Is the 99w-101w-101w-110w-119w arc built into the shape of the dish reflector?

[tkrandall]

This thought occurred to me the other day when I was in Denver and Colorado Springs. No matter where you are in the country, the satellites lie located in an arc, not a straight line, across the sky. I live near Atlanta where we have plenty of "right" tilt/skew (see my avatar) to the satellites, with the Slimline dishes centered on 101w of course.

Denver being sandwiched between 101w and 110w, I happened to notice, as expected, the slight leftward tilt (from behind) of the DirecTV slimline dishes. Which got me to thinking, as 110w and 119w are west of Denver and hence lower and to the right in the sky than a due south azimuth would provide, how does a "left" skew for 99w/101w/103w also align to pick up 110w and 119w which are, in the sky, to the right? Looking at the LNB assembly for a regular SL5 dish, the LBNs appear to be in a straight line. But that would not appear to be optimum for aligning with the sattlite arc unless the dish itself is optimized and shaped to consider that effect. Is that indeed the case?

[doctor j]

The short answer is than even with 20 degrees or arc at 23000 miles out it's "almost" a straight line .

doctor j

[tkrandall]

Not quite.

The dish is located at ~105w longitude for Denver. The dish is centered on 101w, with tilt, it would appear since it is slightly to the left in that market, set for picking up the 99w-103w positions. The tilt setting is 94.8 according dishpointer, or 4.8 degrees - to the east - from level. 110w is about the same distance from Denver, but in the opposite direction to the west, and 119w even further down and to the right in the sky. That is, 119W is a couple degrees lower in the sky than is 101w. So you have a 4.8 degrees to the left tilt centered on 101w, which tilts the right side (looking from behind) of the dish for an upward arc to the right/west, yet 119w is located down and to the right 2 degrees lower in elevation. That would put 119w off of the 99w-103w line by the 4.8 degrees tilt setting plus the the angle subtended by its 2 degrees lower elevation than 101w.

Consider another example that might be easy to visulalize, such as a location exacty at 101W like Holcomb, Kansas. The tilt/skew setting is zero (i.e. level, or 90 on the dial). That will align the LNBs to be level, which the line from 99w to 103w is when you are located at 101w. But 119w is 4 degrees lower in the sky.

So all I am pondering is how they shape the dish reflector to accommodate these type of variances.

[jdspencer]

It's the skew/tilt setting that allows the dish to accommodate these variances.

[veryoldschool]

So all I am pondering is how they shape the dish reflector to accommodate these type of variances.
At 23,000 miles above the equator, they are in a "straight line", so tilting the dish does bring them "in line". Only when you're at the extremes [Alaska & Hawaii] , do you fall out of what the dish can do.

[BattleZone]

So all I am pondering is how they shape the dish reflector to accommodate these type of variances.

They accomplish this by making the dish wider, and calculating the curve angles to "average" the arc. With Ku, the wavelength allows for quite a bit of leeway. It would be much harder to design a single dish to do nationwide coverage with Ka if they were spread out as far (18+ degrees).

[tkrandall]

They accomplish this by making the dish wider, and calculating the curve angles to "average" the arc. With Ku, the wavelength allows for quite a bit of leeway. It would be much harder to design a single dish to do nationwide coverage with Ka if they were spread out as far (18+ degrees).

Thanks, that makes sense.