WO2006110026A1

WO2006110026A1 - Antenna system and method for changing a resulting polarisation of an antenna beam

Info

Publication number: WO2006110026A1
Application number: PCT/NL2005/000281
Authority: WO
Inventors: Tomas Carl Wilhelmsson
Original assignee: Stichting Astron
Priority date: 2005-04-14
Filing date: 2005-04-14
Publication date: 2006-10-19

Abstract

Antenna system (1) and method for changing a resulting polarisation of an antenna beam generated by an antenna system (1) of the phased array type including a first antenna group (2) of at least two first antenna units (6.1-6.3) connected to a first time or phase shifting circuit (9), for creating a first sub-beam with a first polarisation a second antenna group (4) of at least two second antenna units (8) connected to a second time or phase shifting circuit (11), for creating a second sub-beam with a second polarisation which second polarisation is different from the first polarisation, which first and second polarisation are non-linear, and which first and second sub-beam combine into a third beam with a third polarisation, the orientation of the third polarisation being at least partially dependent on the phase of the first sub-beam relative to the second sub-beam, and a control unit (16) which, for controlling both a direction of the first and/or second sub-beam and the orientation of the third polarisation, is connected to the first and second time or phase shifting circuit (9, 11).

Description

Title: Antenna system and method for changing a resulting polarisation of an antenna beam

BACKGROUND OF THE INVENTION

The invention relates to an antenna system of the phased array type. The invention further relates to a method for changing a resulting polarisation of an antenna beam.

Antenna systems are known and used for receiving and emitting electro-magnetic radiation. Antenna systems may, for example, be employed in radar and other direction finding systems, astronomical observatories and satellite receiving equipment. Often, an antenna system has to receive or emit electro-magnetic radiation with differing spatial properties, for example elecitro-magnetic radiation from different directions and/or having different polarisations and/or electro-magnetic radiation stemming from different sources (and, accordingly, emitted from different positions).

Often, differing directions of polarisation are used to differentiate between different signals, e.g. different TV channels emitted by the same satellite. Also, the direction of polarisation of a signal depends on the orientation of the antenna with respect to the signal source. Control of the direction of the polarisation to which the antenna system is sensitive, is therefore desirable.

From US patent application 2002/0167449, a phased array antenna system is known in which the direction of polarisation is controlled. This prior art document discloses an antenna system which includes a rotating phased array antenna. The rotating phased array antenna uses dual linearly polarised transmitters/receivers to create horizontally and vertically polarised sub- beams. The linearly polarised sub-beams form in combination a linearly, elliptically or circularly polarised antenna beam. The known antenna system uses (MEMS) phase shifters and (MEMS) switches and/or phase shifters for beam steering and polarisation control, respectively. The antenna system known from US 2002/0167449 has the disadvantage that it comprises many switches and/or phase shifters for beam steering and polarisation control and is, therefore, relatively expensive. In addition, the known phased array system is mechanically rotated and therefore susceptible to wear.

SUMMARY OF THE INVENTION

An object of the invention is to provide an antenna system of the phased array type which can be manufactured at reduced costs. Accordingly, according to the invention, an antenna system according to claim 1 is provided

Such an antenna system can be manufactured at reduced costs, because the control unit is arranged to control both the direction of the third beam and the orientation of the third polarisation via the first and/or second time or phase shifting circuit. The antenna system thus uses the same time or phase shifting circuits for performing two functions. Therefore, separate, relatively expensive, phase shifting devices for controlling the orientation of the third polarisation can be omitted, and the number of time or phase shifting circuits in the antenna system can be reduced, thereby reducing the cost of the antenna system. A further advantage that can be obtained with such an antenna system is that it may contain less mechanically moving parts and is, therefore, less susceptible to wear.

The first and the second polarisation may be orthogonal with respect to each other. This offers the advantage that the third polarisation can be controlled in an effective manner. In addition, the first polarisation may be circular polarisation and the second polarisation may be circular polarisation. This offers the advantage that the third linear polarisation can be obtained as a combination of the first and the second polarisation without a need for amplitude control of the first and/or second sub-beam. It is, therefore, possible to provide the antenna system without amplitude control devices for controlling the orientation of the third linear polarisation, thus reducing costs of the antenna system even further.

The control unit may be arranged to control a phase offset of the individual antenna units, and the resolution of the first and/or second beam phase may be finer than a resolution of the phase offset of the individual antenna units. The orientation of the third polarisation can be controlled in a precise manner, by controlling the phase offset of the individual antenna units such that the beam phase of the sub-beams is controlled with a fine resolution that is finer than the resolution of the phase offset of the individual antenna units. This offers the advantage that the orientation of the third polarisation can be controlled in a precise manner without phase shifter devices with the fine resolution, but with phase shifter devices with a coarser resolution. The phase shifter devices with the coarser resolution are usually less expensive than the phase shifter devices with the fine resolution, thereby reducing costs of the antenna system.

