Stacker/ De-Stacker

Can you shove two cables down one?

^{Published on 28th July 2010 at 06:42 by Brian Butterworth on UK Free TV}
Jordy writes:

Over the past year or so two almost forgotten pieces of equipment have re-emerged in the installation trade... Stacker/ De-Stacker equipment and brown Coaxial Cable, brown cable i'm not so worried. DIY and even Professional Installers seem to be fitting Stacker/ De-Stacker units in ever increasing numbers most believe they "Can shove two cables down one".

Whilst that is the principal of the system their are great benefits to such a system and there is great pitfalls. The invention of the Sky plus box brought extra challenges such as an additional cable run from dish to box, some people don't have the luxury of getting in another cable run. Possibilities they've built an extension to their home or the the most common they've moved into a flat with a communal aerial/ satellite system.



Two stories - Mr DIY has moved into his nice new flat and discovered he's one satellite cable, he's want Sky HD and Sky refuse the install as they normally do. He hatches a plan to buy a stacker/ destacker the cost of which is 70 quid hooks it up as per what he's read on the internet and it never works. Mr Fitter attends a job in a block of flats discovering he can't easily get another cable in or the multiswitch has no spare output. He goes about setting up a stacker/ destacker unit and discovers it won't work either.

Technical bit - A Stacker/ De-Stacker unit is a brilliant piece of kit if used right... Nine times out of 10 it is either botched or used with inferior cable. Cable type is the most important piece of equipment in the system, cheap poor coaxial cable known as RG6/ 60 is often high loss over distance and has the impedance of a sponge cake. CT/ TX100 or 125 is far superior because it's double shielded and suffers much lower loss. RG6 type cable tends to have a rating of about 1ghz, CT100 has a rating of about 3ghz. The job of the stacker is to use additional bandwidth to combine two satellite feeds, it does this by using a higher stretch of the band and effectively stretch's out's the other feed on a higher frequency, a lower grade of cable with a poorer rating can't cope with the job of performing this. The de-stacker pulls the two frequencies back down and separates them into two separate feeds ready for decoding at the set top box.

Factors that play apart are cable length - The cable run much be longer than 7 metres to account for impedance and to regulate for the insertion loss.

Cable Loss Calculation:

Loss per metre at frequency “X” = loss per metre of a known frequency x the square root of (frequency “X” divided by known frequency)

CT100 type cable has a typical loss of .21dB per metre at 1000 MHz. Therefore, at 3850Mhz the loss would be .41 dB per metre.

RG6 type cable has a typical loss of .22dB per metre at 1000 MHz.Therefore, at 3850 MHz the loss would be .43 dB per metre.

To conclude if this type of installation is to be used insure if you're using a multiswitch that it has a spare port - the amount of times i've seen two taped together. Ensure the multiswitch does not have a dc block function as the stacker itself requires a 9v power supply. Ensure you're using the correct cable and the correct length of cable. You cannot cut corners with there types of installations...

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