GB2180406A - Antenna tracking system - Google Patents

Abstract

A microwave feed for an antenna, particularly a front-fed symmetric reflector antenna, comprises a primary feed waveguide horn (1) and a plurality of peripheral waveguide horns (2-5) which extend longitudinally alongside the primary horn with their apertures in or near the aperture plane of the primary horn. Each peripheral horn includes means (8), such as a diode, for selectively switching the horn between an operative state in which fundamental mode energy propagating in the primary and peripheral horns at a predetermined beacon frequency has a quadrature phase relationship in the aperture plane of the primary horn which will cause a shift of the secondary beam axis in a predetermined direction at the beacon frequency, and an inoperative state in which the peripheral horn has no effect on the secondary beam axis. By operation of the switching means to render one or more of the peripheral horns operative in a predetermined sequence while the others are inoperative, the secondary beam axis will be scanned. Analysis of the beacon signal strength corresponding to each beam shift will provide tracking information. <IMAGE>

Description

SPECIFICATION Antenna tracking system In communications systems, particularly satellite communications systems, in which microwave RF signals are received or relayed by an antenna, it is essential for efficient operation of the system that the axis of the antenna is maintained pointing in a predetermined direction. When the antenna is pointing correctly it is said to be on boresight and even small deviations of the antenna axis off boresight lead to a marked decrease in signal strength and purity. Consequently, such antennas are provided with an automatic tracking system designed to detect any pointing error and to correct the pointing of the antenna accordingly.There are many known forms of tracking system which may be used depending on the nature of the antenna, but virtually all suffer from one or more limitations from the point of view of pointing accuracy, response time, mechanical or electronic complexity, size, or high cost.

Recently a tracking system has been developed in which a number of mode generators coupled to the primary feed of the antenna are controlled electronically to sequentially produce from higher order modes generated in the feed at a beacon frequency a secondary beam squint in a series of predetermined directions. The secondary beam axis is thus caused to shift at high speed from one predetermined direction to another, effectively step scanning the secondary beam at the beacon frequency without moving the antenna or the primary beam. The beacon signal is extracted from the feed with the communications channel signals and is delivered to a tracking receiver which measures the signal strength on each shifted axis and uses the variations in signal strength to derive pointing error signals for correcting the antenna point ing through the antenna drive motors.This form of beam shift tracking system offers the possibility of achieving a relatively compact tracking system using relatively simple and inexpensive microwave and electronic compo nents, and having a pointing accuracy and re sponse time which is very much better than conventional conical scanning or step tracking systems and which approaches that of con ventional monopulse systems. However, be cause the system relies on the generation of selected higher order modes in addition to the fundamental mode in the primary feed horn of the antenna, it can only be used with anten nas utilising relatively large aperture feeds, such as Cassegrain dual reflector antennas.

The present invention relates to a similar tracking system which can be used also with front fed symmetric reflector antennas, which are characterized by feeds having a relatively small aperture for producing the large primary beam width necessary to illuminate the reflector efficiently. These small aperture feeds, typically 0.7R, are capable of propagating only a fundamental mode at the operating frequencies, and the aim of the invention is to modify the above electronic beam shift tracking system so that it is particularly suitable for use with front fed symmetric reflector antennas.

To this end, a microwave feed for an antenna, particularly a front fed symmetric reflector antenna comprises a primary feed waveguide horn and a number of peripheral waveguide horns which extend longitudinally alongside the primary horn with their apertures in or near the aperture plane of the primary horn, each peripheral horn having means for selectively switching the horn between an operative state in which fundamental mode energy propagating in the primary and peripheral horns at a predetermined beacon frequency has a quadrature phase relationship in the aperture plane of the primary horn which will cause a shift of the secondary beam axis in a predetermined direction at the beacon frequency, and an inoperative state in which it has no effect on the secondary beam axis.