Specific embodiments of the invention as set forth in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is further elucidated with reference to the drawing, in which, by way of non-limiting example,

Fig. 1 shows a schematic view of an example of an embodiment of an antenna system according to the invention,

Fig. 2 shows a schematic view of a control circuit suitable for use in the example of Fig. 1,

Fig. 3 shows a schematic view of an other example of a control circuit,

Fig. 4 shows a schematic view of an example of a first embodiment of an antenna group of the antenna system according to the invention, Fig. 5 shows a schematic view of an example of a second embodiment of the antenna group of the antenna system according to the invention,

Fig. 6 shows a schematic view of an example of a third embodiment of the antenna group of the antenna system according to the invention, and Fig. 7 shows a schematic view of an example of another embodiment of the antenna system according to the invention.

In the Figures, corresponding features are indicated with similar reference numbers.

DETAILED DESCRIPTION

In Fig. 1, an example of an embodiment of an antenna system according to the invention is indicated with reference numeral 1. The shown antenna system 1 is a phased array antenna and includes a first antenna group 2 and a second antenna group 4. The first antenna group 2 is provided with a plurality, in this example three, of first antenna units 6.i (i=l, 2, 3...). The second antenna group 4 is provided with a plurality, in this example three, of second antenna units 8.

The first antenna group 2 is connected to a first time or phase shifting circuit 9. In this example, each first antenna unit 6.i in the first antenna group 2 is connected to a separate first phase shifter device 10 in the first time or phase shifting circuit 9.

The second antenna group 4 is connected to a second time or phase shifting circuit 11. In this example, each second antenna unit 8 in the second antenna group 4 is connected to a separate second phase shifter device 12 in the second time or phase shifting circuit 11.

In this example, the first phase shifter devices 10 are connected to a first combining circuit 13. The second phase shifter devices 12 are connected to a second combining circuit 15. In this example the first and second combining circuit 13, 15 are combined into a single combining circuit 14. In the combining circuit, signals received by the first and second antenna group are combined into a received antenna signal. Techniques for combining those signals are generally known in the art of phased array antennas and for the sake of brevity the combiner circuit is not described in full detail. The first and second phase shifter devices 10, 12 are further connected to a control unit 16.

The antenna system 1 shown in Fig. 1 works as follows. The first antenna group 2 and the second antenna group 4 both form a phased antenna array. In operation, the first antenna group 2 is set to be sensitive to electromagnetic radiation received from a predetermined direction, indicated in Fig. 1 with arrow R, by steering an antenna beam in the direction of the arrow R. It should be noted, that antenna beam steering techniques are generally known in the art of phased antenna arrays, and any suitable beam steering technique may be used.

In this example, thereto the first phase shifter devices 10 are set by the control unit 16 to generate a phase difference between the first antenna units 6.i. In Fig. 1, the phase difference between the central first antenna unit 6.2 and the rightmost first antenna unit 6.3 is indicated by φ, i.e. a central antenna-unit-beam 5.2 and a rightmost antenna-unit-beam 5.3 have a phase difference φ with respect to each other. The phase difference between the leftmost first antenna unit 6.1 and the rightmost first antenna unit 6.3 is indicated by 2φ, which is substantially equal to twice the phase difference φ between the central and the rightmost first antenna unit, i.e. a leftmost antenna-unit-beam 5.1 and the rightmost antenna-unit-beam 5.3 have a phase difference 2φ with respect to each other. The second antenna group 4 is also set to receive electromagnetic radiation from the predetermined direction R. The antenna units 6.i are arranged to receive a non-linearly polarised, in this example Left Hand Circularly Polarised (LHCP), electromagnetic radiation. The second antenna units 8 are arranged to receive a non-linearly polarised, in this example Right Hand Circularly Polarised (RHCP), electromagnetic radiation. Accordingly, the first and second antenna group both have a non-linearly polarised sub-beam, in this example polarised LHCP and RHCP respectively.

As both the first and the second antenna group are steered in the same direction, the sub-beam of the first and second antenna group can be combined into a single, e.g. (more) linearly polarised, antenna beam for receiving electromagnetic radiation from direction R. The direction in which the antenna beam is steered can thus be controlled by suitable control of the sub-beams via the phase difference between the phase shifter devices within an antenna group. For example, supposing that the antenna system 1 shown in Fig. 1 has to receive Linearly polarised signals, the antenna system 1 can receive the LHCP component with the first antenna units 6.i and receive the RHCP component with the second antenna units 8. The first, LHCP, signal received by the first antenna group 2 can then be fed to the combiner 14. The second, RHCP, signal received by the second antenna group 4 can also be fed to the combiner 14. In this example the first and the second signal are combined by the combiner 14, resulting in the linearly polarised signal.

In the example of Fig. 1, the orientation of the polarisation of the antenna beam can be controlled. Accordingly, the system is particularly suited for applications in which such a control is desired, e.g. in satellite TV reception where the orientation of the linearly polarised signal to be received may depend upon e.g. the orientation or the location of the antenna system on earth.

In the example of Fig. 1, for control of the orientation of the antenna beam, the control unit 16 is connected to a control input of the first phase shifter devices. 10 The control unit 16 can provide a control signal to the control input, which control signal controls the phase setting of the respective first phase shifter devices 10.

