OB fibre cable is a ruggedised fibre cable laid out for a broadcast job and reeled in afterwards. Use tight-buffered or armoured tactical cable with an abrasion-resistant jacket, single-mode OS2 fibre for most new work, and sealed or expanded beam connectors. Reel hub size, cap discipline and connector cleaning matter more than cable attenuation.
What outside broadcast puts a fibre cable through
An outside broadcast (OB) fibre run is a temporary link between a production truck or flypack and the cameras, mics and remote boxes around a venue. It is laid out the morning of the event, used for a few hours or a few days, and then rolled up and driven to the next job. A cable in a building is installed once and left alone. An OB cable is installed, abused and recovered over and over, so the design question is different: how many deployments will it survive, not how long it will sit still.
That changes what matters in the cable. Attenuation per kilometre is almost irrelevant on runs of a few hundred metres. What decides the life of the cable is crush from wheels, abrasion on concrete and gravel, kinks from fast coiling, water and mud on the connectors, and the pull applied when someone drags a reel through a crowd. The rest of this guide sorts those hazards into construction choices, connector choices and handling rules. For the wider family of armoured and tactical types, start at the armoured fibre optic cables page, and for the broadcast side of the business see fibre for broadcast and live production.
Hazards and what each one asks of the cable
Write the hazards down before you pick a cable. The table links each hazard to the feature that answers it, so that you can see which features you are paying for and which you can drop.
| Hazard on an OB job | What it does to the fibre | Cable or connector feature that answers it |
|---|---|---|
| Vehicle wheels crossing the cable | Crush raises loss, and a hard impact can break the glass | Strong core, crush-resistant jacket, cable ramps or covers on the route |
| Dragging over concrete, kerbs and gravel | Jacket wears through to the strength members | Abrasion-resistant jacket such as polyurethane, braided or armoured outer layer |
| Fast coiling and uncoiling | Twist and kinks create tight bends that add loss or crack the fibre later | Flexible, round cable, reel with large core diameter, figure-of-eight coiling |
| Pull when a reel is dragged or snagged | Tension strains the glass and can stretch the jacket | Aramid strength members, strain relief at the connector, a rule to pull on the connector body or the cable, never on one |
| Rain, mud and grit in the connector | Dirt on the end face adds loss that comes and goes | Sealed or lens-based connectors, dust caps on every end, a cleaning kit at every joint |
| Cold nights and hot trucks | Jackets stiffen in the cold and soften in the heat | A cable whose data sheet temperature range covers both handling and operation |
| Rodents, wildlife, curious crowds | Chewing and cutting | Armour layer, or routing over the crowd area under covers |
Two of these rows account for most failures seen on temporary jobs. One is dirt on the connector. The other is a bend below the minimum radius, usually because someone coiled the cable too tight on a short reel. Both are cheap to prevent, and the sections below cover them.
Cable constructions used for field fibre
Field cables fall into three broad groups. The names vary between makers, so read the construction on the data sheet and not the product name.
| Construction | How it is built | Strengths | Limits |
|---|---|---|---|
| Tight-buffered tactical cable | Fibres with 900 micron buffers, aramid yarn, tough outer jacket, often 2 to 12 fibres | Light, flexible, good for repeated coiling, connectors fit directly | Moderate crush resistance unless the jacket is thick |
| Armoured tactical cable | The same core with a braid or tape layer over the aramid, then a jacket | Better resistance to crush and cutting, suits vehicle routes | Heavier, stiffer, needs a bigger reel |
| Hybrid cable | Fibre plus copper conductors for power in one jacket, as in SMPTE camera cable | One cable for signal and camera power | Heavy, short length per reel, special connectors |
Loose-tube cable, the type used for ducts and poles, is rarely suited to repeated reeling because the loose fibres can move and bunch up inside the tube when the cable is coiled hard. Our guide to tactical fibre optic cable explains the core designs in more detail, and the camera-feed variant is covered in the SMPTE hybrid cable guide. If your route crosses a vehicle lane, read the comparison of steel tape, steel wire and GRP armour before deciding between a braided and an armoured jacket.
Radix makes armoured tactical fibre cable and cable reels to the customer's drawing in Pune. The drawing sets the fibre count, the cable length per reel and the connectors at each end, and the quote confirms the construction offered.
