Russell Brown /
10.01.2009 03:36 PM
Satellite TVRO Part 4

Although satellite dishes are used extensively, steerable dishes can be frustratingly difficult to setup correctly.

Polar and HH dishes require more attention to detail when first installed. Their dish mount is designed is such a way that as it moves through its azimuth range, the elevation changes to match the arc of satellites in the sky. This is a more complex mount to design and install, but it is simpler to operate because it only uses one motor.

Declination

On a polar dish mount, you will find a declination adjustment that is only used during installation; this rotates the entire dish mount in an arc, similar to elevation adjustments. Normally, the look angle of the dish would be parallel to the elevation adjustment frame so the two would always match elevation angles, but here a declination adjustment is found that allows for an offset between the elevation adjustment frame’s angle and the dish’s look angle. This is only adjusted at the time of installation. As the dish’s installation site moves north from the equator, the arc it must follow to match the satellites in the Clarke Belt changes in shape. (See Figure 1.)

Declination is the adjustment used to compensate for this change in the arc due to latitude. The range is from 1 degree near the equator to 8.5 degrees near the North Pole. The dish actually rotates on the elevation adjustment frame’s axis angle.

This is an important adjustment that many installers misalign. When not adjusted correctly, the satellites at the zenith of the arc may be correct, but incorrect at either end of the arc. (See Figure 2.)

Polar and HH

Polar dishes use a drive screw that pushes or pulls the dish through its arc. There is a standard setup where polar dishes have the actuator arm (screw) placed on the west side (viewed from the back) of a dish installed in the eastern half of the United States (linear west) and on the east side of the dish in the western half (linear east). This allows the actuator arm to push the dish up the arc and lets gravity help pull it down. In the eastern United States, most satellites are higher toward the west and reversed in the western United States. (See Figure 3.)

To set up a polar dish, its mounting pole must be exactly plum, because any tilt will throw off the dish’s alignment as it moves through the arc. The next step is to move the dish to its highest elevation or zenith and aim due south, or 180 degrees on a corrected compass heading. Keep in mind that compasses will always display an error due to magnetic variations in the Earth. To compensate for this, you must add a correction factor, for your area, to your compass reading to obtain a true north-south heading. Many online Web sites carry such information.

For example, pick a satellite that shares the same longitude in the Clarke Belt as the dish’s longitude. If the dish was located in Denver, its longitude would be 105 degrees, and at 105 degrees west in the Clarke Belt we find AMC 18 and AMC 15. With the dish at its highest elevation in its polar arc, adjust its declination angle to the value on the chart; in this case, Denver’s latitude is 39.7 degrees, which equates to a declination of 5.63 degrees. The declination angle always causes the dish’s elevation to be lower. With declination set, adjust the elevation frame until the dish, not the frame, is at the correct elevation — in this case, 44 degrees. At this point, small adjustments can be made to elevation, but not declination, and rotating the dish on its pole should bring in the best possible signal. Once that is accomplished and the dish is fixed to the pole, find a satellite at either end of the arc and check to see if you get a good signal from them. A small adjustment of the dish east or west (rotating it on its pole just a little) should be all that is necessary. Once you have good signals from satellites at the peak and either end of the dish’s arc, lock down the all adjustments. Note that the arc is rather flat at the zenith and small adjustments east-west will not affect reception very much. Now the dish will see every satellite within its arc; the controller just has to remember the position of the dish at each satellite.

HH setup is very similar. These dish mounts use a chain drive on a half circle frame to rotate the dish through its arc. Declination is set in the same way as above. Some HH dish mounts have a second motor used for declination adjustment to track inclined orbiting satellites. Once again, the declination adjustment must be set at installation as above for it to be able to properly track all the satellites in the Clarke Belt. If you do not track inclined orbiting satellites, you will have no need to adjust this further. Otherwise, you need a way to track any movement of this declination motor so it can be repositioned back to its normal setting so the dish will track the satellite arc correctly. (See Figure 4.)

Az-El

Az-El dishes do not require any alignment other than making sure the azimuth swing is centered on 180 degrees due south. After that, the two actuator arms can move the dish in any direction to find any satellite. (See Figure 5.) This freedom allows the dish to follow any satellite, even those with a high degree of inclination, moving north and south in their orbit. These dishes require a two-motor controller to move them.

Feed horn

As stated previously, the dish is not the antenna. The antenna resides inside the feed horn, which is mounted above the dish where the signals from the satellite are reflected from the dish and into the feed horn. The feed horn’s antenna, which is inside it, must be set at the correct focus point above the dish where the reflected signals come to a focus point. The position of the feed horn is determined by dish manufacturer, or when the low-noise block converters are installed, the focus point can be adjusted by hand for maximum signal strength.

Next Time

The next “Transition to Digital” tutorial will address the search for satellites in the arc.

Continue reading part five of the Satellite TVRO series.



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