In this example, the control unit 16 can change the phase setting of all of the first phase shifter devices 10 by an equal amount. An identical phase offset is, therefore, added to the phase shifter devices 10. In this case, the first antenna group 2 still has a LHCP sub-beam, and is sensitive to LHCP electromagnetic radiation from direction R. The phase of the received LHCP signal, however, is shifted with respect to the RHCP signal received by the second antenna group 4. In the combiner 14 the combination of the, phase shifted, LHCP signal and the RHCP signal again results in a third signal with a linear polarisation. The orientation of the linear polarisation, however, is changed with respect to the linearly polarised signal resulting from a LHCP and RHCP signal having the same phase. It is, therefore, possible to control the orientation of the linear polarisation of the received third signal, and the orientation of polarisation of the antenna beam, by changing the phase of the received first signal with respect to the received second signal.

In the example of Fig. 1, the orientation of the polarisation of the antenna beam is controlled using the same phase shifter devices that also control the direction of the antenna beam. This allows for a reduction of phase shifter devices, thus allowing a reduced complexity of the antenna system, and accordingly, a reduction of antenna system costs.

Fig. 2 shows a control circuit suitable for use in an antenna system 1. The phase shifter devices 10, 12 shown in Fig. 2 are electrically controllable phase shifter devices, in this example four-bit digital phase shifter devices. The phase shifter devices 10, 12 include a plurality of, in this example four, control inputs 17. i (i=l, 2, 3...). The control inputs of the first and second phase shifter devices 10, 12 are connected to the control unit 16 through control lines 18-26. The control lines are used to set a respective bit of the phase shifter device in a manner known per se, and the control unit 16 transmits a four-bit control signal via the control lines 18-26 to each of the respective phase shifter devices 10,12. In this example, each control input 17. i controls the phase shift of the phase shifter device by a different amount. In this example a most significant bit of the four-bit control signal is fed to the highest control input 17.4, and changes the phase offset of the phase shifter device by the largest amount. In this example, the least significant bit is fed to the lowest control input 17.1, and changes the phase offset of the phase shifter device 10, 12 by the smallest amount.

In the example of Fig. 2, the control unit 16 can change the phase of the first antenna units 6.i with respect to each other for steering the first sub- beam as explained with respect to Fig. 1. Thereto, the control unit 16 can transmit mutually different control signals to the first phase shifter devices 10. i (i=l, 2, 3...) connected to the respective first antenna units 6.i via the control lines 18-22. Each of the first phase shifter devices 10. i may, thereto, be connected to the control unit 16 via separate control lines 18-22 (i.e. only forming a connection between the control unit 16 and that specific first phase shifter device 10. i). In the example of Fig. 2, the control unit 16 can also change the phase of the second antenna units 8 with respect to each other for steering the second sub-beam. In the example of Fig. 2, the control unit 16 can change the phase of all first antenna units 6 with respect to all second antenna units 8. The change of phase changes the phase of the first sub-beam with respect to the phase of the second sub-beam. The phase shift of the first sub-beam relative to the second sub-beam controls the orientation of the linear polarisation of the received signal, as described with respect to Fig. 1. As the phase of all first and/or second antenna units 6, 8 is changed by the same amount, the direction from which the electromagnetic radiation is received is not changed.

In the example of Fig. 2, for control of the phase of the first antenna units 6 relative to the phase of the second antenna units 8, a part of the control lines 18-26 is connected to only the antenna units of one of the first or second antenna groups 2, 4, while another part of the control lines is connected to all antenna units 6, 8. More specific, in this example, the least significant bit of the first phase shifter devices 10 is controlled through the control line 24, and the least significant bit of the second phase shifter devices 12 is controlled through the control line 26. The control lines 18-22 for the three highest bits control the first and second phase shifter devices 10, 12 for simultaneous and substantially identical steering of the direction from which the first and second antenna group 2, 4 receive the electromagnetic radiation. By using the same control lines, in this example for the least significant bit, connected to antenna units 6, 8 of both the first and second antenna group 2, 4, the amount of control lines in the antenna system 1 can be reduced. This simplifies antenna system layout and reduces costs.

In the example of Fig. 2, only the lowest bit controls the phase of the first antenna units 6 relative to the phase of the second antenna units 8. It will be appreciated, that it is also possible to have the lowest two, or any number, of bits control the phase of the first antenna units relative to the phase of the second antenna units. In other words, it is also possible that the control unit 16 is connected with a first connection to a first part of the time or phase shifting circuits 10 , 12, for adding a smaller first and/or second time or phase offset, respectively, and the control unit 16 is connected with a second connection to a second part of the time or phase shifting circuits 10, 12, for adding a larger first and/or second time or phase offset, larger than the smaller first and/or second time or phase offset respectively. An average first and/or second phase offset of the smaller first and/or second time or phase offsets substantially corresponds to the first and/or second beam phase, respectively. The larger first and/or second time or phase offsets, substantially corresponds to the first and/or second beam direction, respectively

Fig. 3 shows an alternative embodiment of the control circuit. The phase shifter devices 10, 12 shown in Fig. 3 are also electrically controllable phase shifter devices, in this example four-bit digital phase shifter devices. The first and second phase shifter devices 10, 12 are connected to the control unit 16 through a plurality of control lines 18-22, 28-38.

