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Setting up a multi-feed satellite dish

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2 What Satellite & Digital TV

Setting up a multi-feed satellite dishYou don’t need a motor to get more than one satellite with a single dish. Find out how to make a multi-feed upgrade

To receive the signals from two (or more) satellites with a single dish is a good way of getting

around planning, landlord, or practical constraints of multiple dishes, and it’s neater and cheaper too.

However, specifying and installing such a ‘multi-feed’ dish is a bit trickier than a standard single-LNB fixed dish.

In most multi-feed systems, the primary LNB (for the main satellite) is at the end of the feed arm, as normal, while one or more secondary LNBs (for the additional satellites) are held on an adjustable bracket to one or other side. Brackets are designed for a particular dish or with a ‘universal’ fitting.

It’s worth noting that a multi-feed dish is not the same as a twin (or quad) LNB on a single dish (as used for Sky+, Freesat+, and so on). While a twin LNB has two identical outputs (and operates like two dishes aligned on the same satellite), a multi-feed dish will provide different outputs from different satellites.

How to choose which is the secondary satellite

A dish works best aimed straight at a satellite. It’s less efficient in gathering and focusing the signal from a satellite off to one side – the farther offset the LNB, the less efficient is the reception. A larger dish is required for offset reception with a secondary LNB than would be needed normally.

This means that in most circumstances the dish should be primarily aligned on the weakest satellite and a secondary LNB, using the dish less efficiently, is aimed at the stronger satellite.

To receive signals from several satellites, sometimes you will need to aim the dish at one towards the centre of the group (even if that is not the weakest satellite) so the outer LNBs are not too far from the primary focus of the dish. This may mean using a larger dish, as the weakest signals are now not being received at the peak efficiency.

How to add secondary satellite reception to a dish

You can only add a second LNB to an existing dish if the dish is already pointed at a weaker satellite. Even then, if the satellite spacing is too great then the loss

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of efficiency may be larger than the difference in signal strength, and you will need to replace the dish with a larger antenna aligned on the weaker satellite.

You cannot, for example, add a secondary LNB to a Sky minidish – both because this is physically difficult and because in the UK the Astra 28.2°E satellites are the strongest around, so any other satellite is weaker and requires a bigger dish to which should be primarily aligned on it.

How to estimate the dish size for a multi-feed system

There is no simple way to calculate the dish size for a secondary offset LNB to successfully receive a second satellite. But for normal setups we can use a rule of thumb – you need to increase the primary reception dish size by about 20 per cent for each 5° of separation of the secondary satellite from the primary satellite. In practice, dishes are only easily available in a few key sizes and you should veer towards buying a larger dish.

For example, a dish in London to see Astra 19.2°E normally needs a 60cm dish, and Astra 28.2°E a 45cm dish. So a multi-feed dish should point at 19.2°E and be sized for 28.2°E – which is 9° away from

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ABC GuiDe To

Align a multi-feed dish on the stronger of two satellites and the offset efficiency reduction for the weaker satellite can make it unreceivable (left) but if the dish is aligned on the weaker satellite the stronger one can still be received (right)

Install LNB at primary focus and align whole dish for maximum signal from primary (weakest) satellite

Fix secondary LNB holder roughly in position

Install secondary LNB to receive secondary satellite

Align secondary LNB only (offset distance and height) for maximum signal from secondary satellite

What Satellite & Digital TV 3

ABC guide to...

The farther an offset LNB is from the primary focus, the greater the reduction in reception efficiency

19.2°E and so it needs a 45cm+40%=63cm dish. In practice, you can use a 60cm dish.

But in, say, South Wales – at the edge of the 45cm zone for Astra 28.2°E and with a considerably weaker signal from 19.2°E – you should probably go for the next size up – 80cm.

If in London, you want Hot Bird as well (normally about 60cm) you also need to move up to an 80cm dish whatever the LNB arrangement – a 13°E offset LNB requires a boost of the dish above 60cm, and aligning the dish on 13°E requires a boost to 80cm size for the farther-offset 19.2°E and 28.2°E LNBs.

How to align a multi-feed dishAfter mounting the dish in position,

first align the primary LNB on its satellite in the same way as any fixed dish is aimed (finding first the approximate position for the correct satellite and then homing in on the accurate alignment using a meter or receiver’s display) and adjust the skew to suit. Tighten up all the bolts to fix the dish in this position.

If you are adding a secondary LNB to an existing dish, then this first stage is already done for you.

Now fix the multi-feed bracket to the dish and mount the secondary LNB. Remember that the dish is a mirror, so everything is back to front – facing the dish, a secondary satellite east of the primary one is over your right shoulder but the secondary LNB should go to the left of the primary one. Similarly, if the secondary satellite is lower in the sky than the primary one, the secondary LNB should be raised a little (the bracket will include spacers for this, or it can be tilted).

As a rough guide, a satellite spacing of 6° will usually mean an LNB spacing of about 8cm for most dishes.

Adjust the position of the secondary LNB to ‘find’ the secondary satellite, in the same way as the dish is adjusted, a little at a time, watching the meter or receiver’s display. Do not move the dish itself or the primary satellite will be ‘lost’. When you have found the best distance from the primary LNB and the best height, adjust the skew and then tighten up the bracket fixings to keep the LNB in position.

You can now repeat the process for further secondary LNBs if required.

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glossary

BeamwidthThe aiming accuracy of the dish in degrees, at which it will still receive a signal at up to half the maximum available power. Doubling the size of the dish diameter halves the beamwidth, so a 60cm dish will still receive a half-power signal if it’s 3° off-target, but a 1.2m has a half-power beamwidth of 1.5°.

SidelobeLower-power reflection of a satellite adjacent to the target of the dish, occurring on the opposite side to its position in the sky. Sidelobes can be used to receive multiple satellites on a single dish, and some antennas have been designed to produce several strong sidelobes for this purpose.

When to use a monoblock LNBIf you want to pick up Astra 19.2°E

and Hot Bird 13°E, or Astra 19.2°E and Astra 23.5°E, then there are special double-input LNBs – called monoblock LNBs – available to simplify the process.

These have two feedhorns at the correct distance apart to receive the two satellites and contain a DiSEqC switch to select between them too.

They will only work on the right dish. For 19.2°/13° monoblocks, this is an 80cm

05dish with the ‘standard’ f/D (focal length/diameter) ratio of 0.60. This includes the vast majority of offset dishes that are sold in Europe.

They will usually be effective on 60cm dishes with f/D=0.6 too.

Monoblock LNBs for 19.2°E and 23.5°E are called Astra Duo LNBs. They also use f/D=0.6 dishes and are available in 60cm and 80cm types.

To the receiver, a monoblock appears just like two separate LNBs connected to a DiSEqC switch but it doesn’t need a special bracket and the only adjustment needed is a slight tilt to raise or lower the secondary feedhorn to the correct position.

Because you cannot adjust the skew for each feedhorn or the LNB spacing, for the location, monoblock LNBs are a compromise – but, nevertheless, they work well with the powerful satellites that they are designed for n Geoff Bains