Quick Answer: A BNC to Ethernet adapter is a passive video balun, sold in pairs. It converts a 75 ohm unbalanced coax video signal into a 100 ohm balanced signal on one twisted pair, and converts it back at the other end. It does not convert analog video to IP, does not create a network link and does not carry PoE, so an IP recorder cannot use it. Both ends must be the same balun type. Expect 150 to 300 m of usable range on Cat5e for composite video with a passive pair, and 500 m or more with a powered balun.
The confusion is in the word adapter. Nothing in the pair speaks Ethernet; the RJ45 is a cheap four-pair connector and the signal on it is analog baseband video at 1 V peak to peak. It exists so a coax camera can reuse structured cabling, or so four cameras share one Cat5e run. Coax and twisted pair are different transmission lines, and the balun is the only part that reconciles them.
What the Pair Actually Does
Each balun contains a small broadband transformer. Coax is unbalanced: the shield is ground and the centre conductor carries the signal. Twisted pair is balanced, so the transformer splits the signal into two opposite phases and recombines them at the recorder. Noise picked up along the run is common to both phases and cancels, which is why UTP often looks cleaner in a noisy plant.
The 75 to 100 ohm mismatch
Coax is a 75 ohm system and Cat5e is nominally 100 ohm. A transformer with a 1 to 1.33 turns ratio matches them, and the transformer, not the cable, does the matching. Cheap units skip the matching and rely on the run being short; those show smeared edges and colour loss.
Pinout and pair use
Most baluns use the blue pair on pins 4 and 5, or a screw terminal pair marked video plus and minus. One pair carries one video channel, so a four-pair Cat5e carries four cameras, and the spare copper can power them. Never split a pair across two baluns.
Types and Real Distance Limits
Distance claims on the box are measured on a bench with a clean picture. Field numbers are shorter, and the table below reflects what installers report.
| Type | Signal | Field distance | Power |
|---|
| Passive balun pair | Composite CVBS | 150 to 300 m | None |
| Active balun pair | Composite CVBS | 500 to 700 m | 12 V at one end |
| HD balun pair | TVI, AHD, CVI 1080p | 100 to 200 m | None |
| IP camera on UTP | Ethernet 100BASE-TX | 100 m | PoE or 12 V |
The last row is not a balun. An IP camera needs a real network port, and no passive transformer will turn an analog camera into an ONVIF device.
What the Adapter Cannot Do
It cannot digitise. If the recorder expects an IP stream, the pair delivers an analog waveform the port does not recognise. It cannot pass power unless the balun is wired for it, and it cannot bridge 50 ohm lab coax to a 75 ohm chain without loss.
Ground loops are the third limit. A balun breaks the shield connection, which usually cures hum bars, but it does not isolate a system already earthed at both ends through the power supply. If the picture still rolls, check the 12 V supply.
Installing a Pair Properly
Terminate the Cat5e to a jack or plug, keep the pair count straight, and label which pair feeds which camera. Do not share conduit with a 230 V feed. At the camera, keep the balun tail short: 300 mm of unshielded lead picks up inverter noise.
For the coax side, use the correct connector rather than a stack of adapters. A worn BNC coupler in the middle of a run adds a reflection, and reflections in a video signal show as a ghost edge. Buy coaxial cable adapters with the right gender and impedance, and test the run with a known good camera before you climb the ladder twice.
Practical Scenarios
- Four analog cameras, one existing Cat5e run: fit four balun pairs on separate pairs and send power on a fifth conductor pair from a local supply.
- Retrofit where coax is already in the wall: keep the coax and skip the balun; adding a pair only helps if the coax is damaged or the run is too noisy.
- New IP installation: do not buy baluns, buy CCTV installation cables rated for the run and a PoE switch.
- 250 m to a gate camera: use an active balun pair with a 12 V supply at the camera end, not a passive pair plus hope.
FAQ
Can a BNC to Ethernet adapter connect a camera to a network?
No. It carries an analog video signal over twisted pair and nothing else. To put a camera on a LAN you need an encoder that digitises the video, or an IP camera. A balun plugged into a switch port will not negotiate a link.
How far can analog video run on Cat5e?
With a passive pair, plan on 150 to 300 m for composite video; quality falls off gradually rather than failing at a fixed point. A powered balun roughly doubles that. HD formats over coax-style baluns are much shorter, typically 100 to 200 m.
Does the impedance mismatch damage anything?
No. A 75 to 100 ohm mismatch causes reflections and a softer image, not damage, and a transformer balun corrects most of it. The practical symptom of a badly matched pair is a slight ghost to the right of vertical edges.
