GB1602194A - Tracking structures for antennas - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO TRACKING STRUCTURES FOR ANTENNAS (71) We, THE MARCONI COMPANY LrMrrr:D, a British Company of Marconi House, New Street, Chelmsford, Essex, CMl 1PL. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to tracking structures for antennas, specifically such antennas as are used in microwave communication systems.

There is considerable demand for a microwave antenna two-axis mount which is both relatively simple and inexpensive and at the same time allows a wide sky coverage to be achieved with minimal "re-setting". Indeed, it is desirable that the mount enable the angular coverage of the sky to be arranged to allow access to a multiplicity of orbitting satellites from a single earth station. The present invention seeks to supply such a mount.

Accordingly, this invention provides a tracking structure for an antenna, which structure comprises a base member carrying on a first pivot axis a first sub-assembly which in turn carries on a second pivot axis a second subassembly upon which the antenna may be mounted, the two pivot axes being spaced from and in crossing relationship with each other so that the second sub-assembly and any antenna mounted thereon may move both in azimuth and elevation, wherein both support members for the first sub-assembly and bracing members for the support members extend from the base member to support the two pivot points defining the first pivot axis, and between which is mounted the first sub-assembly, the bracing members forming a V-shaped channel in which the first sub-assembly can swing, and wherein the first sub-assembly on one side carries the second sub-assembly, and on the other side is shaped as a rectangular pyramid in the notional base of which lies the first pivot axis.

Although the base member, first sub-assembly and second sub-assembly may be solid structures, they are each preferably constructed in the form of frameworks.

The base member may have any convenient outline in plan, but the preferred plan shape is substantially square. Moreover, where the plan outline is square then the first sub-assembly is conveniently mounted with its pivot axis across diagonally-opposite corners at the top of the base member.

Because the prime purpose of the structure of the invention is to allow the antenna mounted thereon to be steerably pointed in any chosen direction, it is clearly very desirable that each sub-assembly be associated with drive means by which it can be pivotted in the required sense.

Most conveniently each drive means is a suitably-mounted linear actuator; in the case of the square plan structure just mentioned, for example, a first linear actuator extends from a lower mounting point on another top corner of the base member to a point on the first subassembly spaced from the first pivot axis (in order to provide the first sub-assembly with an alternative arc of rotation about the first pivot axis, an alternative lower mounting point for the linear actuator is provided at the opposite top corner of the base member, and it is even more convenient if the heights of the two lower mounting points correspond), while a second linear actuator extends from the apex of the pyramid to a point on the second sub-assembly spaced from the second pivot axis.

Naturally, the invention extends to an aerial system which comprises a tracking structure as described and claimed herein together with a suitable antenna appropriately mounted thereon.

The invention will now be further described, though only by way of illustration, with reference to the accompanying drawings, in which: Figure 1 is a perspective view with details of a tracking structure in accordance with the present invention; Figures 2a and 2b show orthogonal vertical cross sections of the tracking structure of Figure 1, carrying an antenna; Figure 2c illustrates the angular sky coverage of the antenna tracking structure of Figure 1.

Figures 3a, 3b and 3c illustrate aspects of a modification of the abovementioned tracking structure and its sky coverage.

As can be seen from Figures 1 and 2, the illustrated structure basically consists of three frameworks: (i) a base member, comprising a gantry, or tower (1), supporting two pivot points (5) defining the X-axis, X-X; (ii) a first sub-assembly, comprising a cradle (2) supporting two pivot points (18) defining the Y-axis, Y-Y; and (iii) a second sub-assembly, comprising a reflector adaptor (3), carrying a microwave dish antenna (4). As best seen in Figure 1, the base member (gantry) 1 is square-shaped in plan, and carries X-axis bearings 5 at the top across one diagonal. The height above ground of the bearings 5 is at least sufficient to permit ground clearance for the microwave dish antenna 4 at 0 elevation, as may be seen in Figure 2a with the antenna in position X or X'. On each corner of the other diagonal there are anchorages (6a and 6b) for the lower clevis (7) of an X-axis rotation linear actuator (8). These alternative anchorages 6a, 6b are at a lower level than the X-axis bearings 5. Bracingmembers(9) between the anchorages 6a, 6b and the bearings 5 form a V-shaped aperture or channel (10) within which the Y-axis cradle 2 can swing.

