Simple explanation needed

[1ADAM12]

Simple explanation needed

I understand how an offset dish on a h-h motor moves and adjusts the dish so that it has proper skew for the lnbf on any satellite on the Clarke belt.

Why does a prime focus and actuator not properly skew the lnbf?

Is there a h-h motor that does this for a 10’ BUD?

 

[Lak7]

Why does a prime focus and actuator not properly skew the lnbf?

My LNBF on the BUD w/ Arm is properly skewed as it moves across the Arc.
Are you having a problem?

 

[Anole]

Why does a prime focus and actuator not properly skew the lnbf?
Is there a h-h motor that does this for a 10’ BUD?

1). it does.
- read the Geo-Orbit web site for encouragement
2). there are
- see Stogie's thread on his Paraclipse, (now there's an H-H motor from hell) -
- or search for "AJAK", (they are H-H BUD motors, and Linuxman's got a 10' dish on one)

edit:
I don't have links to all Fred's AJAK pictures, but you should be able to find them, he's certainly got more than I thought:

[10:04] <Anole> Fred - how many AJAK motors are you running now?
[10:14] <Linuxman> i have 3 right now
[10:14] <Linuxman> 2 big ones and one is non-moving
[10:14] <Linuxman> and one medium
[10:14] <Linuxman> all 10 foot dishes
[10:15] <Linuxman> 10 foot unimesh perforated on one big ajak used for scanning
[10:15] <Linuxman> 10 foot pinnacle on [COLOR=Blue]87w[/COLOR] on one big ajak non-moving
[10:15] <Linuxman> 10 foot unimesh mesh on medium ajak motorized for my "system"
[10:15] <Anole> quite the collection!  :)
[10:15] <Linuxman> nah
[10:16] <Linuxman> i need another "2 or 3" :)
[10:16] <Linuxman> all in good time
[10:16] <Anole>  do you mind if I quote the above in the thread, just for "color" ?  hahhahahh
[10:16] <Linuxman> not at all

 

[B.J.]

Simple explanation needed

I understand how an offset dish on a h-h motor moves and adjusts the dish so that it has proper skew for the lnbf on any satellite on the Clarke belt.

Why does a prime focus and actuator not properly skew the lnbf?
...

If you understand how it works on an offset dish, then you understand how it works on a prime focus. It is exactly the same.

It may be that your question is actually why big dishes usually have adjustable polarity, thinking that you need to adjust polarity on each sat, but that isn't the case. Generally, if you have it set on one sat, the same settings are used on all sats, except for odd balls like AMC1-Ku. The adjustable polarity is kind of handy though, because you don't have to climb up on a 10' ladder to set it. You can do it from the recliner in your living room.

 

[caddata]

......Why does a prime focus and actuator not properly skew the lnbf?

You may be confusing "skew" with "polarization" as it relates to Corotor II type feed on a prime focus dish on a "polar" type mount. The servo motor, by way of wiring from the receiver, physically rotates a probe (small antenna) within the feethorn 90 degrees to align with horizontal or vertical satellite transponders because of the single amplifier using a single coax. Some "Orthomode" feedhorns use multiple amplifiers and coaxes. Your offset dish actually has two amplifiers in one housing---one for horizontal and one for vertical that are switched electronically using one coax. The skew function of the Corotor II is for fine tuning. Before the modern satellites were up there, the angle of individual satellites varied slightly between eachother, and to obtain the strongest signal, had to be individually adjusted.

The geometry of a "polar mount" automatically "skews" the feedhorn as a dish rotates across the arc. HtoH motors on offset dishes have an offset (bend) in the shaft that makes it similar to a "polar mount". The axis of rotation, whether its a polar mount or a HtoH motor is pointing at the North Star (Polaris).

Harold

 

[1ADAM12]

It may be that your question is actually why big dishes usually have adjustable polarity, thinking that you need to adjust polarity on each sat, but that isn't the case. Generally, if you have it set on one sat, the same settings are used on all sats, except for odd balls like AMC1-Ku. The adjustable polarity is kind of handy though, because you don't have to climb up on a 10' ladder to set it. You can do it from the recliner in your living room.

BINGO

I was under the impression that the polarity was being changed from bird to bird. So the dish on a polar mount does allow the axis of the dish to rotate clockwise or counterclockwise.

