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VSAT technology and installation >> Dish pointing and alignment >> Dual LNB with 76cm Weingard dish?

Message started by techtom on Oct 7th, 2007 at 12:50am

Title: Dual LNB with 76cm Weingard dish?
Post by techtom on Oct 7th, 2007 at 12:50am
I'd like to add a second lnb to my 76cm dish.  I realize that I will need to construct a custom mount.  I can find the second sat by trial and error, but I was hoping that some of the dish pointing mathmatitions could tell me how to calculate it.

location 32.8 N, 117.2W
Sats 110 and 119
Wiengard DS-2076

The difference in azimuth is about 16.4 degrees.  On a small 55cm dish the two LNBs are 2.65 inches apart.  The 76cm has a longer focal distance (about twice).  So I would guess the second LNB needs to be mounted about 5 inches from the first one (albeit at a loss in antenna gain).




Title: Re: Dual LNB with 76cm Weingard dish?
Post by USN - Retired on Oct 7th, 2007 at 4:32am
If there's a question  in there, I missed it. But distance is not the factor you need to consider. The dish will remain pointed at the original satellite, and the 2nd LNB will simply have to be offset. That is, aimed at the dish at an angle required to catch the 2nd signal on the first bounce.

Let's say Sat1 is at 88W and Sat2 is at 98W (10 degrees west).  Dish doesn't get moved, so LNB1 is still pointed at 88W. You want LNB2 mounted on  the east side of LNB1 - how far away doesn't really matter - but it has to be offset by that same 10 degrees. So the signal coming in from 98W will come in from the west -  bounce once - and LNB2 will be there on the east side, ready to catch it.

And yes, you'll lose some signal strength because of not taking full advantage of the parabolic gain. But if you're willing to spend a few extra bucks, there are better quality LNBs that can make up the difference.


Title: Re: Dual LNB with 76cm Weingard dish?
Post by techtom on Oct 7th, 2007 at 9:24am
The dish is pointed at the 110W (LNB1).   I'm trying to figure out where I need to position LNB2 relative to LNB1 to receive the 119W sat.  Using a multi LNB bracket, LNB2 is placed a few inches right(or left) of the main lnb.  LNB2 will also be at a slightly different height wrt LNB1.

What I would like to know is, given a 76cm dish, with the primary LNB pointed at 119W.  At what offset distance (wrt lnb1) do I mount the second lnb to pickup the 110W sat?

I'm using an LNB bracket like this one:

With the above bracket, I need at least two numbers to mount LNB2 for a given satellite: 1) the distance to the right/left of LNB1 and  2) the height above/below LNB1.   The third number is a small tilt angle for LNB2.

I can just take a good guess at the initial position of LNB2 and adjust the bracket by trial and error,  but it would be nice to calculate the aproximate LNB2 position on the bracket and then "peak" for max signal on LNB2 manually.


Title: Re: Dual LNB with 76cm Weingard dish?
Post by Eric Johnston on Oct 7th, 2007 at 11:54am
Calculate the azimuth, elevation and polarisation angles for both satellites using:  Write down the answers.

Work out the angle differences for the elevation (up/down) and azimuth(sideways)

Measure the approx focal length of your dish by measuring the distance from the lower edge of the dish to the feed.  (assumes an offset fed type dish with a feed arm from the lower edge, NOT an axi-symmetric circular dish with the feed in the centre, held by three equal length struts.)

Work out the sideways and up down feed distances to set for the additional feed by doing Focal length x tan(difference angle).

For the tan calculation do Start, All programs, accessories, Calculator and select scientific mode.  Test using 12 degrees; result should be tan(12 deg) = 0.21

The above assumes you will keep the dish reflector and primary feed fixed and unchanged.   The performance of the primary feed will be unchanged.  The performance of the additional, sideways positioned feed will be low, particularly if the sideways angle exceeds 8 degrees.  A compromise might be to locate both feeds, say 4 deg,  either side of the perfect focal point.

This page is worth a look:

Best regards, Eric.

Title: Re: Dual LNB with 76cm Weingard dish?
Post by USN - Retired on Oct 7th, 2007 at 12:00pm
So - you bought this thing and it didn't come with installation instructions? If so, perhaps you missed this link on the eBay page too:

This too will illustrate that distance is not the primary consideration. It's offset angle. The dish is currently pointed at 110W. You want to receive signals from another satellite at 119W without moving the dish. Mount the offset bracket on the east side of the feedarm. Install LNB2 on it.

If it's going to the the only LNB on that arm, the distance between LNB1 and LNB2 is not important. Personally I'd consider weight distribution on the feedarm, and mount LNB2 as close to LNB1 as possible - just far enough away so they're not touching at the end of the job.

Find the parabolic center of the dish with two intersecting pieces of string; one top to bottom, the other side to side. Use a straightedge for LNB2 vertical alignment. That is, a line straight through the center of  the LNB2 feed window should intersect with the horizontal string on the dish face. Then set LNB2 horizontal alignment by rotating it until the straightedge points where the two strings intersect in the center of the dish face.

Remove the strings, put away the straightedge, connect a satellite signal meter. Fine tune LNB2 on the horizontal and vertical to obtain signal peaks. Tighten the bolts. Remove signal meter, connect LNB2 to receiver.

Suggestion; just in case your dish has moved slightly over the years, it might be a good idea to use the signal meter to optimize the LNB1 pointing angles after first installing this new offset bracket and before fine tuning the LNB2 angles.


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