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Many installers have asked my advice about "sparklies" on Astra analogue pictures and, although not an installer myself, I've tried to advise as best I can.

Signal Too Weak

A weak signal can have many different causes. The most common are:

  • Heavy rainfall
  • Obstruction such as trees
  • Buckled or rusty dish
  • Cable fault (water inside, kinked or bad connection at LNB or a joint or connection in the cable)
  • Faulty LNB or water inside
  • very infrequently, a faulty tuner module inside receiver

The effects of rainfall and trees are all too obvious but they fool some people! If the picture is bad when it rains then a sparkly picture can result directly from the rain itself (because it reduces the signal coming from the satellite) or from water getting into the LNB or cable. The effect of trees is more obvious in summer when the leaves are present. The interference will vary as the branches move in the wind.

Dish faults are quite common. Even a brand new dish straight from the box can be twisted, bent or buckled. Even a slight amount of distortion is sufficient to move the focal point away from the LNB feed horn. You may be able to correct the distortion but the best answer is to change the dish.

To determine the exact cause of your problem, it's best to change each element, one by one. Try a different receiver first. The next, simplest step is to run a new length of cable alongside the old one. Connect it at each end and test the system. While you are there, check the old cable and LNB for corrosion at the connectors - a sure sign that water has crept inside. Occasionally an LNB can be dried out and will work perfectly again but, if it's difficult to reach the dish, my advice would be to renew the LNB. Be sure to use self-amalgamation tape to make a waterproof connection. It's also a good idea to fasten a polythene bag over the LNB, although this looks unsightly.

If you fit the new cable permanently, DO NOT KINK it by bending it tightly around a corner or by hammering a clip too hard.

Signal Too Strong

My advice has always been that to point a large dish feeding a low threshold receiver at Astra is just asking for trouble because the combined signal strength of all the transponders is likely to overload the tuner input of the receiver.
Recently, I was contacted by Bert Dahlstrom of Sweden who suggested that his tests indicate an additional problem. It seems that most people will combine a large dish with low threshold receiver *and* a high gain LNB. According to Bert, much of the problem arises *inside* the LNB. He says that the high signal input from Astra, collected by a large dish, can overload the output amplifier inside a high gain LNB and cause "sparklies" on pictures.
Fitting an LNB of lower gain (around 45dB) solved his problem. Bert continues, however, by saying that it does not help to reduce the input level to the receiver if it is *not* overloaded (and normally it isn't if you have a reasonably long run of coax cable). However, if a short cable run is used, the input level to the receiver *can* be too high and must be reduced by introducing an attenuator.
Another problem, he points out, is the AGC (Automatic Gain Control) circuitry in the tuner, that reduces the level when you have strong signals coming in, making it difficult to get a low level signal among the strong ones. That is apparently the case in Stockholm with CNN from Astra. It is about 10 dB below the other transponders.This could be helped with a band pass filter at the input, say 200 MHz wide.
You can end up with sparklies in saturated colours when you have too narrow an IF filter. For example, looking at Eutelsat with an Astra receiver. Even if the filters are 36 MHz wide, you can have a signal with a low absolute level into the IF block. Then you use only the tip of the filter (the response is not square in reality) and get a signal that is effectively wider than the filter.
Sparklies on saturated colours is called truncation noise, meaning that the signal is truncated (cut) by the filter. Another place where signal truncation occurs is in the video path; one or more of the video amplifiers is overloaded with signal, e.g. in a decoder. Amplifiers may truncate on the high or low end, (or both!!!!)
In some designs, amplifiers can't handle more than 1 volt video p-p; in
many cases not even that much. It should be all right with 1 volt but if some
signals go higher, you get noise in the picture.
Of course the real answer to the problem is another question: "Why would anyone spend money on a really high-gain, sensitive system, just to watch programmes from Astra?" (Sorry Mr Murdoch & Associates!)
I've also heard of problems at the output end of the receiver. The effect is often described as "sparklies" on saturated colours. From the reports I've heard, it seems to occur with certain makes of TV because they can't tolerate high video levels. The answer is to fit resistors in the Scart plug to attenuate the signal.
To sum up: Sparklies on the picture are not necessarily caused only by a *weak* signal - the signal may be too *strong*. If you have a problem like this, first try an attenuator near the receiver input (use the correct type, not one intended for a terrestrial signal). Buy one and keep it in the van! If this fails to solve the problem, try reducing the amount of signal reaching the LNB (the famous "wet rag" on the LNB?) or fit a lower gain LNB temporarily. Again, you should keep one in the van. If the problem is limited to decoded channels, it may be a problem with the decoder itself. If the picture is better through RF than Scart, get a Scart lead with attenuator resistors in the video lead. If the problem is with the weakest signal amongst strong ones (UK Gold on Astra in the UK I think?) then there may be no workable solution.

Martin Pickering B.Eng. with thanks to Bert Dahlstrom.

You can contact Martin by e-mail

The following information was published in "Television" magazine by a well-known supplier.

A number of points have been raised by TV and satellite engineers, by phone and during visits to our trade counter. Most of the comments relate to the poor quality of satellite TV installations and the effect on LNB gain of poor workmanship, most of which seems to be due to lack of training. Two satellite TV trouble-shooters (north and midlands) estimate that about 42 per cent of installations are poor, leaving no margin of error for equipment deterioration. Most of the problems would be bad practice at u.h.f. and are far worse at the output frequencies of an LNB.

