitted from the high frequency feed (120) which provides the dual- reflector feed to the sub-reflector 130 where it is reflected to the primary reflector 110 and transmitted outwardly . Between Figures IA and IB the sub-reflector 130 and the low frequency feed 140 have been rotated around axis of rotation X to move the antenna to a second configuration where it is possible for the antenna to assume a second mode of operation where the low frequency feed 140 generates a signal path using the primary reflector 110 as indicated by line Z _1B in this arrangement, the Gregorian-type sub-reflector is moved to a position where it does not interfere with the signal path,

substantially behind the low frequency feed in the antenna 100. A person skilled in the art will appreciate that as well as rotating around axis X, it is possible in some implementations for the sub-reflector and the low

frequency feed to be translated along the vertical axis .

Referring to Figures 2A and 2B, a Cassegrain-type antenna 200 is shown in a first configuration. In Figure 2A the antenna 200 is operating in a high frequency mode where the high frequency feed 210 transmits a signal onto the sub-reflector 230 which reflects it off the primary reflector 210 for transmission.

Figure 2B shows a second configuration . To be moved to the second configuration, the low frequency feed 240 and the Cassegrain-type sub-reflector 230 have been rotated around in the axis indicated by arrow X. Again, in some

implementations, there may be axial movement along the axis perpendicular to X. Again the Cassegrain-type sub reflector 230 is moved to a position where it is out of the way of the signal path behind the low frequency feed. Figure 5, shows one implementation of the antenna 200 of Figure 2. The low frequency feed 240 and the sub-reflector 230 are parts of a multi-frequency structure 505 mounted by mount 580 for rotational movement with a rotatable drive shaft 530 mounted between bearing 540 and stepper motor 520, and driven by the stepper motor 520 which operates under control of a control circuit mounted within equipment housing 550 to be driven between two positions 180 degrees apart. Bearing 540 and stepper motor 520 are mounted respectively to a set of struts 510A, 510B

extending from the primary reflector 210. Persons skilled in the art will appreciate that in other embodiments, additional struts may be provided as necessary to securely mount the multi-frequency structure 505. In the

embodiment, a pair of linear actuators 560A, 560B are driven by linear motors to extend or retract the sub- reflector to a desired position. Persons skilled in the art will appreciate that the control circuit can be operated by any appropriate input device (for example, a switch) and can be integrated with the electronics for processing/generating the signals such that switching from one mode of operation to another controls movement of the multi-frequency structure 505 and selects a relevant part of the electronics for operation.

Figure 3A shows an offset type Cassegrain-antenna

300operating in a high frequency mode where the signal is sent via sub-reflector 330 to primary reflector 310 and onwards for transmission. As indicated by arrow M in Figure 3A the sub-reflector is mounted for movement out of the path of the low frequency feed 340. This movement may be linear and/or rotational .

Figure 3B shows the Cassegrain-type sub-reflector having been moved out of the path of the low frequency feed 340 and the low frequency feed in operation with the signal path Z _3B being transmitted via the primary reflector 310.

Figures 4A and 4B show a further arrangement which is an offset-Gregorian type antenna . Figure 4A shows a high frequency mode of operation where the signal path involves the high frequency feed 420 the sub-reflector 430 and the primary reflector 410 such that it is a dual reflector arrangement. As indicated by rotational axis N and translational axis P, the low frequency feed 440 and the sub-reflector 430 are mounted to a mount 450 which can be rotated and the low frequency feed 440 is further mounted for translational movement relative to the mount 450 as indicated by arrow P to move the low frequency feed P to the focus of the primary reflector 410 of the antenna of Figures 4A and 4B such that it operates as shown in Figure 4B in the low frequency mode. It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention, in particular it will be apparent that certain features of embodiments of the invention can be employed to form further embodiments .

Further details on the construction of feeds and

reflectors can be found in "Microwave Horns and Feeds" by AD Olver, PJB Clarricoats, AA Kishk; and L Shafai .

In the claims which follow and in the preceding

description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as

"comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.