Big dish & off set dish question

[skitheberks]

On a traditional BUD it tracks the arc and your dish is pointed to that satellite. Depending on where you are your dish can be up to a 40 degree angle or more looking directly at a satellite.

On my DBS dish for SkyAngel, it was pointed at the 61.5 satellite. The dish looked as if it was pointed level with the ground ( I am in northern CT). In fact it looked like the house was blocking the satellite dish. I knew when it was mounted, it was looking at the satellite fine and even if there was 4 feet of snow on the roof it still had a view of the satellite.

How come these type of dishes are "offset"? Why do they not look at a satellite like a BUD would? I have heard of Ku offset dishes, what is the reason for an offset? For the Dish / DirecTV, is it for us northern climates to keep the snow from filling the dish and degrading the signal? I knew if I left my BUD pointed up during a snow storm, the signal would degrade until I cleaned the snow off of it. I can understand how inconvenient it would be to have a DBS dish to fill up with snow that is mounted up on the roof.

 

[gabshere]

although they both could look at the same satellite the offset is made to basically look at 22 degrees while the big dish is pointed at 40 ( and these are just examples) there is a set figure for each. another thing to consider is that DBS satellites are stronger signals so they can use smaller dishes.

Why do they not look at a satellite like a BUD would? I have heard of Ku offset dishes, what is the reason for an offset?

the shape of a offset dish directs the satellite beam to a set point just like the prime focus dish does just its off set where as the prime focus dish is even all the way around and is centered.

I also feel ( and this is just me guessing ) that its cheaper to offer an offset dish ( cause of its size and ease of installation over a big dish)

they both use different type lnbfs (offset & prime focus)
usually offset dishes are all solids while big dishes are both solids and mesh
primefocus was developed before offset (most older dishes are prime focus) and are usually geared for c-band and later added ku to the feedhorn.
most offset dishes are smaller and geared toward ku (a few add c-band lnb to tinker with it trying to gain c-band on a small dish)

 

[phlatwound]

I should be attempting to setup (stationary) my first Ku offset dish sometime next week, and have read in several threads that the elevation markings can be off a few degrees on scale marked on the dish.

Was wondering:

1.) Is 22 degrees a common offset for most of these small (90 cm, 100 cm, 1.2m) dishes?

2.) Say I'm attempting to point the dish to G16 @ 99W, if the dish elevation for that satellite at my latitude should be 46.6 degrees, and the dish had a 22 degree off set, could I set the face (using inclinometer or protractor/string) of the dish to 24.6 degrees (46.6 - 22 = 24.6) off of vertical to get the desired elevation?

I know I'm just looking for a starting point when I start the aiming process but I've been having some trouble understanding the difference between aiming a prime focus and an offset, trying to understand the geometry. Please help edjimicate this hillbilly, thanks!

 

[mikekohl]

If you have a C-band antenna working nearby, take an elevation reading with an angle finder (also known as an inclinometer). To get the same elevation angle, an offset antenna with a 22 degree offset rating will be 22 degrees more VERTICAL than the prime focus. Most people get hung up on the math and get things backwards with measurement of actual angles on their antennas.

Go to www.global-cm.net and find the Satellite Pointing Angles section for a major city near you.

Find a working antenna for any DBS satellite (as long as you can confirm which satellite it is on (101, 110, 119 or whatever) and measure its elevation angle with an inclinometer. Compare this number with the elevation angles found for that satellite on the chart. Now go to the desired satellite and compare its elevation angle as well as true directional heading. Subtract the differences and point your new antenna accordingly.

 

[phlatwound]

If you have a C-band antenna working nearby, take an elevation reading with an angle finder (also known as an inclinometer). To get the same elevation angle, an offset antenna with a 22 degree offset rating will be 22 degrees more VERTICAL than the prime focus. Most people get hung up on the math and get things backwards with measurement of actual angles on their antennas.

Go to www.global-cm.net and find the Satellite Pointing Angles section for a major city near you.

Find a working antenna for any DBS satellite (as long as you can confirm which satellite it is on (101, 110, 119 or whatever) and measure its elevation angle with an inclinometer. Compare this number with the elevation angles found for that satellite on the chart. Now go to the desired satellite and compare its elevation angle as well as true directional heading. Subtract the differences and point your new antenna accordingly.

