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Explanation of satellite antenna polar mount
For conventional azimuth/elevation mounts (sideways and up/down) please refer to satellite tv dish pointing page which has links to pages about the meaning of elevation and azimuth angles.
A polar mount is designed to allow all visible geostationary satellites to be accessed by swinging the antenna around one axis (the main axis). This allows one positioner or actuator only to be used to remotely point the antenna at any satellite.
Polar mount installation involves pre-setting the two fixed angles shown here. The Main axis angle has a range of movement of approx 90 deg. The antenna swings sideways around this axis. The Dish Offset tilt angle is the amount by which the beam it tipped downwards slightly. This angle is less than 9 degrees. Both of these angles are predetermined exactly by calculation, but it is rarely possible to set the angles with sufficient accuracy (~0.2 deg) using scales and protractor. There is a tendency for dishes to sag somewhat and elevation angle scales may be inaccurate by 1 or 2 deg. Some minor adjustments are therefore necessary once satellites are being observed.
The main motor axis angle is the easiest to understand. It is the angle between the motor axis bearing line and the horizontal, in the nearest polar direction. i.e towards the north if you are north of the equator. If there is a main axis scale, the readings may be marked to correspond to this angle or alternatively to 90 minus the angle. Just see what happens when you set it to say 10 deg. If it does not result in the motor axis line sloping low down to the north then read the scale the other way. Set the scale to 80 instead. The angle is preset by calculation and careful adjustment and you can do this adjustment indoors at leisure till you understand clearly what is happening.
Setting the dish elevation is difficult. The easiest case is for an axi-symmetric dish, which has the LNB/Feed on the dish axis, in the middle, normally on three equal length legs. Just set the dish scale, if there is one, to minus 5 deg, for example.
More likely you have an offset dish. In this case you also probably have a motor with a cranked arm on it, typically bent 30 or 40 deg downwards. In this case if you set the dish scale to 30 or 40 the result is zero elevation. You want it set to Angle = 30 minus the small offset angle above. This means the dish is aimed 5 deg lower than a line at right angles to the polar axis. This will aim you correctly at the due south satellite, with the motor central.
The image above shows an axi-symmetric dish on the left and offset dish on the right. Follow the links below for further examples. If this is your first time, is won't be easy, and it may take a week or more.
Polar mount examples:
Example 1: Axi-symmetric (circular reflector with feed in the middle) polar mount antenna set up
Example 2: Axi-symmetric (circular reflector with feed in the middle) polar mount antenna set up
Example 3: Offset feed (elliptical reflector with feed at the bottom) polar mount antenna set up
Example 4: Polar mount set up with motor drive and cranked bent pipe DiSEqC H-H Motor Model SG-2100
Example 5: Satellite dish Polar Mount set up near the equator
Example 6: HH100 Stab polar mount alignment and set up
Alignment objectives for a polar mount are:
The Main axis angle must be set exactly.
The Dish offset tilt angle must be set exactly.
The central position of the main axis rotation of the mount must accurately point due south (if you are north of the equator)
Set up procedure for polar mount:
An inclinometer (spirit level with scale or similar) is helpful. A plumb bob, i.e. fishing line and weight plus measuring rule and calculator or tan tables is an improvised alternative which can be quite accurate.
Check the ground fixed mount tube is exactly vertical. Apply the inclinometer in several places around the tube. All should read 90.0 deg. Adjust the mount tube if necessary and make sure all foundation bolts are tight.
Set the Main axis angle and the Dish Offset tilt angle as accurately as possible and set the moveable actuator to its middle position. The antenna now points to its highest elevation. If your dish is axi-symmetric (i.e. circular, with the feed in the middle) you can now check that the total elevation angle is correct. Measure elevation angle on the back of the hub of the dish; it should equal (90 - main angle - offset angle).
Rotate the whole assembly until the antenna points to the south. Aim south using a compass, map, pole star, GPS receiver with the sun display or GPS receiver and walking away several hundred yards southwards while keeping the longitude unchanged. The antenna is now approximately correct and you will probably receive signals from whatever satellites are due south from you. Check with the list of satellites for one with the same (or close) longitude as your site; use your receiver and there is a good chance of verifying that the receive system is working. If you can detect a satellite, note and mark the rotation axis and the other angles all very carefully and exactly so that you can return to them subsequently if you become lost. Try moving the actuator to the far east and west. Can you see satellites there as well ?
Now follows the difficult job of optimising the pointing.
There are several methods possible. The following is just one suggestion.
Q1: Considering the satellites due south from you. Apply hand pressure to the top of the dish away from the satellite so as to tilt the beam upwards slightly. Does the signal increase or decrease?
Q2: Considering satellites far to the east. Apply hand pressure to the side of the dish to turn the dish further to the east slightly. Does the signal increase or decrease?
Q3: Considering satellites far to the west. Apply hand pressure to the side of the dish to turn the dish further to the west slightly. Does the signal increase or decrease?
Optimise the south pointing first:
Q2 increase, Q3 increase south is OK, no change
Q2 decrease, Q3 decrease south is OK, no change
Q2 increase, Q3 decrease Rotate whole antenna ~0.3 deg to the east
Q2 decrease, Q3 increase Rotate whole antenna ~0.3 deg to the west
Repeat Q2 and Q3 until same results are obtained either side.
Tighten the main rotation bolts firmly. This may increase the elevation angle slightly, so repeat Q1.
Optimise elevation angle:
Q1 increase. Increase main axis elevation to maximise signal.
Q1 decrease. Decrease main axis elevation to maximise signal.
Repeat all above if necessary until good signals are received from all satellites. Once optimised, hand pressures in all three positions should result in little change of the signals.
The above adjustment sequence may need changing according to the particular features of your antenna including its rigidity, repeatability, beam width, quality of the scales provided and so on.
Good luck and best wishes for a successful installation.
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Page last amended 15 April 2015, 25 Feb 2022 |