Sky HD and Splitter problems

RoryMitch

Active Member
At my parents house, sky is currently installed with the shotgun coax coming straight from the satellite, through a hole in the living room wall and into the sky HD box.
They are currently waiting for a new wooden floor to be installed and we saw it as a good opportunity to put a faceplate in, as well as putting a separate faceplate at a different location in the room, so that is they fancy moving the TV, the sky is already installed there and won't require cables trailing across the skirting.

After researching it, my solution was as follows
Shotgun coax -> two splitters (Labtec FBS402/s)
From each splitter, one output going to point A and one to point B (I understand there are implications with splitters and channel switching, but there will only ever be a box connected to either A or B, never both)
Each of A and B then terminated to a faceplate.

Now my problems are as follows... Sorry if it's confusing, its confusing enough in real life.

We have two feeds, lets call them A and B. If A alone is connected to the box, it works (in input 1 or 2) If B is also connected, it shows as good signal on the signal viewer in settings. However B alone does not work.

If we have one sat feed connected directly to the box and the other through a splitter, then a picture shows and both feeds show as good signal. This applies with either A or B directly connected.
However, if we put both through splitters then it doesn't work.

This completely baffles me, anyone got any ideas? As a test I connected shotgun lead A alone to port 1 of the box (and nothing to port 2), then tried each of the ports on the LNB and it had signal and showed a picture. However doing the same with B and no signal.

I have a feeling I may have 2 separate problems here, but any help would be much appreciated.

rory
 

logiciel

Moderator
Is a broadband splitter the right kind?!
 

grahamlthompson

Distinguished Member
You need to replace the splitter with a switch like this one.

F Type Satellite Cable A-B Switch FREE P&P - UK SELLER | eBay

The switch will connect only one box at any one time to the lnb.

A lnb has to be controlled by it's connected tuner by sending back both DC and AC signals from the tuner to the lnb. Most Sky boxes even in sby power the lnb so either only one output leg of the splitter passes back these signals or the unused Sky box is interfering with the control signals from the box that is on.

In general you cannot split a lnb feed unless you really understand the limitations.
 

RoryMitch

Active Member
You need to replace the splitter with a switch like this one.

F Type Satellite Cable A-B Switch FREE P&P - UK SELLER | eBay

The switch will connect only one box at any one time to the lnb.

A lnb has to be controlled by it's connected tuner by sending back both DC and AC signals from the tuner to the lnb. Most Sky boxes even in sby power the lnb so either only one output leg of the splitter passes back these signals or the unused Sky box is interfering with the control signals from the box that is on.

In general you cannot split a lnb feed unless you really understand the limitations.
Thanks for the help Graham, I understood that having two boxes connected to the splitter would confuse matters, but during testing I have been simply connecting a single output to from each splitter to the box. The other output on each splitter has had nothing connected to it. That's why I was a bit confused by the strange behaviour.
 

grahamlthompson

Distinguished Member
Thanks for the help Graham, I understood that having two boxes connected to the splitter would confuse matters, but during testing I have been simply connecting a single output to from each splitter to the box. The other output on each splitter has had nothing connected to it.
In that case the splitter is unable to properly pass control signals back to the lnb or the splitter losses at the higher frequency used on satellite is simply reducing the signal level to a point the tuner cannot lock on to the signal.

The splitter will reduce the signal by more than 50% even with nothing connected to it. At the frequencies used by satellite you should terminate the unused output with a 75ohm terminator or simply leave it connected to an unpowered box.


http://www.play.com/Electronics/Ele...3417|rnd:4317706136940390445|dvc:c|adp:1o4|mt:

Basically any impedance mismatch will reflect signals backwards and can affect the output from the splitter.

The topic is complex, this may help in explaining what happens.

Signal reflection - Wikipedia, the free encyclopedia

This explains pretty well how a lnb works.

LNB mysteries explained - part 1
 
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davemurgatroyd2

Distinguished Member
The splitter you are using has a significant insertion loss of around 5.5 to 6 dB (which means your signal is being reduced to around a half) and a return loss of 10 to 12 dB (reducing signals voltages to less than a third). Splitters are the worst thing you could use in this situation - either use a switch as suggested above (which should have a lower loss) OR fit a wallplate with 4 f connector sockets (two connected to existing cable from dish and two to the other connection - then all that is required to switch location is to fit two patch cords between the inputs and outputs) which would have an insignificant loss.
 