In its operative state each peripheral horn causes the secondary beam axis to squint in the desired direction by changing the feed aperture field distribution by effectively inducing cross-polarisation in the aperture plane having the appropriate amplitude and predetermined quadrature phase relationship with respect to the primary horn energy at the beacon frequency. Electronic control of the switching of the peripheral horns to render one or more of the horns operative while the others are inoperative in a predetermined sequence will scan the secondary beam axis from one predetermined direction to another at high speed in the same way as in the tracking system described earlier, and analysis of the beacon signal strength corresponding to each beam shift in a similar manner will provide the tracking information required for determining and correcting the pointing error.

The number and position of the peripheral waveguide horns about the primary feed horn will depend on the signal polarization with which the antenna is to be used, and on the tracking information which is required. Preferably however, there are four peripheral waveguide horns positioned with their axes at 90 intervals around the primary horn, at least in the case of circular and square primary horn waveguides, which will enable full elevation and azimuth plane tracking to be achieved with linear (either vertical or horizontal), dual linear, or circular polarisation signals when the axes of the peripheral horns lie in the orthogo nal polarisation planes.

In one form of the feed in accordance with the invention the peripheral waveguide horns are parasitic in that they are not positively coupled to the primary feed horn and the only energy coupling which can take place between them is at their apertures. In this case the switching means in each peripheral horn comprises a microwave diode, e.g. a pin diode or varactor diode, positioned across the waveguide to form a short-circuit in its conducting state at a distance from the aperture plane of the primary horn which will create the required quadrature phase relationship in the aperture plane at the beacon frequency, and preferably the diodes in diametrically opposite peripheral horns are displaced axially with respect to each other by a distance equivalent to a quarter wavelength at the beacon frequency, thus creating a 1800 phase difference between the two peripheral horns in the primary horn aperture plane which will result in secondary beam shifts in opposite directions. Each peripheral horn is preferably terminated at its end remote from the aperture by a matched load or a suitably positioned short-circuit.

As will be appreciated, if preferred, each peripheral horn may instead be arranged to be operative to produce a secondary beam squint when the diode is off, i.e. non-conducting, by suitable positioning of the diode and a terminal short-circuit with respect to the aperture plane of the primary horn.

In another form of the feed in accordance with the invention each peripheral horn is coupled to the primary waveguide horn by means of a coupling aperture or slot in their adjacent side walls so that fundamental mode energy will couple between the primary and peripheral horns through the coupling slot.

Such coupling is accompanied automatically by a 90" phase change, and hence there will automatically be a quadrature phase relationship between the peripheral and primary horns at the aperture plane of the primary horn provided the cut-off frequencies for the two horns are the same, irrespective of the axial position of the coupling slot. If the cut-off frequencies of the primary and peripheral horns are different however, the coupling slot will be positioned axially so as to achieve the required quadrature phase relationship in the primary horn aperture plane.

In the case of diametrically opposite peripheral horns having their axes lying in the signal polarisation plane the coupling slots extend transversely to the axes, and because the surface currents intersecting the opposite coupling slots will be in anti-phase there will automatically be a 1800 phase difference between the energy propagating in the two peripheral waveguides, which will result in the production of beam shifts in opposite directions. On the other hand, in the case of diametrically opposite peripheral horns having their axes lying in the plane perpendicular to the signal polarisation plane, the coupling slots will be disposed longitudinally with respect to the axes and the surface currents intersecting the two coupling slots are in phase.In this case one of the two peripheral horns is provided with means, such as a dielectric or corrugated section, for introducing a 1800 phase shift in the energy propagating in the peripheral waveguide in order to produce the required phase difference in the primary horn aperture plane which will result in beam shifts in opposite directions.

As in the case of the parasitic peripheral horns described earlier, the switching means of each peripheral horn preferably comprises a microwave diode. This may be positioned across the coupling slot, in which case there will be no coupling (and hence no beam shift) when the diode is on (conducting), and there will be coupling resulting in a secondary beam squint when the diode is off (non-conducting).