In the example of Fig. 3, the control unit 16 can steer the first and second sub-beam as explained with respect to Fig. 2. In this example, the control lines 18-22 for the three highest bits are connected to the antenna units 6, 8, to control the first and second phase shifter devices 10, 12. This allows for simultaneous and substantially identical steering of the direction from which the first and second antenna group 2, 4 receive the electromagnetic radiation. The control lines 28-38 for the least significant bits are each connected to only one of the first and second phase shifter devices 10, 12, to provide a separate control of the phase shift of each phase shifter device.

In the example of Fig. 3 the phase shifter devices 10, 12 are capable of coarse phase shifting, but incapable of fine phase shifting. These coarse phase shifts are usually acceptable for steering the antenna beam. The phase shifter devices shown in Fig. 3 may for instance have a coarse phase resolution of more than 5°, possibly more than 10°, or even more than 20° per bit. Accordingly, the manufacturing costs are reduced, since electrically controllable phase shifter devices are relatively expensive, but usually are less expensive when the phase resolution is coarser.

In the example of Fig. 3, the coarse phase shifters 10, 12 are connected such that a fine phase shift control, and therefore polarisation control, is obtained. Accordingly, the orientation of the linear polarisation, as is explained with respect to Fig. 1, can be controlled in a precise manner. In the example of Fig. 3, the first phase shifter devices 10 and second phase shifter devices 12 each have a separate control line 28-38. It is, therefore, possible to set the least significant bit of the first (and/or second) phase shifter devices within the first (and/or second) phase shifting circuit to different values. In this example, the average value of the phase offset of the least significant bit of the first antenna units 6 relative to the average phase offset of the least significant bit of the second antenna units 8 determines the average phase offset for polarisation control. Accordingly, by suitable setting of the coarse phase shifts, the average phase offset, which controls the polarisation control, can be smaller than the individual coarse phase shifts. In this case, the phase difference between the individual first antenna units 6 within the first antenna group 2 can also be changed by the amount represented by the least significant bit. This results in that a sub- beam of an antenna group may be widened In the example of Fig. 3, the control lines 28-38 for the least significant bits are each connected to only one of the first and second phase shifter devices 10, 12, to provide a separate control of the phase shift of each phase shifter device. It will be appreciated, that it is also possible that each of the control lines 28-38 is connected to a sub-group including a plurality of antenna units, thus removing the need for a separate control line for the least significant bit of each phase shifter device 10, 12. Hence, for a predetermined number of phase shifter devices 10, 12 the amount of control lines can be reduced, while maintaining the possibility to set the average phase offset which controls the polarisation control, by suitably setting the coarse phase shifts of the sub-groups, which average phase offset can be smaller than the individual coarse phase shifts. It will be appreciated that, although in Fig. 3 six control lines 28-38 are shown for control of the least significant bits of the first and second phase shifter devices 10, 12, any number of control lines can be used, depending on the required resolution of the average phase offset. In Figs. 1-3 the first and second antenna group 2, 4 are circularly polarised to yield the linearly polarised third polarisation. However, in practice every antenna device has a polarisation which deviates from an ideal circular polarisation. This causes the polarisation of the antenna beam, which results from the polarisations of the first and second antenna group 2, 4, to deviate from the desired shape. If, for instance the first and second sub-beam have an elliptical polarisation ( e.g. left hand elliptically and right hand elliptically), the resulting third polarisation of the antenna beam will also be elliptical, and not linear. In order to minimise the deviation from the ideal circular polarisation, the antenna units of the antenna group can be oriented relative to each other, such that deviations of individual antenna units at least reduce the deviation from the ideal circular polarisation of the sub-beam, as e.g. known from US 5,006,859.

Fig. 4 shows an example of an antenna group 40 which has a reduced error in the circularity of the first and second polarisation. The antenna group 40 includes a plurality of, in this example four, antenna units 42. In this example, the antenna units 42 are provided with two transmitters/receivers 44, 46, i.e. a first transmitter/receiver 44 and a second transmitter/receiver 46. The transmitters/receivers 44, 46 of each antenna unit 42 are positioned perpendicular with respect to each other. A signal line 50 feeds an electrical signal to the antenna group 40. Each antenna unit 42 is provided with a delay element 52. The delay element 52 causes the second transmitter/receiver 46 to lag substantially 90° in phase behind the first transmitter/receiver 44. Each antenna unit 42 will therefore radiate an elliptically or (nearly) circularly polarised radiation signal. The elliptically polarised signals of the antenna units 42 are indicated with dashed ellipses 54 in Fig. 4.

The antenna units 42 are positioned at an angle, in this example substantially 90°, with respect to each other. As the antenna units 42 are positioned at an angle, the directions of the major axes of the elliptically polarised signals are also rotated over the same angle, i.e. 90°, with respect to each other.