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RoryMitch

Active Member
The patch cords solution is one I considered but discounted for aesthetic reasons. However, would back to back adapters work as they are the same principle? I.e have a junction box with the terminated end of the satellite feed, then a back to back connector on each cable, and then manually connect both cables from position A or position B as required?
 

grahamlthompson

Distinguished Member
The patch cords solution is one I considered but discounted for aesthetic reasons. However, would back to back adapters work as they are the same principle? I.e have a junction box with the terminated end of the satellite feed, then a back to back connector on each cable, and then manually connect both cables from position A or position B as required?
It will work (essentially a patch cord solution as suggested by DM) but the switch will be a lot more convenient and save multiple swapping of cabling.

A switch doesn't care which way you connect it.

You can use it to select a single input to either output, or in reverse dual inputs selectable to a single output.

Rather depends on how often you want to swap them over.
 

winston2010

Well-known Member
At my parents house, sky is currently installed with the shotgun coax coming straight from the satellite,
A lead 23,000 miles long would have an awful lot of loss.

Why not simply run 2 shotgun cables from the dish (not the satellite), one to each location? I assume you have a quad LNB.

When using a splitter with one leg with an unterminated cable there will be 100% reflections from the unterminated cable which will pass back through the splitter and interfere with the other output. The effect will be unpredictable and vary with frequency.
 

RoryMitch

Active Member
It will work (essentially a patch cord solution as suggested by DM) but the switch will be a lot more convenient and save multiple swapping of cabling.

A switch doesn't care which way you connect it.

You can use it to select a single input to either output, or in reverse dual inputs selectable to a single output.

Rather depends on how often you want to swap them over.
If they are ver swapped it will be once permanently, we are just putting the second one in as a 'just in case' we want to move the TV.

A lead 23,000 miles long would have an awful lot of loss.

Why not simply run 2 shotgun cables from the dish (not the satellite), one to each location? I assume you have a quad LNB.

When using a splitter with one leg with an unterminated cable there will be 100% reflections from the unterminated cable which will pass back through the splitter and interfere with the other output. The effect will be unpredictable and vary with frequency.
Yeah, I think I might end up having to do this. The reason I hadn't from the start is twofold; firstly because it uses the remaining two ports on the LNB, which sits outside my bedroom and I quite fancied sky in my room, secondly because the second point is about 8 feet from the first.

Having done what I thought was adequate research (but I now know clearly wasn't!) this has become a right pain. Just had another mess around with it, and having one of the leads from the dish connected to the box direct, and the other through a female to female and a flylead works. Good signal on both inputs. Using a female to female and a flylead on each of the leads from the dish and I get no signal on either input. I can't fathom an explanation... Do I need a specific female to female connector? I found out to my peril earlier that I needed specific F connectors, after being confused for 2 hours as to why the feed was now broken...

Again, many thanks to all for your help
 

logiciel

Moderator
Just to repeat your link was to a "broadband splitter".
I don't get the point about connectors being different either.
 

RoryMitch

Active Member
It's on the screwfix website as a VHF/UHF splitter so I thought it would be ok.


Sent from my iPhone using AVForums
 

grahamlthompson

Distinguished Member
It's on the screwfix website as a VHF/UHF splitter so I thought it would be ok.


Sent from my iPhone using AVForums
Satellite IF is above UHF in frequency.
 

logiciel

Moderator
The link is to Labgear and to a broadband splitter.
 

MartinPickering

Well-known Member
You need to replace the splitter with a switch like this one.
I think I've mentioned previously: that type of switch was designed for cable TV UHF signals. Its losses at LNB frequencies are quite high - maybe worse than a splitter.

However, any form of switch or unterminated splitter can cause weird problems such as "one multiplex missing" - just the same as with a kinked or damaged coaxial cable.
Described here: What if my coaxial cable is kinked?
 

grahamlthompson

Distinguished Member
I think I've mentioned previously: that type of switch was designed for cable TV UHF signals. Its losses at LNB frequencies are quite high - maybe worse than a splitter.

However, any form of switch or unterminated splitter can cause weird problems such as "one multiplex missing" - just the same as with a kinked or damaged coaxial cable.
Described here: What if my coaxial cable is kinked?
I used one as a test way back when the Foxsat-hdr wouldn't upgrade it's firmware using only 1 cable. You had to initiate the update with the cable connected to tuner 1 in and then swap to tuner 1 in. The switch allowed this without altering live connections. It had no noticeable effect on signal strength or quality.
 

MartinPickering

Well-known Member
It will vary from installation to installation, dependent on cable length and characteristics. You need to measure signal strength and quality for every available transponder frequency in order to make a sensible comparison. It may have no noticeable effect on reception - or it may wipe out one or more transponders. You can never be certain without trying it and that's why I don't recommend LNB switches or splitters for a permanent installation.
 

grahamlthompson

Distinguished Member
Found the spec - insertion loss is < 1.5dB at 1750MHz. Roughly equivalent to 6M of WF100 cable.
 

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