Why can't you split a satellite signal cable?

March 26, 2017
by
Tom Smart
Picture of a satellite dish on brickwork

Why can’t you split a satellite cable?

 

So here’s a question I get asked frequently. The question arises when a client or customer either wants to upgrade their satellite receiver for a recordable version like Freesat+ or Sky+ which requires two dedicated satellite cables to use this function or would like an extra satellite cable in a nearby room for an extra TV.

 

Why can’t I just split the satellite cable that is already there like you would do with an aerial?

 

Well to begin let me confuse the situation slightly and say that you can. You will need a DC passing splitter that will allow the voltage to flow up one output of the splitter towards the satellite dish and block the voltage flowing up the other output.

 

The problem with doing it this way is that you will have one of your satellite boxes or tuners acting as a master and the other behaving like a slave, please read through this blog and this will make sense.

 

Another answer and a method relatively overlooked here in the UK, this could be done would be using a Smart Splitter and an SCR (Single Cable Router) compatible LNB or satellite multi-switch amplifier and SCR enables satellite receiver, the majority of new Sky+HD boxes come equipped with this facility accessed within the hidden installer menu. The problem with doing it this way is that you would either have to replace your LNB for a model not commonly used here in the UK and install a Smart Splitter, costs are adding up here already and for communal TV systems  you would have to replace the TV multi-switch amplifiers for models which allow the SCR function. Although this may be viable for upgrading single feed communal TV systems it would not be usually be viable for domestic situations. Either way although this method would work, it still wouldn’t be as simple as just “splitting the cable”.

 

I will stop waffling on and let you know why you can’t just split the satellite cable!

 

Because of the huge amount of services available on satellite, I know it doesn’t seem this way these days with an internet connection, but satellite has always been capable of delivering far more services than a standard terrestrial signal. The downlead cannot supply all of the channels from the satellite dish at any one time. Depending on which channel you pick, the receiver will send commands up the LNB (low noise block) to alter between a quarter of the channels at any one time. Each of the four quarters are called Vertical Low, Vertical High, Horizontal Low, Horizontal High and the receiver altering the supply voltage between 13V and 18V will change the LNB request from Vertical to Horizontal and a 22KHz tone will alter between a high and low band.

 

It is because of this that the first answer to the question above that you would have to have a master and slave box. The master would send the commands to the LNB as to which polarisation and band that the LNB would be set to and the slave would then have a choice of a quarter of the TV channels you could view, when you would try to access a channel not on that polarisation and band you would just get a “no signal” as the LNB would not be able to receive the commands that allows it to change channels. Using this set up you would need to change the master receiver to a channel on that polarity and band for the second receiver to be able to receive it and of course the master receiver would need to be on at all times for the slave receiver to get any signal at all. Hardly practical. Problems that would obviously have to know which channels to select on the master receiver for you to be able to pick the correct channels on the slave receiver. You could over time learn the channels to select or investigate yourself and this could and has been a solution for some of my customers there is no doubting that this would become a nuisance. Some receiver actually come with an ‘LNB LOOP’ connection that behaves in exactly this way. This connection is often mistaken for a secondary satellite connection but obviously behaves completely differently.

 

This is also why when you have a Sky box installed in ‘single feed mode’ and a single satellite cable connected that you can still view some of the channels when recording a programme but not most of them. So I’m sure some of you are curious as to:

 

Why does the LNB need to switch between the channels? Why can’t all the channels be available on the satellite cable at any one time?

 

1)     The horizontal and vertical channels polarised channels would interfere with each other.

2)     The bandwidth available. Satellite broadcasts are beamed down from the satellite in the KU band, between but exactly 10.5Ghz and 13Hz, this is then oscillated at the LNB to a lower frequency band that the coaxial cable can contain with far less bandwidth, the Intermediate frequency bad used is 950Mhz-2150Mhz which provides about half the bandwidth required.  If the LNB did not oscillate the signals the cable losses on the cable would be too great for the TV to work as the coaxial cable is not capable of carrying frequencies this high. Because of all of this brings the need for the high and low band which switch via a 22Khz tone provided by the satellite receiver.

 

So I’m sure at this point you can understand that when using a universal LNB which the vast majority of satellite dishes in the UK have already installed. It is far easier to just replace the LNB for a four (Quad) or eight (Octo) output version which allows each output cable to independently alter between the satellite polarisation and band and install extra cables between the satellite dish and the room where the extra satellite signal is required. This is starting to change is domestic situations with the introduction of Sky Q which uses a completely different type of LNB altogether. But I will save that for another blog.

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