Coax to fiber: matching the conversion to the signal

A facilities manager asks for a quote to convert the coax in a warehouse to fiber, and the answer depends on a question nobody has asked: which signal is on the coax. A CCTV camera feed, a cable TV trunk and an Ethernet link over MoCA all ride 75 ohm coax, and each one converts to fiber with completely different hardware and a different distance limit. Buying the wrong converter wastes budget and, worse, produces a link that passes a bench test and fails in the field. Start with the signal, then the distance, then the power available at the far end.

Three jobs that get called coax to fiber

The phrase covers a family of products that share a connector and nothing else. In one case a technician has existing coax in a wall and wants an Ethernet link at the other end, which is a media conversion problem solved with MoCA or an active converter. In another, a headend needs to move a block of television channels or a satellite intermediate frequency across a campus without the loss of a long coax run, which is RF over fiber, where the signal stays analog and is carried as an intensity modulated optical waveform. In a third, someone is running composite video from a camera to a monitor and has heard that baluns are cheaper than coax, which is twisted pair video, not fiber. Each has its own budget, distance limit and failure mode.

Ethernet over coax with MoCA or a converter

MoCA is the standard answer for Ethernet over existing coax. A pair of adapters presents an RJ45 port at each end and negotiates a shared coax segment, typically at 1 Gbps with bonded MoCA 2.0 channels or 2.5 Gbps with MoCA 2.5. The adapters behave as a transparent bridge, so VLAN tags, multicast and jumbo frames pass in most implementations. A coax to Ethernet converter does the same job with a proprietary PHY, usually at 100 Mbps or 1 Gbps, and is common in building retrofit kits where the coax is a single home run per unit. Neither device is a fiber product; fiber enters the picture only when the run leaves the building or exceeds what coax loss allows.

Where a converter beats MoCA

A single home run of coax from a basement to a rooftop cabinet is a case where a point to point converter is simpler than MoCA. MoCA needs a shared segment and a functioning splitter tree, and it shares bandwidth with any television service on the same cable. A converter creates a dedicated link on that one cable and does not care what else sits on the tree, which matters where an ISP still uses the coax for DOCSIS.

What the coax has to be

Either approach needs 75 ohm coax in reasonable condition, and quad shield RG6 is the usual floor. MoCA 2.5 occupies 1125 MHz to 1675 MHz, so splitters must be rated above that range and the cable should be swept to 3 GHz for a clean margin. Old RG59 with a corroded shield will negotiate at a lower rate or not at all, and no adapter setting will repair a shield that is open behind a wall plate.

RF over fiber for long 75 ohm runs

RF over fiber keeps the RF signal intact and uses an optical carrier as transport. A transmitter takes the 75 ohm RF input, modulates a laser diode and launches the light into single mode fiber; a receiver at the far end recovers the RF envelope and presents 75 ohm again. Because fiber loss is about 0.2 dB per kilometer at 1310 nm and does not climb steeply with RF frequency, the technique moves a full CATV or satellite band across distances that coax cannot reach, from a few hundred meters to tens of kilometers with the right optics. The trade off is analog performance: noise figure, spurious free dynamic range and launch power all have to be budgeted, and the link is not interchangeable with a digital SFP.

Link budget and optical loss

Build the budget in two directions. Optical loss is fiber attenuation plus connector and splice losses, and an RFoF link typically tolerates a limited optical range before the carrier to noise ratio drops below the service requirement. RF loss is the flat insertion loss of the link, often quoted per product family, plus tilt across the band. Keep total optical loss inside the transmitter and receiver pair specification, and remember that a dirty connector can add 1 dB or more at a single point.

Splitters and taps on the RF side

An RFoF transmitter expects a defined input level, so the splitter tree in front of it has to be designed rather than inherited. Each two way split costs about 3.5 dB, and a tap costs its published value plus the through loss. Set the transmitter input with a pad instead of an amplifier running at maximum, because an overloaded laser produces distortion that no receiver can remove. Check the composite level, not just one channel.

Power, grounding and the remote end

The far end of a coax to fiber link usually needs power, and that is where retrofits run into trouble. A MoCA adapter needs a local outlet, which may not exist in a telecom closet or above a ceiling. A converter with PoE can be powered from the switch end, but only if the converter supports the standard and the injector is sized for the distance. An RFoF receiver draws a few watts and needs a clean supply, and a remote site without mains needs a solar or PoE arrangement designed for continuous draw. Grounding is the other half: coax shields arriving from outdoors must be bonded at the entry point, and adding a fiber link does not remove that requirement because the coax is still there.

Conversion types and what each one actually carries

Real-World Application Scenarios

Three deployments. A hotel with coax to every room uses MoCA adapters at each floor closet to deliver guest Wi-Fi backhaul over the existing cable, avoiding a full recable. A stadium distributes a satellite and CATV lineup to twelve concession stands across 2.4 km of single mode fiber using RF over fiber transmitters at the headend and receivers at each stand. A machine shop replaces a 300 ft composite video run from a camera to a control room with a fiber transmitter and receiver pair, because the coax route passes beside a variable frequency drive that wipes out the picture.

Related Guides on This Site

Choose the converter after you choose the signal. The copper fiber media converter guide covers the Ethernet side, including the speed, duplex and PoE questions that decide the part number, and the fiber media converter guide explains single mode against multimode and the optics that set distance. If the coax stays in service for the LAN, read the MoCA over coax guide first.

Frequently Asked Questions (FAQ)