06-07-2013, 09:50 AM
Thanks for your thoughts, guys.
I solved the problem this morning as I couldn't see much last night in the dark.
I hope this may be useful to others who may experience a similar problem.
Satellite channels are normally 'polarised'; some horizontal and other vertically to avoid interference between adjacent frequencies. Polarisation is perfomed within the LNB by software in the receiver changing the supply voltage down the coax cable. However, Sky HD boxes are made cheaply for a mass market aimed at the Sky package only, and as far as I know, this feature has been omitted. (I think) all the channels are transmitted with horizontal polarisation.
When I have set up 'proper' non-Sky satellite systems, there is an extra adjustment called 'skew' where the LNB can be physically rotated in it's clamp on the dish to fine tune the polarisation so that the V and H channels are separated as much as possible. This is not nnecessary with a Sky HD box and most LNBs are fixed vertically in the clamp so any rain runs down the cable and not into the LNB connector.
The hot weather had expanded the cheap, plastic LNB clamp and the LNB itself had rotated slightly under the weight of the cables. Had there been any vertically polarised channels, they would all have been lost. Only the weakest horizontal channels were lost on one half of the LNB only.
I rotated the LNB for the strongest signal strength using the on-screen Sky HD Box digital display tuned to the weakest channel (CBS Reality+1). All channels are now working perfectly.
The signal strength of the stronger channels will not increase above 90% and signal quality remains at 80% so it is a waste of time to try and get more (for me anyway).
Like many digital TV systems, you don't get a reduction in picture quality, just complete loss of picture when the signal drops below a certain threshold.
This experience leads me to believe that the usual signal strength meter is useless for setting up a dish and LNB properly. It cannot be channel-tuned so must pick up the whole spectrum which is why you get all-or-nothing when you use one of these devices.
In my experience, once you have aligned your dish, there are a couple of extra taskes to complete. One is to rotate the LNB back-and-forth for the strongest signal and then tighten the clamp only enough to hold it. The second is to slide the LNB backwards-and-forwards in the clamp for the strongest signal to find the focal point for that particular dish. Once both adjustments have been made, and no further signal improvement can be made, the clamp can be tightened a bit more to prevent the LNB from moving on a hot day.
So in conclusion, the recent hot weather had allowed the cheap and nasty plastic clamp to expand which had allowed the LNB to rotate and to lose the weakest signal.
Hope that helps.
Sorry to be long-winded, but installers charge stupid money to fix these faults which everyone can do themselves if they know where to look.
I solved the problem this morning as I couldn't see much last night in the dark.
I hope this may be useful to others who may experience a similar problem.
Satellite channels are normally 'polarised'; some horizontal and other vertically to avoid interference between adjacent frequencies. Polarisation is perfomed within the LNB by software in the receiver changing the supply voltage down the coax cable. However, Sky HD boxes are made cheaply for a mass market aimed at the Sky package only, and as far as I know, this feature has been omitted. (I think) all the channels are transmitted with horizontal polarisation.
When I have set up 'proper' non-Sky satellite systems, there is an extra adjustment called 'skew' where the LNB can be physically rotated in it's clamp on the dish to fine tune the polarisation so that the V and H channels are separated as much as possible. This is not nnecessary with a Sky HD box and most LNBs are fixed vertically in the clamp so any rain runs down the cable and not into the LNB connector.
The hot weather had expanded the cheap, plastic LNB clamp and the LNB itself had rotated slightly under the weight of the cables. Had there been any vertically polarised channels, they would all have been lost. Only the weakest horizontal channels were lost on one half of the LNB only.
I rotated the LNB for the strongest signal strength using the on-screen Sky HD Box digital display tuned to the weakest channel (CBS Reality+1). All channels are now working perfectly.
The signal strength of the stronger channels will not increase above 90% and signal quality remains at 80% so it is a waste of time to try and get more (for me anyway).
Like many digital TV systems, you don't get a reduction in picture quality, just complete loss of picture when the signal drops below a certain threshold.
This experience leads me to believe that the usual signal strength meter is useless for setting up a dish and LNB properly. It cannot be channel-tuned so must pick up the whole spectrum which is why you get all-or-nothing when you use one of these devices.
In my experience, once you have aligned your dish, there are a couple of extra taskes to complete. One is to rotate the LNB back-and-forth for the strongest signal and then tighten the clamp only enough to hold it. The second is to slide the LNB backwards-and-forwards in the clamp for the strongest signal to find the focal point for that particular dish. Once both adjustments have been made, and no further signal improvement can be made, the clamp can be tightened a bit more to prevent the LNB from moving on a hot day.
So in conclusion, the recent hot weather had allowed the cheap and nasty plastic clamp to expand which had allowed the LNB to rotate and to lose the weakest signal.
Hope that helps.
Sorry to be long-winded, but installers charge stupid money to fix these faults which everyone can do themselves if they know where to look.
