Help me understand the LNB angle. (1 Viewer)

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DirectDishNet

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SatelliteGuys Pro
Nov 25, 2007
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Davenport Iowa
If you look at a dish 500 or a dish 1000 you will notice that the LNB's are angled slightly upwards pointing to the focal point of the reflector/dish. So with that said, Does the LNB have to be at that certain height just to have that particular angle? Is this angle a must?
The reason I ask is because I would like to have multiple lnb's on one dish and by using a straight bar, pipe clamps & threaded rods. I could position 3 or 4 linear lnb's on one dish but I do not have any LNB holders to accommodate these lnb's. So I guess my question is, Could I have the LNB's straight/level and just raise the height of the LNB's to focus on the focal point?
 

B.J.

SatelliteGuys Pro
Oct 15, 2008
2,029
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Western Maine
If you look at a dish 500 or a dish 1000 you will notice that the LNB's are angled slightly upwards pointing to the focal point of the reflector/dish. So with that said, Does the LNB have to be at that certain height just to have that particular angle? Is this angle a must?
...

I'll leave the art of putting more than one lnbf on a dish to others, but I just wanted to comment on the above, ie you say the LNBf is angled upward pointing to the focal point. On a dish with one lnbf, the lnbf won't "point" at the focal point, it will BE at the focal point. I assume that what you mean by pointing to the focal point is pointed toward the center of the dish? (If not, I'm not sure what you're referring to.) But the reason for pointing to the center of the dish is simple, ie just to get the maximum quantity of signal since feedhorns generally get maximum signat from the direction they are pointed. A lot of people are adamant that a prime focus feedhorn should always be pointed at the center of the paraboloid, and some even use laser devices to get this perfect, but as long as the lnbf is at the focal point, the angle of the feed really isn't really that important unless part of the dish is blocked or MISSING (or unless the angle is so great that you're picking up lots of ground noise from around the edges of the dish). I once had the bottom half of my BUD blocked by some big pine trees, and I found that I could improve reception dramatically by tilting my feed up by some 15 or 20 deg, ie aiming at the top half of the dish that was receiving signal above the pine trees. This is very much analogous to the performance of an offset dish. Ie an offset dish is really a prime focus dish where the bottom half of the dish is gone. If you aim the lnbf at the center of the paraboloid, you'd be aiming it off the lower edge of the dish, and you'd be receiving ground noise, and missing considerable signal coming from the dish. That's why the lnbfs are tilted up, just so that they're aiming generally at the dish so that it sees only what's reflected from the dish, and as little as possible of the noise coming from around the dish.

Now, re putting more than one lnbf on the dish, since there is only one focal point of a dish, this is pretty much just a matter of trying to find the best compromise position for the secondary lnbfs. I think it's more of an art than a science. Some people seem to have good results, but I was never much good at it myself.
 

DirectDishNet

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SatelliteGuys Pro
Nov 25, 2007
1,052
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Davenport Iowa
Thank you B.J. You explained it very well, I have a better understanding of the purpose of the angle of the LNB now.

Yesterday I tried to place a second LNB on a 30" dish. I first hooked up a lnb to my birdog, hand held the lnb and pointed at the dish using the same angle as the original fixed lnb. I found the sat I was looking (97W) for but could not get more than 20% signal quality. I moved the lnb by hand up,down,left,right,backward & forward for over 30 minutes. Stiil, Max was only 20%. I guess I need a guy in a white lab coat to come over and show me how to do this.
 

Keith Brannen

SatelliteGuys Pro
Jun 2, 2006
1,788
495
Southwestern Ontario
First off, what satellite is the dish aimed at currently? Six degrees off-centre (depending on the satellite, some can be more) is usually optimal for multiple LNBs.

Secondly, a straight bar will work, but you could lose some signal, as you are then putting the LNB slightly outside the focal point. The focal point follows the curvature of the dish (same distance from the dish). As well, you may have to angle the LNB slightly (left or right) towards the dish.

Those are just general rules, sometimes you will find the optimal placing of the LNB is more towards the dish, or angled slightly different than you would expect.

Searching on "multiple LNBs" will bring up some good threads on the subject.
 
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