signal strength v quality

FRAZER

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#1
I have been looking through some of the archived threads about the difference between signal strength and quality, and think I get the gist .

But I'm still not 100% sure what the most likely reason is that on some satellites the signal quality I'm achieving is considerably lower than on others (and occasionally too low for a picture), whereas the signal strength is pretty consistent across the board.

Is the root of this discrepency more likely to lie in my set up (or any partially obstructed view to certain satellites), or in the quality of the signal from source?

Also, having now acquired an analogue receiver to pick up the French channels on 5 West, I'm assuming that the best way to connect my three receivers is by using a twin lnb, with one output connected to my Nokia controlling the motor, and the other to a priority switch connecting the analogue and the Sky digibox.

If this is right, though, would a twin lnb further compromise the quality of the signal I'm getting?

Thanks for any ideas,

Frazer
 

rolfw

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#2
Hi Frazer,

There can be several factors affecting signal quality, partial obstruction as you've mentioned is an obvious one, incorrect alignment is another, as is LNB skew. It would be interesting to know which satellites on your arc are affected, but also bear in mind that inbuilt receiver strength meters are notiorously inaccurate.

With regard to the LNB, the multiple outlet types tend to have a higher noise figure than single outputs, but if you are going that route, you may even consider a quad, then you wouldn't need the smart priority switch.
 

Old Satellite

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#3
Additionally form Rolf's reply

With respect to both the signal strength and Quality, this is very dependent also upon the FEC rate ( field error correction) if you are attempting to receive a channel that has a listing of FEC 7/8 then your dish size would be required to be larger than that for a FEC rate of 1/2
The most commonly used is a FEC of 3/4 if you are attempting to receive a transponder with channels that have a listing of 7/8 for example then your dish size would need to increase proportionally to that of the change in FEC rate to maintain the correct quality of re- encoding of the channel.

LNB noise at specific areas of the KU band spectrum can also additionally create problems when trying to receive digital signals.

While many of the older DRO ( direct readout ) local oscillator based LNB,s were stable for general Digital signal reception, when the requirements are tougher - lower signal levels for example then the digital signal re-encoding starts to exhibit the famous moving block or artifact type problems.

Additionally many of the older - first generation digital receivers were also equipped with non to impressive tuner modules this also additional added to the re-encoding problems.

The simplest way to resolve your reception problems would be to first check on the signals that you would like to receive versus those that you do - what parameters are different, while also ensuring that your system is fully operationally and setup correctly.

Regards

Old Satellite
 

Channel Hopper

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#4
Signal Strength (as shown on a bar graph on most receiver displays) is more to do with the gain of the system plus the signal of the satellite. You will find that on most systems the signal rarely drops below 50% and never rises above 80%. Changing the LNB for a higher gain output unit will have some effect on the signal graph but wont make any difference to the eventual reception

Bit Error rates shown on domestic satellite receivers are similarly erroneous, as they are taken before the FEC is added to the equiation, though the higher this figure is (equating to a lower BER) the better the eventual reception

There is no substitute for a good dish (bigger the better), stable LNBs with proven track record, good connectors, and good quality low loss cable.
 

FRAZER

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#5
Thanks to the big hitters for such a speedy response, which has spurred me on to some more methodical research.

I have found that, whilst the maximum signal strength readings are pretty much the same across all satellites, at around 50% (which doesn't seem particularly high), the optimum signal quality varies from around 70% on some of the major european satellites (Sky, Astra, Hotbird), down to 30-40% on others, which means some of the weaker signals don't get through at all. Particularly bad are 7 East, 12.5 West and 15 West.

I have tried to find the strongest signal on each satellite for each polarization, which seems to mean one with a high symbol rate. Having said that, though, there is a particularly stubborn feed on 12.5 West I have never been able to pick up at all, even though there is another with a far lower symbol rate (3,260, as opposed to 20,150), on the same polarization which I just about can. Is there no rule to this, or is that likely to be a case of noise at certain parts of the spectrum as Old Satellite suggested?

Anyway, I also noticed that moving the dish slightly affected the quality readings, but not necessarily the strength. Is this likely to be due to the inaccuracy of these meter readings, or is signal strength actually less sensitve to correct alignment than signal quality? If this is true, does this apply to obstructions as well, which would seem on the face of it counter-intuitive- why should only noise be affected by an obstruction and not strength?

