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Polarisation Problems for analogue satellite TV

A common question is "I get only horizontal..." or "I get only vertically polarised channels".

Even worse is "I get only some channels".

You need to understand a few very simple concepts:

The LNB on your dish is a Low Noise Block downconverter. The reason for the name is this:

Low Noise because it must introduce very little electrical noise to the very weak signal which is captured by the dish and focussed on the LNB. If the LNB produces electrical noise then the signal will be blotted out. What is electrical noise? OK, imagine you have a guitar amplifier and you turn the volume right up without a guitar connected. You hear a hissing noise. That is the electrical noise which is produced by all the little transistors inside the amplifier. You can't get rid of it but you can reduce it by careful design so it does not swamp out the sound of the guitar.

Well, at very high frequencies (so called "microwave" frequencies) it is quite difficult to reduce the electrical noise. Even a warm brick wall produces "noise". The LNB is designed to produce bery little noise.

Why "Block downconverter"? Well, the LNB captures the microwave signals from ALL the available satellite channels then converts them in one single block of frequencies to a lower block of frequencies that will travel down the coaxial cable. (Microwaves won't go down cable - only down pipes - which makes plumbing a bit difficult!)

The LNB also amplifies all the frequencies to compensate for losses in the coaxial cable. The amount of amplification is called the "gain" of the LNB. The amount of noise it produces is called - yes - the "noise figure". Easy huh? So back to polarisation....

In order to squeeze as many channels as possible into a limited frequency block (or bandwidth) the satellite transmits alternate channels in alternate polarisations. Think about your polaroid sunglasses for a moment: If you look at your car clock while wearing polarised lenses, you can't read the clock digits if you tilt your head. This is because your lenses and the filter on the clock are of opposite polarisation when you tilt your head. When the polarisations are 90 degrees different, you can't see the clock digits at all.

This is how LNB polarisation works. A satellite channel is transmitted with (say) vertical polarisation by means of a polarising "filter" on the satellite in the sky. The LNB can receive the signal perfectly if it is mounted on the dish at a certain position. However, if you rotate the LNB through 90 degrees, it receives almost no signal at all. So the first thing we realise is that there is an optimum "skew" or rotational position for the LNB.

Now, because of polarisation, we can transmit a horizontally polarised channel on a frequency very close to that of a vertically polarised signal without one affecting the other. We twist the LNB one way to receive one channel and twist it through 90 degrees to receive the other. In practise, of course, we do not want to climb a ladder to twist the LNB every time we change channel! So we need some way to switch polarisation remotely.

The old fashioned method is to put a tiny stub of wire inside the LNB feed horn. This wire can be rotated by a motor. The wire acts as a "filter" that twists the incoming signal until it matches the polarisation of the LNB. This type of control is infinitely variable - it doesn't matter how the LNB is mounted or how the incoming signal is polarised - the tiny stub can be rotated until the LNB can "see" the signal.

Another method is to wind a coil of wire around the LNB feed horn. This is called a "magnetic polariser". When an electrical current is passed through this coil of wire, it creates a magnetic field. The magnetic field can twist the incoming signal to change its polarisation inside the feed horn. The current can be varied to adjust the strength of the magnetic field and, therefore, the amount of "twist".

The method we use most now is to design the LNB with two tiny aerials inside - one at 90 degrees to the other. Once the LNB is mounted correctly, one aerial can "see" horizontally polarised signals and the other can "see" vertically polarised signals. We can switch between the two internal aerials by means of the voltage that the receiver sends up the coaxial cable to power the LNB. The convention is to use 13 volts for vertical and 17 volts for horizontal polarisation. So every modern satellite receiver can supply either 13 volts or 17 volts in order to select the correct polarisation for each channel. Something goes wrong, here, you will receive channels of only one polarisation.

What can go wrong?

The LNB can develop a fault. Bear in mind that it sits there exposed to rain, snow or hot sun all year round. This combination doesn't do your wood fence any good and it doesn't do much for an LNB !

The cable can develop a fault. A bad connection caused by corrosion can reduce the voltage that reaches the LNB so it can never get 17 volts.

The receiver can develop a fault either in the power supply or in the control circuit that "tells" the power supply what voltage to send up the cable.

What do I do?

You need to carry out tests. Measure the voltage that comes out of the receiver cable connector. If it measures approximately 13 volts for one channel and 17 volts for another, you can be reasonably sure that the receiver is OK. You can double-check by connecting your receiver to a neighbour's dish cable or by borrowing another receiver to connect to your dish cable.

If the receiver is OK, you can connect a new dish cable alongside your existing one to see if that cures the problem. If it doesn't then the LNB is probably faulty.

See how easy it is to figure it out for yourself, once you undersand what is happening!

Now I want to explain a couple more things about LNBs while you are in the mood:

Firstly, I mentioned that the rotational position or "skew" of the LNB is important so it can properly and accurately "see" horizontal and vertical polarisation.

Well, the other important thing is its distance from the dish. The dish is collecting the weak signal from the satellite and focussing it onto the LNB horn. It can do this only if the LNB is positioned accurately at the focal point. Get this wrong and you could get "sparklies" the first time a rain cloud decreases the signal.


I mentioned frequency bands at the beginning. I have to assume that you understand basically what frequency is all about and can picture in your mind that several frequency bands exist ranging all the way from audio frequencies (what you hear) to visual frequencies (what you see). In between are radio frequencies with "long wave" at the bottom and "microwave" near the top. The microwave band itself is split into a number of smaller bands, each of which was originally used by a different satellite.

An LNB has only a limited frequency bandwidth capability - in other words, it can "see" frequencies from a limited number of satellites. If we want to receiver transmissions from other satellites, we need a different LNB - or do we?

Well, we can get a "dual-band" LNB that can switch between bands. The modern version of this is called a "Universal LNB" and can switch between two bands. You can get more information from the "LNB FAQ" but, briefly, it works like this:

The LNB receives a band of frequencies and can alter polarisation by using 13 or 17 volts.

If the receiver sends a 22kHz tone up the cable the LNB switches to a higher band but can still alter polarisation by detecting 13 or 17 volts. It's as simple as that. You need a Universal LNB and a method of supplying either 13 volts or 17 volts and either a 22kHz tone or no tone. Most modern receivers can do this. Some older ones need add-on boxes.

If you don't know whether your LNB is "standard 10.0" or "enhanced 9.75" or "universal 9.75/10.6" then find out and come back! If you don't know if your receiver can handle a universal LNB, find out. While you are at it, check to see if your car should use leaded or unleaded fuel!


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Version 1.1 updated on 3/5/99
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