Offset LNB & Monobloc LNb Guide - Pdf

satelliteman

Quick theory on multi-bracket LNb offset arrangement & monobloc LNb on a fixed dish.

SM

Attached Files

Offset multi-bracket - SM.pdf (26.6 KB, 1506 views)

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vjkumar

hello

Nice to see the article.Can you give the formula to calculate the distance at which the Lnb's to be located,this will give bit easiness on mounting.

Vjkuamr

Robbo

It depends on the focal length of the dish and the position of the offset satellite relative to the focal one.

As a rough guide, this is what I use, and then adjust accurately with a meter.

Let L= dish focal length
Let a = azimuth angular difference between offset sat and focal one.
Let d = spacing of LNB (from centres)

Equation:-

d= L x tan(a)

N.B, if the focal length is unkown, just measure the distance between the LNB and the centre of the dish as a rough guide.

This method will aslo work for obtaining a rough estimate of the height difference required also, but this is not as difficult, as it will normally be a lot smaller.


Robbo

routerunner

Hi,

very interesting indeed...but I have a question, is it "a" the Azimuth distance relative to the two satellites or to the dish location? Example lets say I want main focus on 13E and offset to 28.2E, then the relative distance will be 15.2 degrees, but from my physical location which depends on my LONG/LAT the distance will be 18 degrees, do you agree?

cheers

Channel Hopper

The angle depends on more than just a couple of points derived from your position. All satellites are in a geostationary orbit which is measured from the centre of the earth (give or take a hundred miles or so). This is somewhat more than the distance of the observer (and their dish) which is some 3700 miles closer to the geostationary arc.


The main variable is the changing distance apart of the two satellites from each other, if they are close then the distance of the two LNBs will be roughly equal (3 to 9 degrees for example), but if the distance is as far as say both horizons (almost 180 degrees to the observer) the satellites are still no more than 140 degrees apart.

The other main variable is the distance from the equator of the observer, moving further north or south reduces down the difference between the two angles, to a maximum of 10% of the original when at the two poles.

There is a very fine line between "hobby" and "mental illness"

Robbo

Possibly yes, but it doesn't matter really, as you fiddle with the offset LNB position to get the best signal. Whether it's 15 degrees or 18 degrees, they are both in the same ballpark.

routerunner

well, I wish it could be that simple ;-)

In my case that 3º differences, made a huge impact on my reception capabilities. It probably depends on my dish as well, I reckon that a Triax with its multiclamp does a better job as the clamp itself is evenly curved so that LNBs at the right or left position relative to the main one are always at the best possible focus position, whilst mine which is a straight bracket need to be curved toward the dish to achieve best results limiting the optimal reception capabilities to just one LNB

cheers

JUVE

hi just looked at the "Quick theory on multi-bracket LNb offset arrangement on a fixed dish". Which of the 2 lnb's is fixed by the bracket and is this the way it would be if the lnb was recieve hotbird?

Robbo

In the diagram, the dish is foccussed on 19.2E, with the LNB for that on the central clamp.

The offset LNB is to the left and is to receive Hotbird, which is to the west of 19.2E.

If you want Hotbird and Astra, set it up as per the picture.

Satcom1

I have set the primary lnbf positioned (By SG2100) to a FTA bird and used Lyngsat to determine the degrees offset (say 6 degrees) for my first offset lnbf. Then I place a protractor at the "bore Site" of the center of the dish. With a string from the protractor I run a string to minus 6 degrees and measure on the bracket where the lnbf should be. If I have a motor, why do this? Cause it saves the motor and tunes the new bird faster.

If your satellite is 6 degrees west of your primary then you want the lnbf offset to be six degrees east.

In my case I'm able to use three positions for six birds.

I am not a complete idiot, some parts are missing.