Thanks for the well thought out explanation, Mike, sounds like I was on the right track. Being my first setup, I not only want it to work, I want to understand how and why it works, so I can have the basics down before I go motorized.

 

[Anole]

... and have read in several threads that the elevation markings can be off a few degrees on scale marked on the dish.

1.) Is 22 degrees a common offset for most of these small (90 cm, 100 cm, 1.2m) dishes?

2.) Say I'm attempting to point the dish to G16 @ 99W, if the dish elevation for that satellite at my latitude should be 46.6 degrees, and the dish had a 22 degree off set, could I set the face (using inclinometer or protractor/string) of the dish to 24.6 degrees (46.6 - 22 = 24.6) off of vertical to get the desired elevation?

1. a lot of dishes are 22º or 23º offset. Especially when you look at all the DBS (pay TV) dishes.
The commercial offset dishes seem to be in the 22º to 26º range, but that's by no means a comprehensive review.
You should be able to find specs on any particular dish, and go by that.

2. yes, except . . .
The whole reason the dish elevation is off in the first place, is because the LNB arm is bent.
If that is so, using an inclinometer on the dish won't work , either.

Another reason for the elevation being off is due to a home-made LNB mount.
One which places the LNB too high or too low.
And an inclinometer won't help with that, either.

One way to check a dish, if you really all that concerned, is to put it on a plumb pole withOUT a motor mount, and aim it at any one bird.
Check the displayed elevation against the known elevation for that bird.
Then, either mark your mount with the error, log the error, or tweak the LNB arm/mount, as you see fit.

If you really cannot find any FSS birds for your adjustment, you could always take the coward's way out ...
Put on a circular LNB for the DBS (pay TV) service.
And then if you still cannot find one of those birds...
.

 

[phlatwound]

1. a lot of dishes are 22º or 23º offset. Especially when you look at all the DBS (pay TV) dishes.
The commercial offset dishes seem to be in the 22º to 26º range, but that's by no means a comprehensive review.
You should be able to find specs on any particular dish, and go by that.

2. yes, except . . .
The whole reason the dish elevation is off in the first place, is because the LNB arm is bent.
If that is so, using an inclinometer on the dish won't work , either.

Another reason for the elevation being off is due to a home-made LNB mount.
One which places the LNB too high or too low.
And an inclinometer won't help with that, either.

One way to check a dish, if you really all that concerned, is to put it on a plumb pole withOUT a motor mount, and aim it at any one bird.
Check the displayed elevation against the known elevation for that bird.
Then, either mark your mount with the error, log the error, or tweak the LNB arm/mount, as you see fit.

If you really cannot find any FSS birds for your adjustment, you could always take the coward's way out ...
Put on a circular LNB for the DBS (pay TV) service.
And then if you still cannot find one of those birds...
.

Thanks Anole, I had seen the 22 degree figure mentioned a few times and wondered if there was anything "magical" about that number, I see now it is just a matter of manufacturer preference. I was having trouble understanding that the elevation scale on the offset dish was already taking into account the offset angle of the dish....duhhh! And, of course the other variables (LNB arm & LNB placement, etc.) must be taken into account.

Gotcha on the bent arm causing the displayed elevation to be off, I haven't had one of these small dishes in my hands yet, but have looked at a lot of pics and they look pretty flimsy. And I understand what you mean about reindexing the elevation scale, or changing the other variables to make the existing scale correct.

I have a 3ABN dish on the way, also I have been offered an older (looks like 36") DTN weather service dish w/LNB, and will be going with a stationary setup for now. Don't know which is going to be my first "victim" but I can't wait to give one a whirl, appreciate your thoughts, as always.

 

[Anole]

I have been offered an older (looks like 36") DTN weather service dish w/LNB

Post a picture (front and rear) and actual measurements.
There are so many experienced folks on here.
Someone will probably recognize it.

 

[phlatwound]

Post a picture (front and rear) and actual measurements.
There are so many experienced folks on here.
Someone will probably recognize it.

Will do, just got a new batt for my camera. I've only seen it through a fence from a distance, it's a pretty common looking steel dish....and it's gratis! :up

I know the owners discontinued the DTN satellite service 2-3 years ago, they now get the same product via the 'net, does anyone know what sat and Tp that service was on? It may not exist any more but I'll see if I can find it on Lyngsat.