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Focal length of parabolic dish |
Setting up a polar mount satellite dish (Example 1)
In this example, the reflector is circular with feed on centreline. The beam is at right angles to the dish back structure ring. This is probably the easiest polar mount design to set up since the parts are large and the distances are easy to measure accurately.
Measure distance B
Measure this distance A
Calculate what length C should be and then adjust the nuts till the distance is correct.
This sets the small downwards tilt angle correctly - sometimes called the declination angle
Polar mount main axis angle.
Identify your latitude from a map or GPS receiver. North of the equator latitude is positive, the further away from the equator the greater the number. South latitudes are negative. Work it out as a decimal number. There are 60 minutes in a degree.
Examples:
51 deg 30 minutes north = 51.5 deg
50 deg 15 minutes north = 50.25 deg
45 deg 45 minutes north = 45.75 deg
20 deg 10 minutes south = - 20.166 deg
30 deg 20 minutes south = - 30.336 deg
Go to the satellite TV dish pointing angle calculator and input your latitude. You can input zeros for the longitude of your site and the orbit longitude of the satellite, since longitude is irrelevant to the setting up of a polar mount. You are only interested in pre-calculating the polar mount main axis angle and the polar mount dish offset tilt angle.
Set the polar mount main axis angle using a protractor or an inclinometer.
In the above design of polar mount the polar mount dish offset tilt angle (or declination angle) is achieved by setting length C greater than length A, so as to tilt the beam downwards a little.
Write down the required polar mount dish offset tilt angle (it is a small angle less than 9 degrees) and calculate the tan of angle using a calculator. A suitable calculator is at start - programs - accessories - calculator - view - scientific. Check the result is consistent with the values in the table below.
| tilt angle in degrees | tan(tilt angle) |
| 0 | 0 |
| 1 | 0.0174 |
| 2 | 0.035 |
| 3 | 0.052 |
| 4 | 0.07 |
| 5 | 0.087 |
| 6 | 0.105 |
| 7 | 0.123 |
| 8 | 0.14 |
| 9 | 0.158 |
Measure lengths A and B.
Note that tan (tilt angle) = opposite/adjacent = (C-A)/B
so B * tan (tilt angle) = C - A
so C = B* tan (tilt) + A
Work this out and find what C is supposed to be. Then set distance C.
Having set the two angles really carefully, centre the actuator and point the whole head assembly to due south by rotating the canister on the pipe at the bottom. the canister on the pipe will have several tightening screws. Do these up carefully so that the canister is exactly vertical and the metal on top exactly horizontal.
Photos thanks to Neil Duggins.
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Page amended 16 Feb 2005, amended 24th Feb 2015 |