Satellite LNBs - dual LNBs for Sky (Astra)

[Anonymous]

I presently have a 1.3m dish with single LNB pointed to the Astra chain, to receive Sky UK transmissions on the 'North Beam'. Being down in southern Spain the signal is very low (deliberate, Sky have to make it difficult to receive outside the UK). We are in a marina and live on our boat.

At the moment, we are getting quite a good signal - we only get the Free to View programmes but can get Channel 5 which is one of the weakest signals down here.

My next door neighbour has asked me if he may put another LNB onto my dish, for him to use with his own Sky box. He says that he's done it before. In principle I don't mind as long as our service is not reduced in any way and actually we are borderline with ITV dropping out in heavy cloud.

I've done some research online and it seems that nobody makes a bracket for a second LNB and in any case both could not be right at the focal point, so I don't know where he got that idea from? Has anyone heard of that arrangement and does it in any way compromise the signal on the original LNB?

What I have found is dual and quad LNBs for sale. To use this we would have to replace my LNB by a new one. Very quick and easy to do but how does the gain and noise figure of a quad or dual compare with a decent single LNB? These LNBs are the luck of the draw, some are better than others and the average are useless down here so it is all very marginal, unlike the UK where you have enough signal to boil a kettle!

I don't want to say no to this fellow if I can help him without reducing our service and I don't want him to mess around and for me to fit his dual/quad LNB only to have to take it off a few days later when cloud cover leaves our signal too weak for a decent picture.

Does anyone here have enough experience to tell me whether the dual/quads are, generally, at least as good or better than the singles or, generally, not quite as good?

Many thanks.

 

[BlueChip]

There is a guy on the Costa Blanca who is active on the CB forums and knows a lot about satellite setup in Spain and seems to be something of a guru. He runs the TV in Spain newsgroup and his name is Bill Hayles. I wont post his email but if you google Bill Hayles TV he pops up. I am sure he will answer your question and you may some of the stuff he has published useful.

From my own experience, when we had satellite broadband installed some years ago, the supplier supplied a quad LNB, two outputs were for Sky and one was for the Satellite data download. I am sure they wouldnt have fitted a quad if there was any signal degradation as we were really struggling for performance most times

 

[Strathglass]

Yes you can use dual and quad LNB without any problems.

A seperate sat box is needed for each output and they do not interfere with each other.

There should be no loss of signal if one with a low noise figure is used.

When SKY+ is installed a dual LNB is used.

A google search will give many suppliers and quantify the performance of their products.

Iain

 

[prescott56]

Hi Lemain,
A nice simple solution we have used on land is to split the output of the single LNB with a splitter. Each TV would need its own sky box. Way down south you would probably need an amp, install this before the splitter. (generally).
Am in the workshop later so will get part numbers of what we use.We get good enough signal here in SW Mallorca.
Later
Roy

 

[Anonymous]

That is really helpful, I am most grateful. As is so often the case, one or more people on this board have 'been there, done that', and sorted out the problems.

Thank you, everyone.

 

[sailinglegend420]

[ QUOTE ]
As is so often the case, one or more people on this board have 'been there, done that', and sorted out the problems.

[/ QUOTE ]

Not always true - especially in this case!!!!!

The most important thing to understand is that the receiver box actually tells the the active part of an satellite antenna - the LNB - which channels it wants by sending a DC voltage up the cable. Each channel is broadcast from the satellite with either a vertical or horizantal circular polarization. An LNB can only receive one type of polarization at a time. Roughly half of the channels are one one polarization and half are on the other.

You CAN feed two receivers from a single LNB using a special splitter where one of the outputs has a DC pass, and the receiver connected to this supplies power to the LNB. Only the master receiver will be able to select the polarity, and thus determine the channels available to the slave receiver. When the master changes the channels available to the other will sometimes also change.

Twin LNBs are the only answer.

 

[Anonymous]

Thanks for that - that's how I understood it to work as well but...[ QUOTE ]
Not always true - especially in this case!!!!!
Twin LNBs are the only answer.

[/ QUOTE ]From the research I have done following Blue Chip's post, it seems that quads and duals (as opposed to a 'twin' which is a rare beast and often used to point to different satellites) allow individual control of 4/2 independent channels. e.g. One box can call for Sky News and another can call for BBC1. Is that not so?

 

[sailinglegend420]

I'm not an expert on everything!!!!

Not sure the absolute difference between twin and dual or quad. By twin maybe you mean two LNBs on one antenna - probably not the best solution as you can't focus both LNBs perfectly.

Dual or quads must have the detectors more perfectly centred in one LNB pod - but I don't know if you lose any gain - which is not what you want.

