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Dish on W3A 7°E but no service
Parameters correct, dish OK but no signal bars
Intro
When Eutelsat changed the Opensky transponder on W3A on7°E to 11.303GHz, some sites could simply enter the new parameters (11.303GHz, 27500 s/r) and receive a perfectly adequate signal and some could not. This was a puzzle. The puzzle deepened when we checked that an 80cm dish should be fine for the 48dbW footprint of the Europe A beam. And then we found a number of sites (including our own test sites) where the TV channels on the same satellite were OK but the new data transponder wasn't. How could this be?
Eutelsat and the company running the uplink/IP hub, Broadsat, kept saying there should be no change, the new transponder frequency was on the same beam so there should be no problem. But several of our customers just couldn't get any data lock (black signal bars on the BSM2/F10). At first, neither could we.
So we called in several industry experts here in the UK and in France and we think they have diagnosed it. If they're right, there is a solution for all our UK customers who currently have a problem.
The Problem
Eutelsat are running two transponders too close together with the wrong FEC. This situation means the data transponder on 11.303 GHz (11303000) is very prone to interference from the adjacent transponder. An analogue or simple digital meter set for 11.303 GHz will tell you / your installer there is a good signal strength and BER but your data receiver doesn't work. That's because the meter measures all the signal within its bandwidth - both right and the wrong transponder. The only way to spot if this is happening is with a digital signal meter with a spectrum analyser.
| Both these spectra were taken by Roger Miles on a Promax meter on the same 80cm dish with Universal LNB at the same site within 5 minutes of each other. The one on the left is aligned very slightly to the East compared to the one on the right. Only the alignment is different - and that only by, as Humph would say, a gnat's crotchet, | |
Data works fine |
Data doesn't work |
 |
 |
TV Channels on 7°E received fine |
TV Channels on 7°E received fine |
Signal strength of 70.2 less than maximum of 74.7 |
Signal strength peaked at 74.7dBmicroV BER 1 x 10e3 = marginal C/N 8dB = marginal |
Both signal strengths are within CAI Code of Practice limits of 52 - 77dB - they both "should" work. But the one on the left has OK values for BER and carrier/noise while the one on the right doesn't. What's the difference? Look at the "troughs" (white dotted lines) left and right of the centre frequency (red dotted line). |
|
Trough OK to the left |
almost no trough on the left |
receiver can tell the difference between transponder on the left and the one we want |
receiver can't tell the difference between the transponder on the left and the one we want |
Without a spectrum analyser like this, the alignment on the right looks stronger but just won't work for data. The offset alignment on the left actually works fine for both data and TV but has a lower overall signal strength.
The Solution
The dish will need to be aligned accurately to get the troughs as well defined as possible. This is done by fine pointing the dish (in our case, a very small amount to the East) while checking the spectrum. The first time, it was done by guesswork and then Roger confirmed whar was happaning with his meter.
So you need a qualified and experienced installer who has a meter with spectrum analyser and who understands the above. Or Eutelsat could run their parameters properly. You can see good standard troughs (minima) to the right of "our" transponder. If they ran their satellite to this spec (like every other transponder we have ever used since we started with them in 1997) the problem would not be happening.
Further Information - Sun 29 June 08
Our technical experts and installers have been testing and rectifying at sites in the UK and abroad from Europe to Pakistan. Some sites are receiving fine on a Zone 2 (60cm equivalent) and some don't get good signal with 80cm. It seems that the type of dish may also be a factor. So far, the solid Triax 78d seems to give best results without having to repoint. The transponders are well-defined. There shouldn't, in theory, be this much difference between a solid and a mesh - maybe it's something to do with the 3/4 FEC.
Thanks are Due
Kan - Technical Manager, Eurosat
Roger Miles - Satsearch
Andrew Slee - AVS Digital
Les Hampson - CAI and TV & Satellite
Paul Stephens - Euro Installations
Paul Roberts - Channel Aerials
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