We find that at 1,450MHz fitting a line coupler with two connectors results in a loss of 5dB, one over-tight cable clip introduces a loss of 2dB, a tight cable bend introduces a loss of 1dB, a trapped cable results in.a loss of 3.5dB or more and a taped joint in the down lead a loss of 6dB or more.

With a typical installation using 15m of good cable and an LNB with a gain of 53dB, the losses for cable, tape joint and two over-tight cable clips would be 3dB, 5dB and 4dB respectively. This would result in an apparent LNB gain of 41dB, i.e. a 75 per cent signal loss. With poor cable the cable loss increases to 4.5dB, the result being an apparent LNB gain of 39.5dB, i.e. a 79 per cent signal loss. Using very poor cable increases the cable loss to 6dB, the apparent LNB gain falling to 38dB with an 82.5 per cent signal loss.

In view of these figures there's little point in manufacturers bringing out ever higher-gain, lower-noise LNBs if all that happens is that poorer cable is used. We've noted from faulty LNBs returned for repair, as well as from the results of our tests, that poorer, thinner cables are at best a slack fit in the standard F-type plug, giving rise to poor connections and later to weather problems. A test installation with 15m of good cable and an LNB with a gain of 53dB introduced a cable loss of 3dB, reducing the apparent LNB gain to 50dB, a signal loss of 25 per cent.

Antiference 5540 Quadair cable was used, with RS crimp F connectors and crimp pliers. No losses were noted during the crimping. When heavy pliers were used instead a 1dB loss was introduced at each end - the use of pliers also distorts the crimp seal, leading to weather problems. Standard plastic cable clips were used on the down lead. On a ladder it was not easy to avoid over driving the nails, producing losses. Perhaps someone should produce saddles for this purpose now! In conclusion, while our examples have been exaggerated to show the losses which can occur it's the small losses here and there that in the end lead to complaints.


Thanks for you last email about NOT using the loft. I got no signal at all that I could find. I hadn't realised that an additional problem was that my loft has boards backing the clay tiles so the chances were pretty poor.

I reverted to the garden and compromised with my wife on the side of the summer house. I considered your suggestion of sending her to join her mother but we are in our 60's and her mother passed on in 1975 so there could have been complications.(:<).

I ran a feed from the dish on the summer house to my office. I fed it out through the air vent which was the first problem The "F" connector was too big to go through the grill which had to be filed. The lead was then run down the wall, round the corner, across the gap to the garage where the garage power cable runs, along the roof of the garage, along the side of the summerhouse, to the dish.

It is hard to find long ready-made leads and I didn't want to fit connectors myself so the main length is 10 metres which was a metre short of the 4 or 5 metre length connected to the dish already. Rerouting the lead from the dish solved that problem.

I fixed up the satellite decoder in my office and connected to the bedroom TV.

Result reception with "sparklies even though it had been fine with the TV etc. set up in the summer house. I was afraid this was due to the extra length of the feeder lead or bad connections. I tightened the connections with no improvement and suspected the dish had shifted as the mounting bolts although fairly tight are under a lot of pressure from the wind. If I moved everything back to the summer house to reposition the dish I was afraid the same thing would happen again. Solution. I faced the TV to the office window (at night) and opened the curtain. Then I stood by the dish in the garden with my wife's birdwatching binoculars and found I could see the picture well enough to see the sparkies. So I was able to correct the dish position and then tightened the mounting bolts a bit more.

Now to try recording. First the small TV had to be tuned to both recorder channel and Satellite decoder channels which needed the manuals. I found myself confused operating three remote controls and getting them mixed up but got there in the end.

That done it all worked. Da Dah.

Question - The LNB on my dish can move about half an inch closer or further from the dish within its clamp before the clamp is tightened. This seemed to have no affect on reception. Does it matter?

Thanks for your help, particularly for pointing out that reception does not take place at right angles to the face of the dish.


Brian Gilbert

Brian, the ingenuity of people like yourself never ceases to amaze me! And all to avoid paying some poor installer twenty five quid to do it properly ;o)

Still, you obviously had fun and learned a lot in the process.

The LNB should ideally be placed at the focal point of the dish. This would be difficult to calculate without knowing the dimensions of the full parabola of which an offset dish is only a part. Trial and error works best. Ideally you should use a spectrum analyser but it's often possible to Sellotape pieces of card over the LNB cover until sparklies appear. Now you can get out your binoculars and slide the LNB back and forth for minimum sparklies on the TV picture. As the LNB skew angle is also critical, you can repeat this exercise whilst twisting the LNB in its clamp.

Why go to all this trouble to optimise it? Well, it minimises sparklies in bad weather.

You should also use the best cable you can afford for the same reason. CT100 is about the minimum specification you should consider. However, I see people using ordinary brown TV aerial coax and getting acceptable results (analogue NOT digital!). If you get sparklies on some channels then good cable and dish/LNB adjustment is the answer. If you don't, then why bother :o)

Fitting an "F" connector to a cable is simply a matter of removing insulation and screwing on the connector by hand. Easy. Just make sure that the strands of shielding braid can't touch the inner conductor, otherwise you could cause damage inside the receiver which supplies 13/17v up that cable!

You should also read the INTERFERENCE page

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Version 1.1 updated on 3/5/99
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Key words sparklies, interference, fischen, stoeren, stoerungen, stören, störungen, sparkles