Splicing a satellite lead
the DIY method



The correct way is not to have to splice the lead - at all.

If you absolutely must join two leads together, then the proper way is to fit those little F plugs to the ends and use an F junction to join them.

If, like me, however, such things are out of your budget (those F plugs and joinings are actually rather pricey!), then the method described below should allow you to join two pieces of satellite cable with little loss in quality. Between the single satellite cable (alone) and the cable with the splice, I only noticed around 5% drop on the signal quality meter, and 5%-15% drop on the signal strength meter.
Your mileage will vary, of course.

That said, if the alternative is no reception due to various obstacles, then making the lead longer with a splice may provide you with some reception, which is better than nothing at all!


Some theory

Before we go any further, a little bit of theory. I have actually seen somebody solder together the satellite cable onto a tiny piece of veroboard and mount the lot into a plastic box. It worked, terribly, for analogue reception (from the audio you could make out Sky Premier, just, but the VideoCrypt never gave a proper picture).


The answer is quite simple. If you are splicing cable then you probably have a bit of cable in front of you. Examine it, end on. There is an outer wrapping of squishy-but-tough plastic. Just under that is a metal braid. Right in the middle is a solid piece of wire. Between the wire and the braid is a waxy-looking plastic.

Here's a really bad ASCII diagram for those without a wire to look at:


  X - outer wrapping      
  # - metal braid         
  + - waxy-looking plastic
  o - inner wire          
The outer braid acts as a 'ground' wire, and (being a braid around the insides, it acts as a shield to stop things interfering with the received signal).

The inner wire has only two purposes in life. The main purpose is to supply the 13 or 18 volts required to power the LNB and to select the polarisation. The second purpose, in modern receivers, is to send the 22kHz tone to select the upper band (tone present) or lower band (tone absent). In some cases this tone is modulated to transmit control commands to dish rotators and the like (for example, DiSEqC).
That is all that the middle wire does.

So where does the satellite signal go? The satellite signal is broadcast at many gigahertz (9.7 to 12.75, roughly). The thing is, such high frequencies can pass tens of thousands of miles through the air, but getting that sort of signal down a wire is more problematic. The LNB actually acts as a down-converter, exchanging the high frequencies of the satellite broadcast for a lower set, around 1-2GHz. This can then be sent down the cable, but not down the wire.

Ever wonder why satellite equipment has special cable and those fiddly little plugs? Why not a phono plug with two-core bell wire? Why was my friend's soldered join so bad?

The secret is that the high frequency signal actually travels down the waxy-looking stuff between the inner wire and the braid. Please don't ask me how this happens, it smacks of ghostly goings-on to me, but rest assured that it is true. I'm sure people hoping for tenure have written long and involved theses on this very subject, explaining the phenomenon in more detail than anybody ever wanted to know!


Ready... Steady...

This, then, is the secret to making a good splice. You can't simply twist the bits together and tape 'em up. While it may be perfectly good in electrical terms, it won't allow the signal to pass. We must go to pains to preserve our signal path. In fact, above all else, our signal path is the most important thing.



You will need:
The stuff you'll need...


As shown in the picture below, you should remove about 2cm of the outer cover and twist back the braid. Then strip about 8mm of the waxy plastic to expose a short length of the inner wire.

Initial preparations.


Now comes a seriously fiddly bit. You must place the inner wire of both pieces end to end (not overlapping) and then wrap a piece of aluminium foil around the wires. This should be done as sparsely as possible to avoid making the diameter of the inner wire much thicker.
As you can see, I've taped down my example. You might like to do the same, because at this stage the cables can easily slip apart.
I have, unfortunately, had to tape my wires to a piece of light blue paper. It would have been much more logical to tape them to the table, but the table is black and no good for taking pictures...

The first join.


The next thing to do is notice that a side-view of your join would be something like:

++++++++++++              ++++++++++++
++++++++++++              ++++++++++++
==================  ==================
++++++++++++              ++++++++++++
++++++++++++              ++++++++++++

  + - waxy-looking plastic
  - - your aluminium foil 
  = - the inner wire      
What we must do now (and this, also, is fiddly) is to cut some tape to fit into the gap between the waxy-looking plastic. Then, wind this round and round to fill the gap.
I chose to wind mine fairly loosely, I don't know if a tight wind is better or worse.
Note that the cables can come apart, still, at this stage.
Filling in the gap.


Wrap a small piece of tape around the entirety of the inner join. This not only keeps the cables together for the shield-work, but it also protects against short circuits in the splice.

Protecting against short-circuits.


The outer shield must now be restored. My method of doing this is to wrap a piece of aluminium foil carefully around the inner join, and then lay the braid wires over this, and then wrap another piece of foil around it all. The reason for this is to try to present a 'smooth' (or as smooth as we can make) inner surface to the outer shield. If you examine various cable types, you'll notice many have a similar arrangement with a thin piece of metal-coated tape between the waxy-looking plastic and the actual braid. We should attempt to reproduce this as best we can.

The outer shield.


The final step is to enclose the whole lot in the tape. Please be extremely careful not to squeeze the splice too tightly as you put the tape around the cables - it is no good taking care to make a good splice to then mess it up by squeezing the cable!

However... This tape has to do double-duty in both keeping the splice waterproof and also protecting the cables against accidental pulls. It won't take much strain (so don't try to use this method for slung cables) but it'll take movements such as a person tripping over the cable a few metres away.

The finished splice.


It should go without saying that you must carefully measure the cable using a multimeter before connecting to your receiver. Some receivers will say '!! LNB SHORT !!', but others might just go pop-fizzle. Don't take a risk - check it carefully.


There you have it. I have had this splice in operation here for over two years. While my Digibox is temperamental with the BBC/Sky-based services (28.2°E), it performed quite well in torrential rain while tuned to the older 19.2°E constellation.


I will, though, re-iterate that the best way is not to splice. If you can afford new cabling, do that. It is always going to outperform any form of splicing.
If splicing is necessary, a proper with-plugs-and-stuff splice is the preferred method.

This works, but it is intended for those on a shoestring...


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Copyright © 2004 Richard Murray