You need a BONA FIDE sat expert - maybe to fit one for you and "try before you buy".

 

[sailinglegend420]

PS.

With more than one LNB you can of course select more than one channel on different receivers - SKY or whatever - all from the same satellite. Maybe a twin LNB could be positioned to also pick up a reaonable signal strength from an adjacent satellite - but I'm not sure if you would compromise on the signal strength from both.

 

[alan]

I think you will find that when you use a dual or quad LNB, that each one acts independantly. In fact what you are doing is putting a number of OMTs around the feed horn with a separate control for the polarisation provided by their respective receivers ........... not the same as splitting the output from a single LNB.
In theory any degradation in S/N0 (Eb/N0) due to using a quad LNB should be undetectable to the receiver (100ths of a dB).

Alan.

 

[Strathglass]

As has been said by other posters This is not the solution.

The adding of a splitter will seriously degrade the signal to noise ratio.

Adding amplifiers after a splitter will not help this in week signal areas.

Also with one LNB only one station can be tuned into at any one time , irrespective of how many sat boxes are connected to it.

There are two different families of LNB.

Twin ones have one of the LNB set at a different optical position within the dish to allow the reception of a satalite on a different bearing from the main satalite to be recieved. Typical of those is one which allows the reception of both Astra and Hot bird satalites on the same dish without rotating the dish.

The other type of LNB is the dual or quad one. In this type all of the sections are aligned on the same bearing.
Each section can be controlled by a seperate sat box to different channels without interferance between sections.

This is the type of LNB the original poster requires.

Iain

 

[Anonymous]

Yes, Iain, that's what I need and that's how I understood it to work. The double LNB for Hotbird is an oddball but could be quite useful down here if - and only if - the quality was superb on the main LNB. The signal down here is very marginal and every so often they work on the transponders to tweak the signal back a bit.

One of these days, Sky will come to an arrangement with the copyright owners so that they can legally transmit throughout Europe. I once read that one problem is an international treaty to prevent one state from 'indoctrinating' other states - but then the Beeb have been doing that for years, and still are /forums/images/graemlins/confused.gif

 

[alan]

Lemain,
What you need is a dual LNB (as has been said) it will not effect the reception of your signal a discernible amount, and will provide two independant receive chains.
To squeeze the last bit of signal strength of the LNB after fitting, leave the clamp a bit loose and then rotate the LNB axially to "peak" the polarisation. The polarisation offset angle is more or less the difference between the satellite longitude and the receive antenna longitude (the actual maths are a little more compliceted but for practical prurposes this is close enough); e.g. Astra at 19E and antenna at 5 E, will give you a polarisation offset of 14 degrees. So the vertical (or horizontal) axis of your LNB needs to be rotated by this amount. An easy way to do this is to rotate the LNB in one direction untill the signal becomes noisy or in the case of a digital signal you lose lock and mark this point on the LNB support, then rotate it in the other direction untill you lose the signal and again mark this point, then rotate the LNB untill it is in the middle of the two marks. An axis of the LNB is usually marked or it will be where the two halves of the plastic case are joined.
It is also worth doing the same with azimuth and elevation movements of the antenna and repeat this a couple of times, to squeeze those last 1/100ths of a dB !!!
After doing this you will have the maximum signal strength that you can get with that antenna/LNB combination.

Alan.

 

[prescott56]

Evening Guys,
previous posters were correct a simple splitter will only allow the same channel to be viewed, which would be interesting on 2 boats heheheh....
Never got back to the workshop today, xmas party and all that. but what i have been doing over here is rather large expensive villa's. We stick a sat dish in the grounds with a quad LNB on, similar to this.
http://www.maplin.co.uk/module.aspx?ModuleNo=47174&doy=search
then feed the 4 output cables to a box similar to one of these,
http://www.satquip.com/product.php?productid=53&cat=17&page=1
these switches then feed up to 4 sat boxes allowing each sat box to choose its own channel.i.e. different channels in each room. the switches can be cascaded to give more outputs, but quality degrades without amps.
I know the switches we use are Grundigs but can't remember teh model numbers.
Sorry i can't be any more specific as i have only got into this stuff this year, and in honestly the jobs i have been working one...eeeerrrr...money is not really a limiting factor. but prices for kit look managable.
If you really wanna watch lots of TV get 2 dishes, (one to Astra 1 and one to Astra 2 i think) and 2 quad LNB's fed into switches and you can get over 940 channels of free S h 1 t. even i was amazed.
ask a local sat guy, am sure they will advise, and most i have met over here are very amiable to us poorer people
Regards
Roy