Alternatively the diode may be positioned across the peripheral waveguide itself, in which case the coupling occurring at the coupling slot will result in the desired secondary beam squint when the diode is off, and the coupling will have no effect on the secondary beam when the diode is on and forms a short-circuit reflection plane for the coupled energy.

Each of the peripheral waveguide horns is preferably terminated at its end remote from the aperture by a matched load at the beacon frequency, and it may also be preferable to provide absorber means at its aperture end to prevent or reduce aperture coupling between the primary and peripheral horns.

In either of the above forms of the feed in accordance with the invention each of the peripheral waveguide horns may be provided with a filter, preferably a bandpass filter, for rejecting the communication band frequencies.

Preferably the coupling magnitude between the primary and peripheral horns is in the region of - 15 dB. In the case of the slot coupled horns, the coupling magnitude can be controlled easily by the size and shape of the coupling slots.

Although it will be usual to arrange the peripheral waveguide horns with their apertures co-planar with the primary horn aperture, and with their axes parallel to the primary horn axis, one or more of the peripheral horns may, if desired, have its aperture displaced axially and/or tilted with respect to the primary horn aperture.

The feed in accordance with the invention may be constructed with any one of a number of different waveguide configurations, for example circular, square, or hexagonal, both for the primary and for the peripheral waveguides. Furthermore, the peripheral waveguides do not need to be of the same type as the primary waveguide so iong as the required aperture distribution is produced in the aperture plane of the primary waveguide horn.

The invention is also applicable to the choked small aperture feed horns which are commonly used with front fed symmetric reflector antennas to reduce cross-polarisation.

In this case the peripheral waveguide horns may be located on the outer ring of the choked primary horn. Alternatively, the peripheral horns may be formed within the outer ring, the ring being divided longitudinally to form segmental peripheral waveguides, and each peripheral waveguide including a filter for reflecting communication band frequencies while passing the beacon frequency positioned axially from the aperture by a distance equivalent to approximately a quarter of a wavelength at the mean operating frequency so that the peripheral waveguides operate as a conventional choke at the communications frequencies. Beyond the filter however, each peripheral horn is provided with an appropriately positioned switching diode and short-cirucit or matching load termination for operation to produce a secondary beam squint as described earlier.

By way of illustration, some examples of possible feed configurations in accordance with the invention are shown diagrammatically in the accompanying drawings.

Figure 1 shows a perspective view of an example comprising a circular primary waveguide horn 1 and four circular peripheral waveguide horns 2, 3, 4 and 5 disposed alongside the primary waveguide 1 with their axes parallel to the primary waveguide axis and positioned at 90 intervals about the primary waveguide axis, the apertures of the primary and peripheral waveguides lying in a common aperture plane 6. Each of the peripheral waveguide horns 2, 3, 4 and 5 is terminated at its end remote from the aperture by a matching load 7 operative at a designated beacon frequency.The top and left-hand peripheral waveguides 2, 4 are each provided with a pin diode 8 extending across it at a distance x from the aperture plane 6 such that, when the diode is on (conducting), a quadrature phase difference at the aperture plane 6 is created between fundamental mode energy propagating at the beacon frequency in the primary and peripheral waveguides.Each of the other two peripheral waveguides 3, 5 (i.e. the bottom and right-hand peripheral waveguides) is also provided with a pin diode 8 extending across it, but at a distance from the aperture plane 6 which is greater than x by a quarter wavelength at the beacon frequency, so that when the diode 8 is on a quadrature phase difference at the aperture plane 6 is created between fundamental mode energy propagating in the primary and peripheral waveguides at the beacon frequency which is of antiphase to the quadrature relationship created between the opposite peripheral waveguides 2, 4 and the primary waveguide 1.

Figure 2 is a horizontal section through the example shown in Figure 1 illustrating the positions of the pin diodes 8 of the right and left peripheral waveguides 4, 5 relative to the aperture plane 6, and a vertical section through the example would appear the same.