But if this is indeed true, it might suggest that the problem does lie with the (now completely bare) tree in front of my dish, which has already taken some pruning, and which I would happily cut down were it not a rented house! The fact that Sirius is weaker than I would expect, and this lies close to 7 east, as 12.5 and 15 west are close to each other too, may suggest that there are clusters of branches getting in the way. But this is not immediately obvious to the eye, and when I hung on a suspicious branch earlier on, it made no difference at all! So I would like to be sure this is a prime suspect before risking any more pruning.

Thanks again for the help,

Frazer
 

Channel Hopper

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#6
Best test (if the tree is not too large) is to tie a rope to the top branceh and pull to one side, see if the signal gets better.

Then give everyone here an idea of the system you are using (dish size, LNB cable run etc etc, ) and your location

That way we can all decide whether its hardware or environment
 

FRAZER

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#7
OK thanks. If the tree is really a prime suspect then I shall have a go at that, next time someone is around who can help me out- I live alone in the middle of nowhere, so that may have to wait a few days!

As I said before, I was merely surprised that the tree could possibly be affecting signal quality without affecting signal strength. And likewise, if the alignment issues can affect one without affecting the other, then I can give that a good tweaking if and when I change to a twin/quad lnb.

Actually, I don't think the reception I get is too bad considering I only have an 80cm (offset) dish in the south of Scotland. I even picked up a signal off the nordic beam on Sirius when I was testing earlier- although it had gone later. It's just the little anomalies which are a bit frustrating, and seem to be denying me a few things I would expect to get from checking out the coverage charts. Perhaps, though, I expect too much!

Frazer
 

Old Satellite

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#8
From your last reply - I would add that your dish is on the small side at your location for some of the channels that you would like to recieve.

What length of cable run do you have and how old is it ? what typoe of lnb do you have - the signal strength seams a liitle on the low side for several of the satellite's you identify - what reciever do you have?



Channel hopper is also right generally most of the standard readouts for both channel strength and qaulity are a referance use only and are not a real measurement for signal reception conditions at a location- if you were to test six recievers all on the same system and then record the results the likely hood of two of them even being simlar would be a supprise.

A test at any site location requires a professional C/N equipment

regards

Old Satellite
 

FRAZER

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#9
I agree that I'm actually not doing badly considering my location and equipment- by the way, I'm using a Nokia 9902S and Satscan motor, a solid 80cm offset dish, I don't know the make of the lnb, but it was advertised as a universal gold 0.6dB (it came as part of a package with the motor and dish), and about 15 metres or so of cable at a guess, all of which was bought less than a year ago.

But again it is the discrepencies which have confused me- from the coverage charts, for instance, I would have expected 7 East and 5 West to give a similar reception, yet (and I accept the probable inexactness of the readings), whilst the signal strength on 5 West is only very marginally superior, the quality is significantly better. And the signal quality on 15 West, which the charts suggest should be one of the better signals, is lower than both of them (and too low to receive some channels), although the strength is again only marginally lower.

Likewise, Sirius is giving a markedly lower quality signal than Thor from the same signal strength, although I might have expected the reverse to be true from the charts. And the same signal strength for Astra (19 degrees)- I'm talking about roughly 50% in all these cases- gives the clearest signal quality of all.

So it is these little contrdictions I'm trying to iron out, though maybe it wil turn out that they are not contradictions at all!

Thanks again for the advice,

Frazer
 

Old Satellite

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#10
Have you checked the alignment recently to ensure that the dish is tracking correctly - from your comments that 15.0 west is weaker ?

Additionally many of the footprints also are not entirely accurate, and many examples exist.

Example
I am able to recieve both analogue and also several digital channels on 78.5 east from the Thiacom 2 satellite from the Asia footprint - ( this is at a location in Belgium with a 3.7mtr and not in the UK the satellite is below the horizon in the Uk)
When you estimate the ERIP level; in europe they should not be recievable.

Additionally if you have easy access to your dish and have a friend that can help check out the skew adjustment ( by rotating the lnb slightly) to see if this also improves the reception.

regards

Old Satellite
 

FRAZER

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#11
Thank you. I am presuming that, as Pas 43 West, which is my western limit, is giving a signal quality not far short of Sky, my eastern limit, and Thor and 5 West somewhere in the middle are pretty good too, the tracking is OK. Would that make sense? It may be, of course, that they could all be somewhat stronger if things were adjusted a little, and I would certainly like to try this when I get the chance (my set up was though installed by an enthusiastic professional who spent a good time adjusting things-I think he was happy not to be doing yet another Sky dish!).

