I've read quite a few threads and websites talking about polar mounts, and I'm confused on a key point... is a dish on a polar mount with a linear actuator able to point to satellites on both sides of true south, or not? Or does it depend on the dish you're using? Is it dependent on the length of the actuator you buy? Intuitively, it seems like a longer actuator should give you a wider span at the expense of less pointing precision (assuming a constant number of pulses per inch.)
Polar mount/actuator - arc question
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Yes.
The linear actuator will reach a point towards the ends of the dish's movement where the dish becomes close to going over center. At these points the dish becomes more difficult to control because the forces required increase and the dish speed increases. The rate at which the dish moves in relation to the actuator movement is not constant due to the geometry of the linkage. If you think of the degrees of dish movement per actuator pulses, you may have 2 degrees for 30 pulses at due south, but 10 degrees per 30 pulses near the ends of the arc.
Both sides: yes
Length: some run an 18" & go both ways from TS.
Longer: actuators are rated in pulses per inch of travel, so longer doesn't lose accuracy.
It is all based on the geometry of the mount.
If your actuator is attached to the east side of the dish, you can get more travel east of TS.
And vice versa.
There's more to this discussion, but I don't have a good reference to site to send ya to right off hand.
But happy to answer more questions if this is still confusing.
Length: some run an 18" & go both ways from TS.
Longer: actuators are rated in pulses per inch of travel, so longer doesn't lose accuracy.
It is all based on the geometry of the mount.
If your actuator is attached to the east side of the dish, you can get more travel east of TS.
And vice versa.
There's more to this discussion, but I don't have a good reference to site to send ya to right off hand.
But happy to answer more questions if this is still confusing.
This sounds promising. To clarify: I'm at 91 W longitude. There are sats with C-band transponders with coverage here as far east as the 40's W and as far west as the 130's W. I know that little or none of what's on the birds out over the oceans is FTA, but the goal would be to get as much coverage of the arc as I can. What should I be looking for when shopping for a BUD, mount, or actuator to give myself the greatest amount of coverage? I have never seen a polar mount advertised as anything more than "a polar mount."
gpflepsen made reference to the dish "going over center". I have also read about people "flopping the dish", without further explanation. I imagined that this would refer to going over the true south azimuth, since that's the only place the elevation changes direction.
gpflepsen made reference to the dish "going over center". I have also read about people "flopping the dish", without further explanation. I imagined that this would refer to going over the true south azimuth, since that's the only place the elevation changes direction.
Think of the Earth as a round ball. The satellites are more or less right above the equator at a certain distance of miles in height. This satellite arc goes around the entire world, and the satellites maintain that distance above the equator. Now, say you are in Indiana, and you are standing in your yard, facing due south. If you could SEE the satellites, they would appear to curve in an arc from East to West. The "Polar Mount" is engineered to follow that arc with a linear motor (actuator). It also has a feature in it called "Declination". This is where the dish itself is tilted permanently some degrees to lower itself from otherwise looking at the stars (where it would point were it not for declination), to hit the sats. It's made in a way so that as it travels from it's lowest East azimuth, it raises up as it goes South to follow that arc. It then starts going lower again as it goes towards it's furthest West.
Here is a picture: http://www.geo-orbit.org/sizepgs/decchartp.html
Here is a picture: http://www.geo-orbit.org/sizepgs/decchartp.html
You'd want to track down what is known as a "Horizon to Horizon" mount. It's a type of polar mount that uses a motor and gears to move the dish from the lowest East, to the lowest West. Fairly hard to track down. There probably was like 100 linear motor polar mounts to every horizon to horizon mount that was ever installed.
If your linear polar mount is installed and adjusted properly, it's almost impossible to "flop the dish over". Most that do flop, just aren't installed correctly.
This is what I meant by going over center or flopping the dish. Think of the actuator and dish as the drive rod and wheel on a steam locomotive. When the rod is at the extent of it's reach, it gets stuck and can't pull or push the wheel back. The same thing can happen to the dish if it's over extended. You get stuck and have to physically turn it back so the actuator can again move it.
Same thing with an internal combustion engine and the piston rod and crank. At top or bottom dead center, the rod can't force the movement of the crank.
Same thing with an internal combustion engine and the piston rod and crank. At top or bottom dead center, the rod can't force the movement of the crank.
I always wondered what a horizon-to-horizon mount is. Can this term be applied to small ku-type motors like an SG2100?
Not really, as they don't have quite the range of a true horizon to horizon mount. They are much closer to a standard polar mount, just condensed into a package like an old-school tv antenna rotator.
The explanation of 'gears used to move the dish' reminded me that one of the houses I looked into buying had a C-band dish on the roof that utilized an H-H mount.
What the?
Are those pics upside-down? I've never seen a polar mount set up like that! Could be it's in China, in which case upside-down would make sense! lol.
Hummm, after another look, maybe you moved the actuator arm to the declination bolt?
We don't, but we should - you have the right idea.
Due to it's geometry, a little Ku motor and a BUD H-H mount both move a fixed number of degrees per step.
BUDs with actuators move different number of degrees per step across their range.
Likewise, actuators have more travel on one side than the other.
If the actuator is attached to the east side of the dish, you can aim further east from TS.
On that same dish, you cannot go so far the other side of TS, west (and this is the side subject to flop over)
Of course, the geometry does give reasonable travel to the weak side.
Typically an east coast user would attach to the west side of the dish.
A west coaster hooks his actuator to the east side of the dish.
If you are in the middle of the country, you choose based on which direction has lower birds of interest.
(or get an H-H and don't worry about it!).
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