Single-mode or multimode for an OB link
For new OB kit, use single-mode (OS2) fibre unless the whole venue is short and fixed. A single-mode cable carries 3G, 6G and 12G SDI, IP video and intercom over any distance a venue is likely to need, and it does not tie you to one speed. Multimode (OM3 or OM4) still appears in older trucks and on short feeds, but optics for it are less common in broadcast gear and the bandwidth limit at 850 nm shortens the reach at higher speeds.
| Question | OS2 single-mode | OM3 or OM4 multimode |
|---|---|---|
| Core size | About 9 microns | 50 microns |
| Typical broadcast use | Truck to stage box, venue to compound, 12G SDI, IP trunks | Short links inside a truck or a small studio |
| Cable attenuation (industry maximum) | 0.4 dB/km at 1310 and 1550 nm in ISO/IEC 11801 for OS2 | 3.5 dB/km at 850 nm, 1.5 dB/km at 1300 nm |
| Tolerance of dirty connectors | Low: a speck covers a larger share of the core | Moderate |
| Future speeds | No cable limit at any broadcast rate | Reach falls as the rate rises |
The single-mode core is small, so a speck of dirt on the end face does more harm than on multimode. That is the argument for lens-based expanded beam connectors in the field, covered below. The trade-off between the two fibre families is set out in single-mode vs multimode fibre, and the SDI side of the choice is in the guide to 12G, 3G and HD SDI over fibre.
Connectors that survive a wet field
The connector fails before the cable does on most OB jobs. A butt-coupled physical contact connector depends on two clean, flat glass faces touching. Put it in a field with rain, grass seed and fingers, and the faces collect dirt on every mating.
- Expanded beam connectors. A lens in each half widens the light beam to a spot much larger than the core and the two halves do not need to touch. A speck of dirt blocks a small part of a large spot. They tolerate dust and many mating cycles, and cost more per pair than a plain LC. The principle is explained in expanded beam connectors explained and compared with contact types in expanded beam vs physical contact.
- Ruggedised physical contact connectors. A housing with a seal and a locking collar around an LC or similar contact inside. They are lighter and cheaper than expanded beam, but they need cleaning before every mating. Examples are the opticalCON range described in the opticalCON Quad, Duo and Advance comparison.
- Plain LC or SC with dust caps. Fine inside a truck or a rack, poor on the ground. Where used outside, put the joint inside a splice box or a sealed enclosure.
Whatever the style, the cap discipline is the same. Every end carries a cap, caps go back on at once, and the cap is cleaned or replaced when it gets wet. The cleaning method is in connector end-face cleaning and inspection. A cleaning pen and a swab pack in the pocket of the person who makes the joint is a better investment than a larger spare cable.
Reels, length and handling
The reel is part of the cable system. A reel with a small core diameter forces the cable into a tight bend for its whole stored life, and the loss from that bend shows up as a pass-fail problem months later. Check the minimum bend radius on the cable data sheet, and make sure the reel hub is larger than twice that radius. Many makers quote a figure in the region of ten times the cable diameter for a cable at rest and about twenty times while it is under tension or being pulled. Treat those as typical figures, and use the data sheet value for your cable.
Length per reel is a balance. Long reels mean fewer joints, and every joint costs loss and a chance of dirt. Short reels are easier to carry and replace. A common compromise is to pick one standard length for the fleet, so that any reel can stand in for any other and joints can be added in 100 or 200 metre steps. Our guides on tactical fibre cable reels and deployment and on drum and reel handling go through winding, braking and storage in detail.
- Unroll with the reel on a stand or a spindle, never by pulling cable off the side of a fixed drum, which puts one twist in the cable for every turn.
- Walk the cable out, do not drag it, and lay it flat without loops.
- Keep the route clear of kerbs and gates, or protect the cable with ramps and ducting at those points.
- Wind back on to the reel under light tension, with the cable guided into even layers.
- Cap both ends before the reel is picked up, and put the reel in the truck tied down, not loose.
Worked example: a 650 metre feed to a venue compound
The numbers below are assumed values chosen to show the method. Replace them with the data sheet values of your cable, connectors and optics.
A truck sits in a compound 650 m from the stage. Two single-mode reels, one of 300 m and one of 350 m, are joined in the middle. The route has these mated connector pairs: the truck bulkhead, the reel joint, a patch to the stage box, and the stage box output. That is four pairs, and the truck-side patch cord adds two more, so six pairs in total. Take 0.4 dB/km for the cable, a field allowance of 0.5 dB per mated pair (a conservative figure for connectors that are mated and unmated often), and a module budget of 9 dB.
| Item | Calculation | Loss |
|---|---|---|
| Cable | 0.65 km x 0.4 dB/km | 0.26 dB |
| Six mated pairs | 6 x 0.5 dB | 3.00 dB |
| Total | 3.26 dB | |
| Assumed module budget | 9.00 dB | |
| Margin left | 9.00 minus 3.26 | 5.74 dB |
The margin looks comfortable, but note where the loss sits: connectors cost more than eleven times as much as the cable. Adding a third reel for a longer run adds 0.5 dB for the joint and almost nothing for the extra fibre. Dirt is the larger risk, because one dirty pair can add several dB on its own and use up the margin. This is why the effort in an OB plan goes into connector choice and cleaning, not into the cable attenuation figure. The calculation follows the pattern in our fibre loss budget guide, and you can run your own numbers in the loss budget calculator.
