Clarke-belt projected shape: Does polar-mounts work?

  • Thread starter Thread starter archive10
  • Start date Start date
A

archive10

Guest
If you consider that the axis of a a polar mount is essentially displaced from the axis of Earth, there is a distortion of the Clarke belt tracking of a dish on a polar mount.

The effect is minimum either at the North Pole (from where you probably cannot see the Clarke Belt unless you have a tall mast, and I am sure no polar ,ount has even been there, the name sadly notwithstanding), or at the Equator.

Now, being roughly midway in-between these two points, the displacement of the two axis of rotation means that my polar mount is not tracking the precise clarke belt. I have not calculated the exact difference, but surely the tracking is not optimal at either E-W end (assuming the due-south satellite has been properly aligned).

Question: How to adjust for this?

Even if I fit the polar mount with an additional actuator for inclination control, is there any software for set-top boxes or even PC cards that can manage the two as a whole?

Just wondering....
 
search for declination :)
 
Hi ST1,

I don't quite understand what you're asking. I was under a distinct impression that polar mount inclination along with dish declination would shape the arc quite accurately?
 
If you have set the three adjustments of elevation, declination and East/West alignment correctly for your location (other caveat is probably a completely vertical pole, but I've never had to stick with this rule) then you should get the vast majority of geostationary satellites with an average size dish on a polarmount.

The three other exceptions that spring to mind are

1) (Offset dish only) - the clamping arrangement holding the reflector and declination system to the polarmount has to be exactly perpendicular to the face of the tracking system, otherwise you end up with an arc that will never mimic what's on the geostationary one.
2) With larger dishes (and increasingly more common the Ka band broadcasts on mid-sized reflectors), the small powerd movements of the satellites themselves to keep on station will result in signal variations on the ground.
3) As a satellite nears the end of it's service life, those movements in space will mean a single motor system will no longer track them correctly more than twice a day.

Two last points
4a)the angle of the feed in relation to the polarity of transponders is different on each satellite. On older domestic systems you had the option of a polariser which could align these up to a point, but since the invention and mass supply of the fixed polarity Universal LNB for use on motorised systems this is no longer something that is incorporated - except on the upper quality outlets.

4b) Even though you can get the polarity aligned correctly on one polarisation, owing to the perceived angle of satellites not directly above the observer , the opposite polarity of transmission will not arrive at exactly 90 degrees, and so this will require a further mechanical adjustment when selecting a transponder or broadcast.
 
Channel Hopper said:
If you have set the three adjustments of elevation, declination and East/West alignment correctly for your location (other caveat is probably a completely vertical pole, but I've never had to stick with this rule) then you should get the vast majority of geostationary satellites with an average size dish on a polarmount.
Click to expand...

I think the completely vertical pole is not so significant with a perfectly round dish but an oval shaped offset dish may be more off axis and slightly out of the ideal focal plane (not important with digital) beyond the +/- 30 degrees of the arc from 0 than a round dish. Axial rotations of the signal and geomagnetic and electro smog effects would be more severe with an oval dish at the ends of the arc, not helped by the silly polarization angles on some satellites, though this is probably taking it too seriously.

Anyway no matter the dish I swear by a 'perfectly' vertical pole, to be on the safe side.
 
Last edited:
Channel Hopper said:
Two last points
4a)the angle of the feed in relation to the polarity of transponders is different on each satellite. On older domestic systems you had the option of a polariser which could align these up to a point, but since the invention and mass supply of the fixed polarity Universal LNB for use on motorised systems this is no longer something that is incorporated - except on the upper quality outlets.

4b) Even though you can get the polarity aligned correctly on one polarisation, owing to the perceived angle of satellites not directly above the observer , the opposite polarity of transmission will not arrive at exactly 90 degrees, and so this will require a further mechanical adjustment when selecting a transponder or broadcast.
Click to expand...

This is very true and it is a pain.

It is also proof that change does not necessarily mean evolution and for serious satellite use it is a massive step back.

Also the granularity of any actuator is nothing close to a threaded bar, they are not comparable, as anyone who has used both knows very well.

Though obviously threaded bars and skewing the LNB precisely is the ideal, the need for precision soon wears off after the third satellite change of the day in December in the pissing freezing rain.
 
Last edited:
Captain Jack said:
Hi ST1,

I don't quite understand what you're asking. I was under a distinct impression that polar mount inclination along with dish declination would shape the arc quite accurately?
Click to expand...

In rough terms, I am thinking that the path drawn by what the dish is "pointing to" as it sweeps the sky does not map exactly with the arc of the clarke belt as seen from a point on the earth sphere.
The exceptions are where the dish is either directly underneath the arc (Equator - the arc appears to be a straight line), or on the north pole, where the axis of rotation of the satellites and the polar mount would be the same (the arc is a perfect circle). Anywhere in between, the arc is a more a more squashed circle (aka an oval) as you approach the equator.

I haven't sat down and tried doing the maths to figure out the deviation (and if it has anything to say), but 3D geometry tells me that the two circles do not intersect unless they have a common axis of rotation. So a polar-mount with only one axis of adjustment (rotation), will draw out and approximation of the the clarke beltin the sky, and only be spot-on in maximum two places on the arc.

So, what I am wondering is how bad the deviation is. As CH points out in another post here, it is probably neglible for the part of the arc that we can see due to the focusing limitations of the <1m dishes that are so often used for motorised installations.
However, the narrower beam with of larger reflectors (especially prime focus ones with lower f/d ratios) would make these more susceptible to errors on the fringes.

In other words, how bad do we need inclination control for the sats that are actually not "inclined", but supposed to be bang on the arc?
 
Movable big professional dishes for broadcasting have inclination actuators due to the sharp beamwidth and need for maximum gain. Those used with strong satellites are normally fixed.

If you want to regularly watch channels on weak end of arc satellites then fixed dishes are your best bet.

If you want the best signal from weak satellites anywhere on the arc a dish farm is the best.
 
If you set the polar mount at the correct angle it will track perfectly. But yes you are correct. Popular wisdom is to set the polar axis of the mount to be parallel with the polar axis of the planet but that is incorrect. So if you were at 52ºN your motor elevation would be 38º. But due to your non-geocentric location that is incorrect. Correct figure motor elevation for that latitude is 37.32º. So if you go for conventional wisdom you would be almost 0.7º off at the ends of the arc.
 
Back
Top