In operation, the antenna units 42 transmit the elliptically polarised signal in phase. To that end, in this example, phase shifters 56 are included to allow the transmitted signals to be in phase, by compensating for the phase shift introduced by the rotation of the antenna units. In the far field, i.e. so far from the antenna system that the plurality of antenna units is perceived as a single transmitting or receiving antenna, the elliptically polarised signals of the antenna units 42 are summed. As the major axes of the elliptically polarised signals are distributed substantially uniformly over 360 degrees in total, the elliptically polarised signals add to a substantially circularly polarised signal, with the ellipticity at least being reduced.

Fig. 5 shows an alternative embodiment of an antenna group 60 according to the invention. The antenna group 60 includes three antenna units 42. Each antenna unit 42 is provided with a first transmitter/receiver 44, a second transmitter/receiver 46 and a third transmitter/receiver 48. The transmitters/receivers 44, 46, 48 of each antenna unit 42 are positioned at substantially 120° with respect to each other. In this example the transmitters/receivers are sequentially rotated over 120° with respect to each other, i.e. incrementally rotated over 120° with respect to each other. The transmitters/receivers 44,46,48 are similar, so that the antenna units 42 substantially demonstrate symmetry about boresight, i.e. a virtual axis substantially perpendicular to the plane in which the transmitters/receivers extend, through the centre of the antenna unit 42. The geometrical arrangement of three transmitters/receivers positioned at substantially 120° with respect to each other, shown in Fig. 5 is known per se, and described in more detail in IEEE Transactions on Antennas and Propagation, Vol. 50, No. 3, March 2002, pp. 398-399.

A signal line 50 feeds an electrical signal to the antenna array. In this example the transmitters/receivers are fed in a sequentially rotated phasing arrangement, i.e. in this example each antenna unit 42 is provided with a first delay element 62 and a second delay element 64, wherein the first delay element 62 causes the second transmitter/receiver 46 to lag substantially 120° in phase behind the first transmitter/receiver 44 and the second delay element 64 causes the third transmitter/receiver 48 to lag substantially 240° in phase behind the first transmitter/receiver 44 and thus 120° behind the second transmitter/receiver 46. Each antenna unit 42 will therefore radiate a substantially (nearly) circularly polarised radiation signal.

In practice, mechanical and electrical deviations will cause the polarisation of each antenna unit 42 to be slightly elliptical. The elliptically polarised signals of the antenna units 42 are indicated with dashed ellipses 54 in Fig. 5. The antenna units 42 are positioned at substantially 120° with respect to each other. In this example the antenna units are sequentially rotated over 120° with respect to each other, i.e. incrementally rotated over 120° with respect to each other. This causes directions of the major axes of the ellipses 54 also to be sequentially rotated over 120° with respect to each other.

The three antenna units 42 are made to transmit the elliptically polarised signal in phase. In this example phase shifters 66, 68 are included to allow the transmitted signals to be in phase, by compensating for the phase shift introduced by the rotation of the antenna units. In the far field the three elliptically polarised signals of the antenna units are summed. As the major axes of the three elliptically polarised signals are sequentially rotated over 120°, the three elliptically polarised signals add to a substantially circularly polarised signal, with the ellipticity at least being reduced. In Figs. 4 and 5, the signal lines connecting the antenna units 42 and the transmitters/receivers 44, 46, 48 to the signal feed line 50, are assumed to introduce no additional phase shifts. In practical applications, however, these signal lines may introduce additional phase shifts, which may cause additional non-circularity of the signal received or transmitted by the antenna group. These additional phase shifts can be compensated for, e.g. in the phase shifters 52, 56, 62, 64, 66, 68. It is also possible to counteract the non-circularity arising from these phase differences by positioning the antenna units rotationally distributed with respect to each other as explained with respect to Figg. 4 and 5. The three antenna units 42 shown in Fig. 5 form an antenna unit cluster 70 providing a substantially circular polarisation. The first or second antenna group 2,4 may include a single antenna unit cluster 70 . However, the first or second antenna group 2,4 may include more than one, i.e. at least two antenna unit clusters 70. To further reduce errors in the desired circular polarisation, such a plurality of antenna unit clusters 70 can be arranged such, that the ellipticity of the separate antenna unit clusters, if any, is oriented such that the deviation from circular polarisation is reduced in the combined polarisation of the antenna unit clusters.

For instance in Fig. 6, an antenna group 2' is shown with three antenna unit clusters 70 positioned sequentially rotated over 120° with respect to each other. In the far field, the three elliptically (or substantially circularly) polarised signals of the antenna unit clusters are summed. As the major axes of the three elliptically polarised signals are sequentially rotated over 120°, the three elliptically polarised signals add to a substantially circularly polarised signal, with the ellipticity at least being reduced.

It will be appreciated that reducing the remaining ellipticity by choosing the position and/or orientation of antenna units and/or smaller or larger clusters of antenna units can be used on any scale within the antenna group. In this way, the antenna group can be constructed such that the resulting actual polarisation approaches a perfectly circular polarisation as close as possible. In general, the larger the number of antenna units in the antenna group, and the smaller the ellipticity of the individual antenna units, the better the resultant polarisation approaches a perfect circular polarisation.

