Race Communications, based in Berkshire, are a leading distributor of equipment for both commercial and domestic installers. Managing Director Robert Calaz is a respected figure in the industry, a published author of technical literature, and has spoken at training seminars throughout the country on a range of topics.

Working with the dTV Commercial division, Race were able to sucessfully deliver this complex fibre install project in just six weeks! The specification called for 37 UHF TV channels, 80 TV receivers, and all spread across an estate of 60 acres. This article outlines the system concept and how considerable technical challenges were overcome.

Overview
33 of the programmes originate from 4 satellite locations Nilesat 7oW, Hotbird 13oE, Arabsat 26°E and Astra 28°E. The 4 dishes were to be located in a remote part of the estate with no building nearby, so the customer provided a “Portacabin”, complete with mains power, lighting and even air conditioning!

The relay of 37 programmes in the UHF frequency band (where there are only 47 channels in total) means breaking all the channel allocation rules. The eventual system design specified the use of channels 21 to 33 and 39 to 62, leaving a gap (34 to 38) for VCR channels at viewing locations, and 63 to 68 for adding additional programmes at a later date.
The problem of relaying all these programmes over such a wide area was overcome by specifying the use of fibre optic cables. Fibre optics has several advantages over conventional coaxial cables, the two main ones being a negligible signal loss (less than 0.5dB per kilometre) and absolutely zero frequency slope (most important with so many channels being relayed). Until recently, fibre transmitters and receivers have been prohibitably expensive, but the new Vision range offers a significant price reduction, which made the use of fibre a viable proposition.

Reception and relay equipment
It was decided to use the 4 terrestrial analogue channels (relayed at their incoming frequencies) and digital satellite receivers for the other 33 programmes. The Fracarro KDR modular receiver was used for the free-to-air channels (you need to know all the PIDs in order to programme them) and the new Pace BskyB1000 minibox (fed from a Vision multiswitch) for the Sky programmes.

The most important component in a system relaying this many channels is the VSB modulator. The new Vision V40-102 unit proved to be the ideal choice, since it incorporates a SAW filter. Even so, this was followed by a UHF channel amplifier to ensure reliable operation on adjacent channels.
three such combinations were installed and the outputs combined to give a total of 37 UHF channels, all of exactly the same amplitude.

Fibre equipment
The Vision fibre transmitters and receivers operate on a wavelength of 1310NM and are housed in aluminium die-cast containers, with FC/APC optical connectors. There are no adjustable controls of any kind on either unit and it is vital to inject all UHF channels at 94dBµV in order to achieve optimum operation of the system.

Two transmitter versions were used, one with an optical power of +3dBm (followed by an optical 2way splitter) and the other +6dBm (followed by a 4way splitter). In each case, this gave an allowable loss of up to 10dB on the distribution system, more than enough since the combined fibre and connector losses are very much less than this. The theoretical UHF output level of each receiver is 94dBµV less 2dB for every 1dB of optical loss on the network. A total of 8 receivers were used on the project, one in each of the major buildings on the site, with coax links to receivers in nearby buildings. In nearly every case, it was possible to feed all the TV receivers in each building without the need for additional UHF amplification.

Fibre cables
There are 2 types of fibre cable construction, tight buffered and loose tube. The loose tube version holds the fragile fibres in a semi liquid gel (offering additional protection), which can migrate over long vertical runs. A Webro 12 core loose tube polythene sheathed cable was eventually specified (no vertical runs being involved). Being flexible and only 6mm diameter, this proved to be extremely easy to install. The cable incorporates mylar strands for added strength on longitudinal pulls and can be treated just like a coax, although care must be taken not to kink it.
Fibre joints

There are 2 main ways of jointing fibre, butt splicing (a mechanical crimped joint) and fusion splicing, where the fibre ends are fused together using a microscope and an electric arc. Fusion splicing results in lower joint losses, but necessitates the use of a very expensive fusion splicer. We employed a specialist sub contractor to do the fusion splicing for us and the 38 joints took him 2 days to complete. The end result was that each optical path loss including the connectors was less than 4dB.

You cannot join fibres just like coaxial cables. The composite fibre cable must be rigidly anchored using a suitable cable gland so as not to impose any strain on the delicate fibres that are only one tenth the diameter of a human hair. The environment must be clean and the tails must be long enough to reach the splicing machine. The outer sheath and fibre cladding are removed and each fibre in turn is cleaved and polished, all using specialist tools. The fibre is then spliced to a “pigtail” with a connector already fitted at one end and an open end at the other. The fused joint is then protected with a heat-shrink sleeve incorporating a metallic strip for additional strength, and the whole joint must then be supported to prevent any strain on it. The connector end of the pigtail is then joined via a back-to-back coupler and a patchcord to the transmitter, receiver or splitter. The fibre joints should then be protected in a lockable housing the motto should be “fit-and-forget”.

Fibre testing
We ended up terminating 3 cores in each fibre cable (one to use and two others for spares) the other nine cores being available for future use. Each terminated fibre link was then tested using an Optical Time Domain Reflectometer (OTDR). This remarkable (and very expensive) device sends a light pulse down the fibre and measures the light being reflected back. You can measure the exact cable length and the total optical path loss. The display also shows the quality of each fibre joint and exactly where a fibre cable has been damaged during installation, if at all (thankfully not in our case).

We felt a bit helpless after the jointer had left site, wondering how we would trouble-shoot if we ever had a problem. We have since discovered two remarkable service aids: a laser torch which, when connected at one end of a fibre, makes the other end glow (this checks the integrity of the fibre link), and an optical power meter (to measure the received power).

The end result? Thirty seven channels of perfect quality on every TV receiver, delivered on time, and one very satisfied customer. To see how this can be translated across to your business, contact Robert Calaz at Race Communications on 01344 621561 or David Evans at dTV Commercial on 07720 467307