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Satellite dish pointing using a multi-LNB dish
A dish and feed array designed to receive two or three spaced apart satellites will have two or three feeds, each with an LNB. The feeds need to be spaced apart to suit the satellite azimuth and elevation pointing angle differences.
The dish acts as a mirror and forms an inverted and reversed image of the many satellites in a curved line in the focal region. If you are looking due south the satellites in the sky form a curved line with the highest satellite at the top. In such a case you would set your central feed aimed at the due south satellite. The outer feeds would be slightly higher up. The left feed would work to the right satellite and vice-versa.
Many dishes have pre-made fixed LNB spacings. This assumes that the dishes will be used in the US and for the reception of satellites spaced say 9 deg apart. The entire antenna (dish and LNBs) may be rotated to set the polarisation/skew angle. Optimise this for the central satellite.
General purpose multi-LNB antennas may have perhaps 5 feeds. The central LNB/feed will be set first and then the outer ones individually. Each LNB/feed needs to be adjusted, up/down and sideways and in rotation also to set the polarisation angle.
This figure shows a view of the sky with the various satellites represented by the red dots. Note the top three satellites and the corresponding triple LNB feed array below.
This triple multi LNB dish feed needs to be tilted anticlockwise if used to receive from three satellites in the south east. If used for satellites due south then it needs to be used upright, as illustrated. If used for satellites towards the south west then the feed array should be tilted in a clockwise direction. Note that in this above design the distances between the feeds is fixed, so this unit is suitable for its intended dish focal length only. It is not suitable for use with dishes of longer or shorter focal lengths.
For the enthusiast it is possible to use a fully adjustable multi-LNB arrangement with perhaps as many and 5 LNB/feeds. In this case set the pointing for the central feed first and then adjust the outer feeds individually. Up/down and sideways movements are needed to find the satellites and then individual rotation to set the polarization/skew angles. This fully adjustable design is suitable for a dishes with a variety of sizes and focal lengths.
If you are using an elliptical shaped dish this guide about the orientation of elliptical dishes may be of interest.
If all the satellites are away sideways from south, for example in a generally south west direction, the the feeds will all be in a sloping line going down towards the south west.
As an example. You are on longitude 90 west and you want to point to satellites at 110 and 119 west. These satellites are further to the west than you are, so the satellites will be in a sloping line going downwards towards the south west. See the right hand half of the figure to the top right.
In some cases the individual LNBs may be rotated to individually to set the polarisation/skew angles correctly. In some cases the dual or triple feed assembly is not adjustable and also fixed relative to the dish and the entire dish and feed array must be rotated to set the average polarisation angle correctly. The tilt makes the feed array line up with the sloping line of satellites in the upper diagram.
Because the dish is a reflector the upper feed receives signals from the lower satellite and vice versa. The left feeds receives from the right satellite and vice-versa. The effective reflection point on the dish is the lower central edge of the dish in the case of offset fed designs, or the centre of the dish in the case of axi-symmetric circular dishes with the primary feed in the centre supported by three equal length struts.
When setting up a multi-LNB system it is possible to become confused as to which LNB is in use. I suggest initially wrapping all except one feed with aluminium foil. Then only one LNB will work. Line this up correctly and verify that it really does point at the wanted satellite. Check the list of satellite TV programmes and make sure that you have a unique identifying known satellite TV signal relating to a specific satellite. The Lyngsat list here https://www.lyngsat.com/america.html is recommended. Then leave the central LNB alone and the outer feeds may then be optimised by small adjustments.
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Page last amended 5 Dec 2014, 2 Mar 2022. |