Fig. 7 shows an array antenna 80 according to the invention provided with two antenna groups, the first antenna group 82 having a LHC polarisation and the second antenna group 84 having a RHC polarisation. The first antenna group 82 is provided with first antenna units 86 which each contain three substantially triangular transmitters/receivers, Ll, L2, L3, which are positioned and fed in a 120° phasing relation with respect to each other, e.g. as shown in Fig. 5. The substantially triangular transmitters/receivers Ll, L2, L3 have an asymmetrically arranged lip 90 at the outer perimeter of the antenna unit, and an asymmetrically arranged hole 92 in the face of the element (shown schematically and not to scale). These asymmetrical features define the rotation sense (LHCP/RHCP) of the transmitter/receiver Ll, L2, L3. The second antenna group 84 is provided with second antenna units 88 which each contain three substantially triangular transmitters/receivers, Rl, R2, R3, which are also positioned and fed in a 120° phasing relation with respect to each other.

In Fig. 7, the two antenna groups 82, 84 are positioned in an interleaved fashion, each transmitter/receiver Rl, R2, R3 of the second antenna unit 88 substantially occupying an interstitial space between transmitters/receivers Ll, L2, L3 of the first antenna unit 86, which gives the benefit of a large active antenna area per available surface area.

In the example of Fig. 7, each first antenna unit 86 is positioned rotated over 120° with respect to neighbouring first antenna units 86 and each second antenna unit 88 is positioned rotated over 120° with respect to neighbouring second antenna units 88. Each of the antenna units 86 of the first antenna group 82 can be provided with a phase shifter. By varying a phase offset per antenna unit 86 within the first antenna group 82, the beam of the first antenna group can be steered in two dimensions in a manner known per se. Each of the antenna units 88 of the second antenna group 84 can also be provided with a phase shifter. By varying a phase offset per antenna unit 88 within the second antenna group 84 the beam of the second antenna group can also be steered in two dimensions in a manner known per se.

Preferably, the LHCP and RHCP beams of the first and second antenna groups 82, 84 are steered in the same direction. This results in a single linearly polarised antenna beam. By adding (or subtracting) a phase offset to (or from) all antenna units 86, 88 within the first and/or second antenna group 82, 84, the phase of the LHCP beam can be changed relative to the RHCP beam, thus changing the orientation of the resulting linear polarisation of the antenna beam as described with respect to Figs. 1-3. In this example the phase shifters (not shown) may be e.g. each connected to one antenna unit. It is also possible that each phase shifter is connected to a cluster (with an arbitrary number) of antenna units.

In Fig. 7, the antenna units are positioned rotated over 120° with respect to each other. It is, of course, also possible to position the antenna units rotated over different angles with respect to each other, e.g. 90°, 60°, 45°, 30° or otherwise or random angles. The number of antenna units in the antenna group determines a preferred angle between the respective antenna units. In order to at least reduce part of the non-circularity, two antenna units are preferably positioned at 90° or 180° with respect to each other, three antenna units are preferably positioned at 60° or 120° with respect to each other and four antenna units are preferably positioned at 45° or 90° with respect to each other. In general the angle between the antenna units is preferably 180° divided by the number, n, of antenna units (180°/n) or 360° divided by the number of antenna units (360°/n). Using 180°/n effectively reduces elliptical non-circularity of the individual antenna units, using 360°/n effectively reduces any non-circularity of the individual antenna units. Using an other angle between the transmitters/receivers or a varying angle between the transmitters/receivers may still result in substantial reduction of non- circularity. In Fig. 7, twenty-four antenna units are used per antenna group, so that the optimum angle between the antenna units is 15°. The angle between the antenna units in Fig. 7 is, however, 120°, which allows for simpler antenna layout, while still providing a reduction of the non-circularity of the polarisation of the antenna groups.

Preferably the antenna units within the antenna group are oriented such that the deviation from circular polarisation cancels out. As the antenna units within an antenna group can be arranged such that any deviations from perfect circular polarisation are reduced, the ellipticity of each separate antenna unit may be less important. In fact, the separate antenna units can be allowed to be considerably elliptical, depending on the number of antenna units within the antenna group, if this reduces costs of the antenna units or benefits other design criteria of the antenna array.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design alternatives without departing from the scope of the appended claims. For example, the invention is not limited to transmitting or receiving only. Where in the description transmitting is mentioned, also receiving is meant and vice versa. The term antenna beam refers to both the pattern of transmitted radiation of a transmitting antenna as well as the pattern of radiation which can be received with a receiving antenna.

Claims

1. Antenna system of the phased array type including a first antenna group of at least two first antenna units connected to a first time or phase shifting circuit, for creating a first sub-beam with a first polarisation; a second antenna group of at least two second antenna units connected to a second time or phase shifting circuit, for creating a second sub- beam with a second polarisation which second polarisation is different from the first polarisation, which first and second polarisation are non-linear, and which first and second sub-beam combine into a third beam with a third polarisation, the orientation of the third polarisation being at least partially dependent on the phase of the first sub-beam relative to the second sub-beam, and a control unit which, for controlling both a direction of the first and/or second sub-beam and the orientation of the third polarisation, is connected to the first and second time or phase shifting circuit.

