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SBE 24 GeoSat Pt4

GEOSTATIONARY ORBITS PART 4:
SATELLITE ANTENNA MOUNTS

by Neal McLain, CSBE
Copyright © 1995-2002 by Neal McLain

This is the fourth in a series of articles about geostationary orbits; i.e., the orbits occupied by communications satellites which remain at fixed points in the sky. In this series, we cover basic physical principles, orbital geometry, pointing angles, and antenna mounts.

This fourth article describes antenna mounts.


CLASSIFICATION OF ANTENNA MOUNTS

Antenna mount the name given to the mechanism which supports a ground-based satellite antenna. Ideally, the mount must allow the antenna to be adjusted precisely to the specified pointing angles, and it must hold the antenna securely in position.

Every antenna mount is constructed so that the antenna can be rotated about two or more axes. Each axis allows the antenna to be adjusted to one specific pointing angle.

Two types of antenna mounts are used in the satellite communications industry:

EL/AZ mount. This type of mount allows the antenna to be adjusted in the azimuth and elevation axes.

Polar mount. This type of mount allows the antenna to be adjusted in the hour angle and declination axes.

EL/AZ MOUNTS

The geometry of an EL/AZ mount is illustrated in the following figure:
4.5-meter satellite antenna on EL/AZ mount (Andrew Corporation).


Note that:

Azimuth is adjusted by rotating the antenna about the azimuth axis. The azimuth axis is always vertical; rotation of the antenna about this axis moves the antenna beam along a line which is parallel to the horizon.

Elevation is adjusted by rotating the antenna about the elevation axis. The elevation axis is always horizontal; rotation of the antenna about this axis moves the antenna beam along a vertical line.
An EL/AZ-mounted antenna is relatively difficult to adjust if it must be moved from one geostationary satellite to another, because both azimuth and elevation must be adjusted. This difficulty is illustrated in the following figure. Note that moving an antenna from one satellite to another requires two motions, one along each axis. For example, to move from SAT1 to SAT2, the antenna must be moved to the right, and also moved down.

Azimuth and elevation are EL/AZ mounts are used in many other fields as well; for several examples, click here.


POLAR MOUNTS

The second type of mount is called a polar mount, sometimes called an equatorial mount.

Compared with an EL/AZ mount, a polar mount is more complicated to construct and install.

The geometry of a polar mount is illustrated in the following figure:

3.8-meter satellite antenna on polar mount (Comtech Antenna Systems, Inc.).


For proper operation, a polar-mounted antenna must be carefully aligned with respect to the earth's polar axis.

Note that:

Hour Angle is adjusted by rotating the antenna about the hour angle axis (sometimes called the polar axis). The hour angle axis is always parallel to the earth's polar axis (and hence, perpendicular to the equatorial plane); rotation of the antenna about this axis moves the antenna beam along a line which is parallel to the Celestial Equator.

Declination is adjusted by rotating the antenna about the declination axis. The declination axis is always perpendicular to the hour angle axis (and hence, parallel to the equatorial plane); rotation of the antenna about this axis moves the antenna beam along a line which is perpendicular to the Celestial Equator.

• The angle between the hour angle axis and the earth's surface equals local latitude.

• At hour angle 0° (antenna aimed straight north or south), declination is related to satellite elevation and local latitude as follows:
EL + DEC + LAT = 90°

A polar-mounted antenna is relatively easy to adjust if it must be moved from one geostationary satellite to another, because only hour angle must be adjusted. This is illustrated in the following figure:

Here we see the great advantage of a polar mount over an EL/AZ mount: once the declination angle is set to the correct value, the antenna tracks the Clarke Belt with very little error. As a result, the antenna can be moved from one geostationary satellite to another simply by moving just one axis: hour angle. Note, as an example, the move from SAT1 to SAT2: in this case, the declination angle error is only 0.1°.

Because of this fact, many polar-mount antennas are constructed with fixed declination adjustments: declination is set when the antenna is installed, and locked in place.

Nonetheless, some slight declination error does exist. This error may be significant in the case of high-gain antennas with very narrow beamwidths, particularly at Ku band. These antennas are sometimes fitted with mechanisms which adjust declination over a small angle, typically about ±1° from the average declination value.

The concept of the polar mount was borrowed from the field of astronomy, where it has been used for centuries. For several examples, click here.


CLASSIFICATION BY CONTROL METHOD

Antenna mounts also can be classified by control method, as fixed or steerable:

• A fixed mount can be adjusted only by direct mechanical action. The adjustment may require any or all of the following: turning a hand crank; loosening and retightening bolts; or simply pushing on the reflector to move the antenna into position.

• A steerable mount is equipped some sort of mechanism which moves the antenna from one satellite to another. Electric motors are most common, although some mounts employ hydraulic mechanisms. Most steerable mounts can be controlled from a remote location by a device called an antenna controller. Controllers vary from simple switches to computerized systems which can store multiple satellite positions and operate automatically on a pre-determined schedule.

PUTTING IT ALL TOGETHER

If we combine these classifications, we obtain four possible mount configurations:

• FIXED EL/AZ
• STEERABLE EL/AZ
• FIXED POLAR
• STEERABLE POLAR

As a practical matter:

• Virtually all fixed mounts are EL/AZ. While it is theoretically possible to construct a fixed polar mount, there is little economic incentive to do so. If the antenna is not moved frequently, there is little justification for the added cost of the polar mechanism.

• Small steerable antennas are almost always polar-mounted. If the antenna aperture is less than about 4 meters and the antenna must be moved frequently, use of a polar mount greatly simplifies the process: only the hour angle axis must be motorized. The antenna can be moved through the entire visible portion of the Clarke Belt by changing only the hour angle. Virtually all C-Band "backyard dishes" employ polar mounts.

• Larger steerable antennas may be either EL/AZ- or polar-mounted. Some manufacturers prefer EL/AZ mounts; others prefer polar mounts. The single-axis simplicity of the polar mount makes these antennas easier to adjust; however, the necessity for declination-error correction partially negates this simplicity.
The type of mount used in any particular situation is, of course, dictated by the application. Fixed antennas are generally used only in situations where they are not moved frequently; for example, network-feed downlinks at broadcasting and cable television facilities. In most other applications, steerable antennas are required.


CONTINUE TO PART 5 - ANTENNA MOUNTS
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