Is declination doing this?

 

[melgarga]

BINGO

I was under the impression that the polarity was being changed from bird to bird. So the dish on a polar mount does allow the axis of the dish to rotate clockwise or counterclockwise.

Is declination doing this?

If you are calling the axis of the dish an imaginary line from the center of the dish, perpendicular to it's surface and pointed into space (thru the focalpoint/polarizing element) then yes, the declination adjustment is what causes it to "rotate" CC and CCW relative to a zero polar offset point when at your true south position, which is the highest point in the arc for your location.
Very good, go to the head of the class!

 

[1ADAM12]

If you are calling the axis of the dish an imaginary line from the center of the dish, perpendicular to it's surface and pointed into space (thru the focalpoint/polarizing element) then yes, the declination adjustment is what causes it to "rotate" CC and CCW relative to a zero polar offset point when at your true south position, which is the highest point in the arc for your location.
Very good, go to the head of the class!

Thank you for that explanation.

My teacher always had me sit next to her desk in class.

 

[Anole]

My teacher always had me sit next to her desk in class.

Was that so she could smack your hand with a ruler? - -
No wonder you wound up in law enforcement.

 

[melgarga]

Was that so she could smack your hand with a ruler? - -
No wonder you wound up in law enforcement.

Naaa.... it's because all of us anti-establishment, peace and free love types already took up all the desks as far away from authority as possible. The front of the class was the only room left......lol
Adam12,
Get you a C band setup (if you dont already have one) and watch it as it travels thru the arc. It will make it easier to understand the mechanical geometry than trying to visualize it.
I have a daughter that has a degree in CAD and I've considered trying to explain it to her so she could do an interactive 3D presentation of it to be able to "see" the satellites/polarity of signals and a TVRO station from several perspectives. I dont know if it is within her abilities, but something like that, similar to the sim/3d graphics stuff you see on Discovery Science Ch. etc, would be pretty cool.

 

[1ADAM12]

......I have a daughter that has a degree in CAD and I've considered trying to explain it to her so she could do an interactive 3D presentation of it to be able to "see" the satellites/polarity of signals and a TVRO station from several perspectives. I dont know if it is within her abilities, but something like that, similar to the sim/3d graphics stuff you see on Discovery Science Ch. etc, would be pretty cool.

That would be a very interesting visualization. If it included a perspective of the Clarke Belt from a fixed point on earth, all the better for understanding Skew.

However, the FTA Insanity is virual, you may not want to expose her to it.

 

[B.J.]

BINGO

I was under the impression that the polarity was being changed from bird to bird. So the dish on a polar mount does allow the axis of the dish to rotate clockwise or counterclockwise.

Is declination doing this?

Declination has very little to do with polarity/skew. Declination allows you to see down to the Clarke belt when your rotation axis is aligned close to (not exactly) parallel to the polar axis. Without declination, you'd generally be aiming about 6 degrees above and parallel to the equatorial plane, and you'd be a constant amount off across the arc. THe polarity would be tracked properly however.
WITH declination, you tilt the dish only (not the polar axis) down toward the southern horizon to reach the Clarke belt, best if done AFTER tilting the polar axis about 0.6 degrees, which is a minor adjustment to allow you to track properly across the arc, since the declination needed changes across the arc.

But really, declination has little or no effect on polarity/skew.

 

[melgarga]

A 3D rendition would be pretty useful, since the concept is somewhat complex. I'll talk to my daughter and see if I can get it across to her or she can teach me CAD......

BJ is correct in that without declination to compensate for the "relative spherical line of sight relationship" for lack of a better term you would not be able to track the entire arc, only the point above you. Fortuntely, polar offset is a function of "relative spherical line of sight relationship" as well and the declination adjustment compensates for both variables at the same time.

Suffice it to say that without any declination adjustment you would not be able to properly track the Clarke Belt nor, have the proper polar offset for satellites that were not directly "above" you ( true south, zero polar offset).

If everything was circular polarization, then polar offset would be a moot point, but declination settings would still be necessary to properly track the arc.

 

[1ADAM12]

B.J. and Melgarga,

Thank you both for sharing your knowledge with me and others. I have had several questions answered by your postings here.

Melgarga- " 3D Rendition" That would be a major accomplishment that likey would create a good deal of interesting comment here.