2. Antenna system according to claim 1, wherein the first and the second polarisation are orthogonal with respect to each other.

3. Antenna system according to claim 1 or 2, wherein the first polarisation is circular polarisation and the second polarisation is circular polarisation.

4. Antenna system according to any one of the preceding claims, wherein the third beam has a polarisation which is more linear than said first or second polarisation, for example that the third polarisation is a linear polarisation.

5. Antenna system according to any one of the preceding claims, wherein the control unit is arranged to control a phase offset of the individual antenna units, and the resolution of the first and/or second beam phase is finer than a resolution of the phase offset of the individual antenna units.

6. Antenna system according to claim 5, wherein the control unit is connected with a first connection to a first part of the time or phase shifting circuits, for adding a smaller first and/or second time or phase offset, respectively, and the control unit is connected with a second connection to a second part of the time or phase shifting circuits, for adding a larger first and/or second time or phase offset, larger than the smaller first and/or second time or phase offset respectively, wherein an average first and/or second time or phase offset of the smaller first and/or second time or phase offsets substantially corresponds to the first and/or second -beam phase, respectively, and wherein the larger first and/or second time or phase offsets, substantially corresponds to the first and/or second sub-beam direction, respectively.

7. Antenna system according to any one of the preceding claims, wherein the first and/or second time or phase shifting circuit includes at least two first and/or second time or phase shifting devices, respectively.

8. Antenna system according to claim 7, wherein each first and/or second time or phase shifting device is connected to at least one mutually different first and/or second antenna unit, respectively.

9. Antenna system according to claim 7 or 8, wherein the control unit is arranged to add a first and/or second time or phase offset in each of the at least two first and/or second phase shifter devices, respectively, to control the first and/or second beam phase, respectively.

10. Antenna system according to claim 9, wherein the first and/or second time or phase offset is equal for each of the at least two first and/or second time or phase shifting devices, respectively.

11. Antenna system according to claims 5 and 9, wherein the phase offset of the individual first and/or second time or phase shifting devices is controlled such that the resolution of the first and/or second beam phase is finer than a resolution of the phase offset of the individual time or phase shifting devices.

12. Antenna system according to any one of the preceding claims wherein the first and/or second time or phase shifting circuits include digital phase shifter devices.

13. Antenna system according to any one of the claims 6-12, wherein the control unit is connected to a plurality of control inputs of the first and/or second time or phase shifting circuits, wherein each control input controls a predetermined amount of phase shift.

14. Antenna system according to claim 13, wherein at least two of the control inputs control different amounts of phase shift.

15. Antenna system according to claim 14, wherein the control unit is arranged to control the finest control inputs of the first and/or second time or phase shifting circuits, such that an average value of the resulting phase shift corresponds to a desired phase shift for the first and/or second antenna group, respectively.

16. Antenna system according to claim 12 and 13, wherein the control unit is arranged to change a least significant bit of the digital phase shifter devices of the first and/or second time or phase shifting circuits for changing the orientation of the third polarisation.

17. Antenna system according to claim 16, wherein the control means are arranged to set the least significant bit of a first sub-group of digital phase shifter devices of the first and/or second time or phase shifting circuits to logical value "0" and the least significant bit of a second sub-group of digital phase shifter devices of the first and/or second time or phase shifting circuits to logical value "1", such that an average value of the phase shift controlled by the least significant bit corresponds to a desired first and/or second beam phase, respectively.

18. Antenna system according to any one of the preceding claims, wherein the antenna units each include at least one transmitter/receiver for electromagnetic radiation.

19. Antenna system according to any one of the claims 3-18, wherein the antenna units of the first and/or second antenna group are substantially similar and positioned rotationally distributed with respect to each other, substantially rotated around an axis extending perpendicular to a plane in which the antenna units extend, such that a non-circular component of the polarisation of a primary first and/or second antenna unit is reduced by a non- circular component of an at least secondary first and/or second antenna unit, respectively.

20. Antenna system according to claim 19, wherein the at least two antenna units of the first and/or second antenna group are positioned at an angle with respect to each other, wherein the angle equals 360 degrees divided by the number of antenna units within the respective antenna group.

21. Antenna system according claim 19, wherein the at least two antenna units of the first and/or second antenna group are positioned at an angle with respect to each other, wherein the angle equals 180 degrees divided by the number of antenna units within the respective antenna group .

22. Antenna system according to any one of claims 19-21, wherein the antenna groups include at least three substantially similar antenna units which are rotated over substantially 120 degrees relative to each other.

23. Antenna system according to any one of the claims 19-22, wherein the at least two antenna units each include at least two transmitters/receivers with a fixed time or phase relationship and a fixed position and/or orientation relative to each other.

24. Antenna system according to claim 23, wherein the at least two transmitters/receivers are sequentially rotated and fed in a sequentially rotated phasing arrangement.