Although not essential, the aperture dimensions of the primary and peripheral waveguides are the same, giving them equal cut-off frequencies, and preferably each of the peripheral waveguides is provided with a beacon bandpass filter, not shown.

In operation each of the diodes 8 is switched on in turn while the others are switched off, producing a secondary beam squint in the direction of the active peripheral waveguide. The top and bottom peripheral waveguides 2 and 3 therefore produce beam squints upwards and downwards respectively to provide elevation plane tracking information for vertically polarised signals, and the left and right-hand peripheral waveguides 5 and 5 produce beam squints to the left and right respectively to provide azimuth plane tracking information for vertically polarised signals.

For horizontally polarised signals the feed shown in Figure 1 would be used rotated through 90" about the axis of the primary waveguide 1.

Figure 3 is a vertical section through an example similar to that of Figure 1, but in which both the top and bottom peripheral waveguide horns 2 and 3 are positioned with their aperture planes displaced axially with respect to the aperture plane 6 of the primary waveguide 1, although the distances of their pin diodes 8 from the primary waveguide aperture plane 6 are unchanged, i.e.

IZ x and x + 4 respectively. The left and right peripheral waveguides may be displaced in a similar manner.

Figure 4 is a view similar to that of Figure 3 but illustrating an example in which only the bottom peripheral waveguide 3 (and correspondingly the right-hand peripheral waveguide) is disposed with its aperture plane displaced from the primary waveguide aperture plane 6.

Figure 5 shows in perspective an example which is similar to that of Figure 1 except that the four circular peripheral waveguide horns 2, 3, 4 and 5 are disposed about a single choked primary feed horn 9.

Figure 6 illustrates an alternative to the example shown in Figure 1 in that the four peripheral horns comprise segmental waveguides 12, 13, 14 and 15 formed by septum walls 10 which divide longitudinally at equiangular intervals an outer circular waveguide 11 coaxially surrounding the circular primary waveguide horn 1.

Figure 7 shows an example similar to that of Figure 6 except that the segmental peripheral waveguides 12 to 15 are disposed around a single choke circular primary feed 16.

Figure 8 illustrates an example similar to that of Figure 1 except that the four peripheral waveguide horns disposed about the circular primary waveguide horn 1 are formed by rectangular waveguides 17, 18, 19 and 20.

Figure 9 illustrates an example similar to that of Figure 8 except that the rectangular peripheral waveguides 17 to 20 are disposed about a circular single choked primary feed horn 21.

Figure 10 illustrates a construction which may be adopted for the segmental or rectangular peripheral waveguides of the examples illustrated in Figures 6 to 9. The waveguide has an aperture dimension sufficient to support only the dominant mode in the waveguide at the operating frequencies and, as shown, includes a filter 22 which is designed to pass the designated beacon frequency while reflecting the communication band frequencies and which is positioned at a distance 1 from the aperture 23 of the waveguide equivalent to a quarter wavelength at the mean communication band frequency.Beyond the filter 22 the waveguide has a pin diode 24 extending centrally across it to form a shortcircuit reflecting plane when the diode is on, and beyond that the waveguide has a shortcircuit termination 25, the diode and shortcircuit termination being positioned axially with respect to the aperture 23 so as to induce the required phase at the beacon frequency for the fundamental mode energy at the aperture when the diode is switched on and off as described earlier.

Figure 11 is a perspective view of an example comprising a square primary waveguide feed horn 26 and four square peripheral waveguide horns 27, 28, 29 and 30 having the same aperture dimensions (and hence cutoff frequency) as the primary waveguide 26, the peripheral waveguides being shown (for convenience) slightly spaced from the primary waveguide whereas in practise they will be disposed symmetrically in contact with the outer walls of the primary waveguide. The primary waveguide 26 couples into the top and bottom peripheral waveguides 27 and 28 through opposite transverse rectangular coupling slots 31, 32 respectively, and couples into the left and right-hand peripheral waveguides 29 and 30 through opposite rectangular longitudinal coupling slots 33, 34 respectively, preferably lying in the same coupling plane as the slots 31 and 32.Each of the peripheral waveguides 27 to 30 includes a pin diode 35 extending across it at a position between the waveguide aperture and the coupling slot to the primary waveguide 26, and is terminated at its end remote from the aperture by a matched load 36 at the beacon frequency. In addition the right-hand peripheral waveguide 30 is provided with a dielectric phase shifter positioned between the pin diode 35 and the waveguide aperture designed to produce a 1800 phase shift in energy propagating in the waveguide at the beacon frequency.

