Offset angle of Terk 18" dish?

[bloomdog]

hello all! I am new to FTA. Been running lots of experiments (no hacking!!!!!!) and am addicted to this hobby. I have downloaded and read ALL posts (over 600) and have learned more from you folks than all the books and literature I have read on the topic. You guys are great! Thanks for the info. I read a post and reply last night which said that when aiming the KU dish, one is actually aiming lower than the satellite's actual location. This is very confusing to me. Please explain!!
Thanks!!!!!!

 

[torg0]

 

[iafirebuff]

Good pic! I have wondered myself.

 

[bloomdog]

Nice drawing--does anyone know why?

Am I supposed to use the elevation provided for a given satellite and then adjust downward (if so, how to calculate the adjustment?) OR am I really aiming lower than the satellite (and this is proper)???

Does anyone know why??

My degree is in Physics, although I am an attorney, and the science is important to me.

Thanx

 

[iafirebuff]

Use the elevation that is given for your location. That # has the deviation taken into consideration. www.sadoun.com under their tech section has a place where you key in your zip and it gives you your settings for whatever satellite you are looking for

 

[bloomdog]

Thanks for the information. I am still very curious as to the reason for the deviation.
Is there a calculation for the deviation as this effects line of sight problems, i.e if I am aimed at a distant tree, it may not be a problem becasue the signal is actually coming from over the tree--right???

 

[iafirebuff]

Yes, that is correct. I think it is 22 degrees ofset, but I am not 100% sure.

 

[bloomdog]

Thanks! do you know why the offset?

 

[iafirebuff]

I am not sure. But I bet someone out there will know.

 

[GrumpyGuy]

There are at least benefits to the offset I can think of off the top of my head:

It keeps the LNB and its support from shadowing the dish from the satellite signal.

Snow and rain are less likely to accumulate on the steeper angled reflector.

 

[damaged]

This may help explain:
http://www.satsig.net/22-deg-offset-dish.htm

 

[ronjohn]

If you have a degree in physics, the following has equations for focal point calculations that you may enjoy.

http://www.qsl.net/n1bwt/chap5.pdf#search="offset dish"

The focal point for a parabola(2d overhead view) is in the center of the parabola. On a prime focus dish, the LNB is mounted directly in the center of the shadow of the dish. The LNB causes a shadow of the satellite signal. An offset dish is shaped like a cutout part of a larger parabola. The focal point is lower than the dish. This allows the LNB to be located away from the reflector and not cause a shadow.

 

[bloomdog]

Thanx, ronjohn! That's exactly what I was looking for! Now, I understand. I didn't realize which portion of the parabola the dish represented-I assumed (see what happens when you assume) that it was the bottom of the parabola--since the dish is actually slid up a bit, it all makes perfect sense to. This is a GREAT place. Many thanks again.

 

[bloomdog]

I read the article. Very nice. My new understanding is that if I have an offset dish, like direct Tv dish, that I should aim it 22 degress lower than the elevation number given in all the computer programs on the web which provide dish elevation for a given satellite. PLease confirm or tell me if I am wrong. Thanks a million!

 

[dashaund]

Yes and no. The elevation markings on the dish itself are adjusted to the dish...you adjust those markings to the ones that you are given (if you are given an elevation of 36 degrees you move the dish to 36 degrees). If you are doing to manually (off hand I can't think of why, but you're obviously smarter than me), then, yes, you must compensate for the 22 degrees.

 

[PSB]

The 22º off set on off-set satellite dish is already built in! No need to compensate!

 

[bloomdog]

Sorry, folks, but the last two replies are contradictory. Can someone clarify this for me?? The web pages which all tell you where to aim your dish do not ask if you have an offset dish or a directly-aiming dish. The answer as to where to point the dish (elevation) has to be different, by approx. 22 degress, depending on which type of dish you are using.

 

[Wescopc]

Both prior messages are saying that the elevation markings that are part of the dish, ie imprinted in the metal of dish adapter by the dish manufacturer, are already adjusted for the offset if it is an offset dish. IF you don't have any markings on the dish and have to "wing it" then you will have to use the rim of the dish and compensate for the degrees of offset. BTW not all offset dishes are set to 22 degrees.
Hope this helps, Bob

 

[bloomdog]

(a) Thanks for the clarification. I am doing all of my dish aiming by compass and inclinometer. I examined the dish I am experimenting with and it seems to be offset by approx. 10 degrees, but not sure. It's an 18 inch dish which says " Direct TV" and "Terk" on it.

(b) Please answer this question for me: When I look up a bird's elevation on the web,is the angle they are providing me a direct line to the satellite? In other words, it I am not using any "markings" and am using the horizon as zero degress of elevation, do I aim the dish at the angle that the web provides and then adjust the dish downward based on the amount of the dish's offset?


(c) For the life of me, I could not pull in 119 W at all. Not sure what am I doing wrong. I live in Mass.

 

[bloomdog]

PLEASE answer this dish question!

Please answer this question for me: When I look up a bird's elevation on the web,is the angle they are providing me a direct line to the satellite? In other words, it I am not using any "markings" and am using the horizon as zero degress of elevation, do I aim the dish at the angle that the web provides and then adjust the dish downward based on the amount of the dish's offset?