Weight is the other number worth a quick check. At an assumed 60 kg/km, the 350 m reel carries 21 kg of cable before the reel itself. If the venue crew must carry it by hand over a long route, a lighter unarmoured tactical cable and shorter reels may be the better design.
What to specify when you buy
A purchase order for OB fibre should contain these items. Anything left out will be filled in by the maker's default.
- Fibre type and count, for example OS2 with 4 or 6 fibres, with spares counted.
- Cable construction: tight-buffered or armoured, outer jacket material, outer diameter, and the data sheet to which they refer.
- Connector style at each end, with the polish (APC or UPC for single-mode) stated for each end. Most broadcast optics specify one or the other, so check the module sheet. See UPC vs APC.
- Length per reel, measured as cable length and not route length, with the tolerance you accept.
- Reel or drum type: hub diameter, flange size, brake, handles, and the label.
- Labelling: reel number on both flanges and on the cable at each end, so that a long run can be rebuilt in the same order after each job.
- Test record: end-to-end loss and the wavelengths used. For a field set, ask for the loss of each reel on its own so that you can add them.
- Dust caps and a clean carry case or pouch for each reel end.
Radix builds tactical and armoured fibre assemblies to the drawing and BOM you supply, with quotes by form, phone or WhatsApp. Radix tests each cord before dispatch. The guide to writing an RFQ for fibre assemblies lists the same details in a form that you can paste into an enquiry, and the tactical cable reel page shows the product family.
Mistakes that cost a show
- Buying by fibre count only. Two six-fibre cables can have very different crush and abrasion ratings. Compare jacket and construction, not just the count.
- Mixing APC and UPC connectors. An APC connector mated to a UPC connector gives high loss and damages the end faces. Make every joint in a route the same polish.
- Forgetting the spare reel. One cut cable on air day is a failure. Carry one spare reel of the longest length in use.
- No cleaning kit at the joint. The person who joins the reels needs a pen and swabs in hand, not in a van.
- Coiling over-under on a short reel. A hard coil on a small hub creates a bend below the minimum radius and the damage is invisible until the link fails.
- Testing only at the start of the season. A reel that has been run over once may pass visually and fail on a loss test. Test after any incident and on a schedule.
When not to use field-deployable cable
Do not use tactical or reeled cable for a permanent run. For a fixed stadium link inside ducts, loose-tube or distribution cable of the right fire rating is the sensible choice, and a patch panel replaces the connectors that would otherwise sit on the ground. Reeled cable is also a poor fit for very long single runs over several kilometres, where a loose-tube outdoor cable on drums does the job at a lower cost per metre.
Questions buyers ask
What is OB fibre cable?
OB fibre cable is a ruggedised fibre cable used to link an outside broadcast truck or flypack to a venue. It is designed to be laid out, used and reeled in many times, so it has a tough jacket, aramid strength members and connectors that suit rough ground.
How long a run can OB fibre handle?
Single-mode fibre has cable loss of about 0.4 dB/km at most, so cable length is almost never the limit on a venue run. The limit is the number of connector pairs and the optics budget. Runs of several kilometres are possible when the optics allow it.
Should I use single-mode or multimode for outside broadcast?
Single-mode (OS2) is the usual choice for new kit because it carries 12G SDI and IP video over any venue distance. Multimode is used for short links inside trucks and for older equipment that has multimode optics.
Why are expanded beam connectors common in broadcast?
They use lenses to widen the beam, so dust and small scratches block a small share of the light and the end faces do not touch. This suits wet, dirty and often-mated field joints. They cost more than a standard LC pair.
How tight can I coil tactical fibre cable?
Use the minimum bend radius from the cable data sheet. A common rule on data sheets is around ten times the cable diameter at rest and about twenty times under load, but the figure for your cable governs. Coil in large loops and avoid twist.
Can a vehicle drive over fibre cable?
Only if the cable is rated for crush and the route is protected with ramps or covers. A single crossing of an unprotected cable may pass a visual check and still raise the loss. Test any cable that has been run over.
What should I clean before joining two reels?
Clean both end faces with a pen or swab and inspect them before they mate, even if the dust caps were on. Make this a rule for every joint, because dirt is the most common cause of loss on temporary jobs.
Can Radix make OB reels to my drawing?
Radix makes armoured and tactical fibre cable assemblies and reels to the customer's drawing in Pune. Send the fibre type, count, connectors and length per reel through the quote form, and the reply states what is offered.