Figure 12 illustrates more clearly the arrangement of the right-hand peripheral waveguide 30 of the example shown in Figure 11, and also shows a filter element 38 disposed in the coupling slot 34 designed to prevent coupling of the communication band frequencies while permitting coupling of the beacon frequency. A similar filter element would also be located in each of the coupling slots 31, 32 and 33 to the other peripheral waveguides.

Figure 13 is a view similar to Figure 12 but illustrating an alternative construction for the example of Figures 11 and 12. In this alternative the pin diode 35 of each peripheral waveguide horn is not positioned across the waveguide itself but is instead positioned across the coupling slot to the primary waveguide 26 as shown in Figure 13. In this case, if a beacon frequency bandpass filter is to be included, this will be positioned within the peripheral waveguide.

With the examples of Figures 11 and 13, each of the peripheral waveguides is operative to produce a secondary beam squint when its diode 35 is off (non-conducting). When the diode is on the secondary beam axis is unaffected, the diode reflecting coupled energy back to the coupling slot in the example of Figure 11, and preventing coupling from taking place altogether in the example of Figure 13.

Figure 14 is an exploded perspective view illustrating an example which corresponds to that of Figure 11 except that the primary and peripheral waveguide horns 39 to 43 are all circular waveguides.

Claims (30)

1. A microwave feed for an antenna, particularly a front-fed symmetric reflector antenna, comprising a primary feed waveguide horn and a number of peripheral waveguide horns which extend longitudinally alongside the primary horn with their apertures in or near the aperture plane of the primary horn, each peripheral horn having means for selectively switching the horn between an operative state in which fundamental mode energy propagating in the primary and peripheral horns at a predetermined beacon frequency has a quadrature phase relationship in the aperture plane of the primary horn which will cause a shift of the secondary beam axis in a predetermined direction at the beacon frequency, and an inoperative state in which it has no effect on the secondary beam axis.

2. A microwave feed as claimed in claim comprising four of said peripheral horns positioned with their longitudinal axes spaced apart at 90 intervals around the primary horn.

3. A microwave feed as claimed in claim 1 or claim 2, wherein the peripheral waveguide horns are not positively coupled to the primary horn, and energy coupling between the peripheral horns and the primary horns occurs at the apertures of the horns.

4. A microwave feed as claimed in claim 3, wherein the switching means of each peripheral horn comprises a microwave diode positioned across the horn to form, when the diode is conductive, a short-circuit at a distance from the aperture plane of the primary horn which will create said quadrature phase relationship in that aperture plane at the beacon frequency.

5. A microwave feed as claimed in claim 4, wherein the diodes in diametrically-opposite peripheral horns are displaced axially with respect to each other by a distance equivalent to one quarter wavelength at the beacon frequency, to provide a 180 phase difference between those two peripheral horns in the aperture plane of the primary horn.

6. A microwave feed as claimed in any preceding claim, wherein each peripheral horn is terminated, at its end remote from its aperture, by a matched load.

7. A microwave feed as claimed in any one of claims 1-5, wherein each peripheral horn is terminated, at a predetermined position at or adjacent its end remote from its aperture, by a short-circuit.

8. A microwave feed as claimed in any one of claims 1-3, wherein the switching means of each peripheral horn comprises a microwave diode and a terminal short-circuit so positioned in the peripheral horn relative to the primary horn that the horn is operative to cause shifting of the secondary beam axis when the diode is non-conductive.

