About LNBs

What does "LNB" stand for?

It stands for "Low Noise Block-downconverter" because the function of the device is to convert a block (or band) of microwave frequencies from the satellite, down to a lower block of frequencies that will pass through coaxial cable without too much loss.

What is Noise Figure?

The graph shows a typical result of measuring the unwanted noise generated by two LNBs across their frequency range (horizontal axis).

The red line shows a "0.2dB minimum" LNB and the blue line shows a "0.4dB minimum" plot.

Bearing in mind that the ideal situation is to have a minimum amount of noise across the entire frequency band, which LNB would you prefer?

Every LNB is different because of manufacturing tolerances. Measuring LNB noise is time-consuming and can't be done for LNBs selling for just a few pounds each. Instead, the manufacturer will make a few checks at specific frequencies (not across the whole band) on a few random samples.

Without a full graph showing the actual measurements of the LNB that you buy, it's impossible to know what you've got. The manufacturer may state a "typical" or "minimum" noise figure but that's based on measurements of only a few random samples at a specific frequency.

In the end, you have to rely on results. If a lot of people say (for example) "the zzzzzz quad-output LNB works for me in a fringe reception area with a dish made by xxxx and a receiver made by yyyy" then that's probably the best information you are going to get.

(If you are not in a "fringe reception area", just buy the cheapest. It won't make ANY difference whatsoever to your picture quality or rain margin. the dish size is MUCH more important. If in doubt, buy a "Sky zone 2 minidish" or larger.)

You should also bear in mind that there are other important parameters:

GAIN

This is the amplification factor and, again, it varies across the frequency range, just like the noise factor does. The Gain determines how strong the signal is when it is sent down the cable. Ideally you want a high gain and a low noise factor but the two are mutually exclusive so you won't get it.

CROSS POLAR REJECTION

Satellite signals from the Astra satellites (and most others viewable in the UK) are transmitted in either vertical or horizontal polarisation. A "universal" LNB can switch between the two and this switching is controlled by the LNB voltage supply: 13v gets you vertical and 18v gets you horizontal polarisation. But signals of different polarisation can interfere with each other. The ability of an LNB to prevent this is called its "cross-polar rejection" (CPR) and is measured in deciBells.

CPR is important if you are in an area of fringe reception where one polarisation is much stronger than the other. If the LNB can't reject the unwanted signal, it will swamp the weaker one resulting in "No satellite signal".

BAND

A universal LNB can receive signals from two separate frequency bands - high and low. The associated satellite receiver "tells" the LNB to use High Band by sending a 22kHz tone up the cable. Without the tone, the LNB will revert to Low Band.

POLARISATION/SKEW

The satellite transmits signals which are horizontally polarised and vertically polarised, which minimises interference between adjacent frequencies. The receiver "tells" the LNB which polarisation to "look at" by supplying it with 13 volts (V) or 18 volts (H). In addition, the LNB rotational position ("skew") has to be set to the correct angle so that horizontal and vertical signals are received with equal strength. The best guide to the correct "skew" position on a Sky Digibox is the "signal quality" indicator. Rotate the LNB for the highest reading.

SPLITTING THE FEED

Because the LNB is looking for a steady 13 volt or 18 volt supply and a steady 22kHz tone, you can't "split the feed" to drive two receivers unless you appreciate that one receiver will always "win". The receiver supplying 18 volts and 22kHz tone will win over the other. That's fine if both programmes happen to be on (for example) High Band, Horizontal Polarisation but otherwise you'll get a conflict. The answer is to use an LNB with multiple outputs - usually a "quad-output" LNB.

You can get more information about LNBs on the Satcure site