But if you are all concurring that my troublesome tree could be affecting the signal quality without affecting the signal strength, would I be right in supposing that that would be the most likely explanation of the varying signal quality across the satellites I am receiving, and the first idea to try and disprove?

Frazer
 

Old Satellite

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#12
yes first check that there are no obstructions or partly that are effecting the system- seems likely form your last report that you can recieve 43.0 west.

Regards

Old Satellite
 

FRAZER

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#13
Just to report back what I have managed to find out- unfortunately not very much! I've done a bit of judicious pruning to try and leave a good path for 12.5 west, but it made little or no difference to the quality of signal I'm receiving. Also I played with the lnb skew, and although this did improve the signal quality a little, it made a number of other satellites a lot worse. Finding a happy compromise put me right back where I started- as I said before my set up was professionally installed, so I guess he did a good job. The angle of the lnb seems to be fixed by the plastic moulding that holds it to the arm, so I presume the whole kit is designed so that the lnb points to the centre of the dish. I am a little loathed to mess with anything else now, as it's a very laborious business running to and from the TV after each minor adjustment, and I figure that maybe my installer has set me up to the maximum potential already.

One thing I have noticed, though, is that where the signal quality does vary massively amongst channels on the same satellite, such as those on 12.5 and 15 West, it is, as was suggested before, often those transponders which carry more than one channel that are causing the problem. For instance, on 12.5 W, the MBC Europe channel (11155 H, 2892, 3/4) gives a decent enough signal quality on a very poor signal strength, whereas the three sports feeds on 11014 H, 20150, 3/4 give a signal quality too low to capture despite having a much stronger signal strength. Similarly on 15 W, the BTV channel at 10989 V 4480 3/4 gives a lowish but watchable signal quality, whereas at 10996 V 7110 3/4, the 4 channels (part of the UPC package- including the sports channel which is FTA at present), are again too low to capture despite a similar signal strength.

All of which leads me to wonder whether my system is working pretty near to it's optimum, bar, perhaps, chopping down the tree completely, and it's just sod's law that the channels I'm interested in are the weaker ones. I'm sure that a slightly bigger dish would do the trick, but I'm not sure how popular that would be, so I may take a risk on a low noise lnb when I get round to changing to a twin, and hope that might give me the small boost I need. Has anyone heard anything good or otherwise about the 0.3 db Invacom lnbs which are easily and cheaply available in Germany now? I have read some sceptical posts on a German website, but based on the memory of previous spurious claims by other lnbs, as has been noted on this forum before, rather than experience of this particular brand. Could it perform any worse than a quality 0.6db model? (such as a twin MTI Blueline, which I believe has got a recommendation here before, and which I could get for 40 euros on German e-Bay) .

Anyway, thanks again for all the previous advice I've been given here,

Frazer
 

Old Satellite

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#14
Generally Lnb Noise reported on a majority of LNB's that can be purchased are Typical figures and has a result a majority will at some point though the recieving spectrum exceed the tyipcal value stated.

Many of the alledged 0.3 and 0.4 db lnbs will have a point around the mid band when the lmb noise will actually be that tyipcal specified value.

The additional 0.1 -0.2 db diifernace is in general, normally not worth the additional investment.

The only lnb which generally will produce an improvment are the Professional PLL ( phase locked loop) they normally meet the specified noise value through out the specified range.

The down side is is that they are much more expensive and require additional components to make use of them.

The best way remains is to increase the size of the dish and hence the recieving signal.

Regards

Old Satellite
 

FRAZER

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#15
Point taken, thank you. Strangely, the signal quality on the 12.5W feeds was just about strong enough today to receive a very broken picture for the first time, even though the weather in southern Scotland was awful, and the signal strength a little down on usual. I guess this may be related to some of the interesting points you made in another thread about fluctuating Astra reception, unless the gale force winds were blowing in more of the signal! (Also read in that thread about the articles on Invacom lnb's in this months What Satellite, so I'd better see if I can find a copy anywhere in the Borders).

Frazer
 
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