The first sub-assembly, the 'Y' axis cradle 2, contains the stub shafts (1 la, 1 ib, and 12a, 12b) respectively for both X- and Y-axes. These axes are orthogonal, and the Y-axis is offset from the X-axis to permit clearance between the back of the antenna and the structure (see Figure 2b). On the side of the X-axis in the opposite direction, the Y-axis cradle 2 is in the shape of a inverted rectangular pyramid in the notional base of which lies the X-axis. At the apex of the pyramid there is an anchorage (13) for the bottom clevis (14) of a Y-axis rotation linear actuator (15). Adjacent the Y-axis stub shafts 12a, 12b on each side of the cradle 2 there are alternative anchorages (1 6a, 16b) for the upper clevis (17) of the X-axis linear actuator 8.

The second sub-assembly, the reflector adaptor 3, is nominally a plane triangular frame upon which the main reflector of the microwave antenna 4 is mounted. One side of the triangle carries the Y-axis bearings (18) at two of its comers. Adjacent the third corner, on the opposite side to the reflector, there is an achorage (19) for the top clevis (20) of the Y-axis linear actuator 15.

It would be possible to incorporate this subassembly into the backing structure of the antenna's main reflector, but the added complication to that structure does not make a more economical overall proposition.

The linear actuators 8 and 15, for the X-axis and Y-axis rotation respectively, may be identical, providing that an adjustable limitation to the length of stroke is employed.

Nominally 900 of movement is provided about both axes, but a larger or smaller angle may be employed, depending upon the configuration of the structure. As illustrated in Figures 2a and 2b this is arranged as 00 to 900 about the Xaxis (i.e., to reach position X in Figures 2a) and + 45 on either side of vertical about the Y-axis (see Figure 2b). By repositioning the actuator 8 to the alternative anchorage positions 6b, 16b, the X-axis rotation is altered to reach position X' in Figure 2a at the 0 position.

If it is desired to use the structure to track geostationary satellites (with equatorial orbits), it may only be necessary to look south from the northern hemisphere (or north from the southern hemisphere), and in such a case it may be preferred to give 00 to 900 excursion around the Y-axis. Then, by setting the X-axis north to south a full range of azimuth (0 - 1800) can be covered from any earth station location, though the position of X-axis actuator 8 will still need to be changed to cover the sky completely.

By introducing driving means (not shown) for rotating the base member or gantry 1, full sky coverage can be obtained. A 900 turn will suffice if + 45 on either side of vertical Y-axis rotation (as shown in Figure 2b) is provided.

For an antenna with 0 to 900 Y-axis rotation it will however be necessary to turn the gantry through 1800.

Figures 3a and b illustrate a modification to the embodiment of Figure 1, 2a and 2b, which modification in effect enables the sky coverage to be re-arranged to give optimum coverage of a geostationary orbit. The basic change to the geometrical arrangement, compared to the arrangement of Figures 1 and 2, consists of moving the Y-axis Y-Y from midway between the X-axis bearings 5 to a position where it is adjacent one of the X-axis bearings (see Figure 3a). The X-axis linear actuator 8 has then to be moved to a position in line with the Y-axis.

This is achieved by modifying the gantry structure to produce a strong point (21) on the leg supporting the X-axis bearing 4 adjacent the Y-axis and this strong point 21 is then used for the bottom anchorage of the linear actuator 8.

Since there is now only one lower anchorage for the X-axis actuator 8, the actuator no longer has to be disconnected at the lower anchorage when changing to the alternative X-axis arc of rotation X'.

Practical considerations for this arrangement suggest that it is desirable to limit the Y-axis arc of rotation to something less than 90". A nominal figure of 700 has been chosen for the purpose of illustration, as shown in Figure 3b.

If the X-axis is set to run from north to south, it will be seen from the sky coverage diagram (Figure 3c) that the entire geostationary orbit (that is, what is visible from any earth station location between latitudes 600N and 60"S) is covered by a mount to this design (to cater for equatorial sites a 'Y'-axis arc of rotation of -5 to +700 is provided, making a total of 75").