25. Antenna system according to claim 24, wherein the at least two antenna units each include three transmitters/receivers sequentially rotated and fed in a sequentially rotated phasing arrangement over 120 degrees with respect to each other.

26. Method for changing a resulting polarisation of an antenna beam generated by an antenna system of the phased array type, the antenna system including a first antenna group of at least two first antenna units connected to a first time or phase shifting circuit and a second antenna group of at least two second antenna units connected to a second time or phase shifting circuit; the method including: controlling the first time or phase shifting circuit for creating a first sub-beam with a first polarisation and for controlling a direction of the first sub-beam; controlling the second time or phase shifting circuit for creating a second sub-beam with a second polarisation and for controlling a direction of the second sub-beam, which second polarisation is different from the first polarisation, wherein the first and the second polarisation are non-linear; combining the second and the first sub-beam into the antenna beam with the resulting polarisation; and controlling the first and/or second time or phase shifting circuit for generating a phase difference between the first sub-beam and the second sub- beam for changing the orientation of the resulting polarisation.

27. Method according to claim 26, wherein the first and the second polarisation are orthogonal with respect to each other.

28. Method according to claim 26 or 27, wherein the first polarisation is circular polarisation and the second polarisation is circular polarisation.

29. Method according to any one of claims 26-28, wherein the resulting polarisation is more linear than said first or second polarisation, for example that the resulting polarisation is a linear polarisation.

30. Method according to any one of the claims 25-29, wherein the method further includes: controlling a first and/or second time or phase offset of the individual first and/or second antenna units, respectively, wherein a resolution of the phase difference is finer than a resolution of the phase offset of the individual antenna units.

31. Method according to claim 30, wherein the method further includes: controlling a first part of the first and/or second time or phase shifting circuits for adding a smaller first and/or second time or phase offset, respectively, and controlling a second part of the first and/or second time or phase shifting circuits for adding a larger first and/or second time or phase offset, larger than the smaller first and/or second time or phase offset, respectively, wherein an average first and/or second time or phase offset of the smaller first and/or second time or phase offsets substantially corresponds to the phase difference, and wherein the larger first and/or second time or phase offsets, substantially corresponds to the first and/or second sub-beam direction, respectively.

32. Method according to any one of the claims 26-31, wherein the first and/or second time or phase shifting circuit includes at least two first and/or second time or phase shifting devices respectively, and the method includes: adding a first and/or second time or phase offset in each of the first and/or second time or phase shifter devices.

33. Method according to claim 32, wherein the first and/or second time or phase offset is equal for each of the first and/or second time or phase shifting devices, respectively.

34. Method according to any one of the claims 26-33, wherein the method further comprises: controlling a plurality of control inputs of the first and/or second time or phase shifting circuits.

35. Method according to claim 34, wherein at least two of the control inputs control different amounts of phase shift, wherein the method further includes: controlling the finest control inputs of the first and/or second time or phase shifting circuits, such that an average value of the resulting phase shift corresponds to a desired phase shift for the first and/or second antenna group, respectively.

36. Method according to claims 30 and 34, wherein the method further includes: controlling the phase offset of the individual first and/or second time or phase shifting devices, such that the resolution of the first and/or second beam phase is finer than a resolution of the phase offset of the individual time or phase shifting devices.

37. Method according to any one of the claims 26-36, wherein the first and/or second time or phase shifting circuits include digital phase shifter devices, and wherein the method includes: - changing a least significant bit of the digital phase shifters of the first and/or second time or phase shifting circuits for changing the orientation of the third polarisation.

38. Method according to claim 37, wherein the method further includes: setting the least significant bit of a first and/or second sub-group of digital phase shifter devices of the first and/or second time or phase shifting circuits, respectively, to logical value "0" and the least significant bit of a second sub-group of digital phase shifter devices of the first and/or second time or phase shifting circuits to logical value "1", such that an average value of the phase shift controlled by the least significant bits corresponds to the phase difference.

39. Method according to any one of the claims 28-38, wherein the method further comprises: in each antenna group generating at least two similar non-linearly polarised antenna-unit-beams, which substantially coincide; - adding the at least two antenna-unit-beams, wherein the at least two antenna-unit -beams are generated rotationally distributed, rotated around an axis, extending in a direction perpendicular to a plane in which the antenna units extend, with respect to each other such that a non-circular component of the polarisation of a primary antenna-unit-beam is reduced by a non-circular component of an at least secondary antenna- group-beam.

40. Method according to claim 39, wherein the at least two antenna- unit-beams are generated with the non-circular components at an angle with respect to each other, wherein the angle equals 360 degrees divided by the number of antenna-unit-beams generated by the respective antenna group.

41. Method according to claim 39, wherein the at least two antenna- unit-beams are generated with elliptical components at an angle with respect to each other, wherein the angle equals 180 degrees divided by the number of antenna-unit-beams within the respective antenna group.

42. Method according to any one of the claims 39-41, wherein the method includes generating three similar antenna-unit-beams, the non- circular components of which are rotated over substantially 120 degrees relative to each other.