9. A microwave feed as claimed in claim 1 or claim 2, wherein each peripheral horn is coupled to the primary waveguide horn via a coupling slot through the walls of the peripheral and primary horns whereby fundamental mode energy coupling between those horns is achieved through the slot.

10. A microwave feed as claimed in claim 9, wherein the cut-off frequencies of the primary and peripheral horns are mutually different; and wherein the coupling slot is positioned axially of the horns in a predetermined position to achieve said quadrature phase relationship.

11. A microwave feed as claimed in claim 9 or claim 10, wherein two peripheral horns are spaced apart around the primary horn by 1800 with their longitudinal axes lying in the signal polarisation plane; and wherein the coupling slots between the peripheral horns and the primary horn extend transversely of the horns.

12. A microwave feed as claimed in claim 9 or claim 10, wherein two peripheral horns are spaced apart around the primary horn by 180" with their longitudinal axes lying in a plane perpendicular to the signal polarisation plane; and wherein the coupling slots between the peripheral horns and the primary horn are disposed longitudinally with respect to said axes.

13. A microwave feed as claimed in claim 12, wherein one of said two peripheral horns is provided with means to introduce a 1800 phase shift in the energy propagating in that peripheral horn.

14. A microwave feed as claimed in any one of claims 9-13, wherein the switching means of each peripheral horn comprises a microwave diode.

15. A microwave feed as claimed in claim 14, wherein the microwave diode is positioned across the coupling slot, whereby coupling between the peripheral horn and the primary horn is inhibited when the diode is conductive.

16. A microwave feed as claimed in claim 14, wherein the microwave diode is positioned across the peripheral horn, whereby coupling between the peripheral horn and the primary horn is inhibited when the diode is non-conductive.

17. A microwave feed as claimed in any one of claims 9-16, wherein each of the peripheral horns is terminated at its end remote from its aperture by a matched load at the beacon frequency.

18. A microwave feed as claimed in any one of claims 9-17, wherein each of the peripheral horns includes means at its aperture to absorb microwave energy to at least reduce aperture coupling between the primary and peripheral horns.

19. A microwave feed as claimed in any preceding claim, wherein each of the peripheral horns includes a filter for rejecting communication band frequencies.

20. A microwave feed as claimed in any preceding claim, wherein the apertures of the peripheral waveguide horns are co-planar with the aperture of the primary horn, and the axes of the peripheral waveguide horns are parallel to the axis of the primary horn.

21. A microwave feed as claimed in any one of claims 1-19, wherein the aperture of one or more of the peripheral horns is displaced axially and/or is tilted with respect to the aperture of the primary horn.

22. A microwave feed as claimed in any preceding claim, wherein the cross-sections of the primary and peripheral horns is circular or square or hexagonal.

23. A microwave feed as claimed in claim 22, wherein the peripheral horns are of different cross-section from the primary horn.

24. A microwave feed as claimed in any preceding claim, wherein the primary horn comprises a choked, small aperture horn, having an outer ring.

25. A microwave feed as claimed in claim 24, wherein the peripheral horns are located on the outer ring of the choked horn.

26. A microwave feed as claimed in claim 24, wherein the peripheral horns are formed within the outer ring, which is divided longitudinally to form segmental peripheral waveguides.

27. A microwave feed as claimed in claim 26, wherein each peripheral horn includes a filter for reflecting communication band frequencies, spaced axially from its aperture by a distance equivalent to one quarter wavelength at the mean communication band frequency.

28. A microwave feed as claimed in claim 27, wherein the switching means of each peripheral horn is positioned further from its aperture than the filter.

29. A microwave feed as claimed in any preceding claim, wherein the switching means of the peripheral horns are operated cyclically to render one or more of those horns operative while the others are inoperative.

30. A microwave feed for an antenna, substantially as hereinbefore described with reference to the accompanying drawings.