At latitudes greater than 60 a 'hole' appears in the azimuth coverage. This increases progressively to a maximum of 40 at latitude 900 The azimuth direction at which the 'hole' occurs can of course be chosen prior to initial installation by specifiying a deviation from the north-south direction for the 'X'-axis.

WHAT WE CLAIM IS: 1. A tracking structure for an antenna, which structure comprises a base member carrying on a first pivot axis a first sub-assembly which in turn carries on a second pivot axis a second sub-assembly upon which the antenna may be mounted, the two pivot axes being spaced from and in crossing relationship with each other so that the second sub-assembly and any antenna mounted thereon may move both in azimuth and in elevation, wherein both support members for the first sub-assembly and bracing members for the support members extend from the base member to support two pivot points defining the first pivot axis, and between which is mounted the first subassembly, the bracing members forming a Vshaped channel in which the first sub-assembly can swing, and wherein the first sub-assembly on one side carries the second sub-assembly and on the other side is shaped as a rectangular pyramid in the notional base of which lies the first pivot axis.

2. A tracking structure as claimed in Claim 1, wherein the base member and first and second sub-assemblies are structures in the form of frameworks.

3. A structure as claimed in either of the preceding claims, wherein the base member has an outline which in plan is substantially square, and the first sub-assembly is mounted with its pivot axis across diagonally opposite corners at the top of the base member.

4. A structure as claimed in Claim 3 wherein as drive means for rotating the first sub-assembly about the first pivot axis, a linear actuator extends from a lower mounting point on another upper corner of the base member to a point on the first sub-assembly spaced from the first pivot axis.

5. A structure as claimed in Claim 4, wherein an alternative lower mounting point for the linear actuator is provided at the opposite upper corner of the base member.

6. A structure as claimed in Claim 5, wherein the heights of the two lower mounting points correspond.

7. A structure as claimed in any of the preceding claims wherein, as drive means for rotating the second sub-assembly about the second pivot axis, a linear actuator extends from the apex of the pyramid to a point on the second sub-assembly spaced from the second pivot axis.

8. A tracking structure for an antenna as claimed in any of the preceding claims and substantially as hereinbefore described.

9. An aerial comprising a structure as claimed in any of the preceding claims together with an antenna mounted on the second sub-assembly.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. occurs can of course be chosen prior to initial installation by specifiying a deviation from the north-south direction for the 'X'-axis. WHAT WE CLAIM IS:

1. A tracking structure for an antenna, which structure comprises a base member carrying on a first pivot axis a first sub-assembly which in turn carries on a second pivot axis a second sub-assembly upon which the antenna may be mounted, the two pivot axes being spaced from and in crossing relationship with each other so that the second sub-assembly and any antenna mounted thereon may move both in azimuth and in elevation, wherein both support members for the first sub-assembly and bracing members for the support members extend from the base member to support two pivot points defining the first pivot axis, and between which is mounted the first subassembly, the bracing members forming a Vshaped channel in which the first sub-assembly can swing, and wherein the first sub-assembly on one side carries the second sub-assembly and on the other side is shaped as a rectangular pyramid in the notional base of which lies the first pivot axis.

2. A tracking structure as claimed in Claim 1, wherein the base member and first and second sub-assemblies are structures in the form of frameworks.

3. A structure as claimed in either of the preceding claims, wherein the base member has an outline which in plan is substantially square, and the first sub-assembly is mounted with its pivot axis across diagonally opposite corners at the top of the base member.

4. A structure as claimed in Claim 3 wherein as drive means for rotating the first sub-assembly about the first pivot axis, a linear actuator extends from a lower mounting point on another upper corner of the base member to a point on the first sub-assembly spaced from the first pivot axis.

5. A structure as claimed in Claim 4, wherein an alternative lower mounting point for the linear actuator is provided at the opposite upper corner of the base member.

6. A structure as claimed in Claim 5, wherein the heights of the two lower mounting points correspond.

7. A structure as claimed in any of the preceding claims wherein, as drive means for rotating the second sub-assembly about the second pivot axis, a linear actuator extends from the apex of the pyramid to a point on the second sub-assembly spaced from the second pivot axis.

8. A tracking structure for an antenna as claimed in any of the preceding claims and substantially as hereinbefore described.

9. An aerial comprising a structure as claimed in any of the preceding claims together with an antenna mounted on the second sub-assembly.