Two coaxial cables can look identical, take the same connectors, and behave completely differently in a system. The dividing line is impedance, and for most installations the answer is settled before the cable is chosen.
Quick answer
| 50 ohm | 75 ohm | |
|---|---|---|
| Standard for | Radio, test equipment, antenna feeds, some instrumentation | Video, television, satellite, CCTV, digital audio over coax |
| Why that value | A compromise favoring power handling and low loss in radio systems | A compromise favoring low loss for signal distribution |
| Typical cables | RG58, RG8, RG213, LMR series | RG59, RG6, RG11 |
| Connector families | BNC, N, SMA, TNC | F, and the 75 ohm versions of BNC and RCA |
| Mixing the two | Degrades the signal through reflections; does not damage equipment | Same |
Impedance is geometry, not resistance
Characteristic impedance is set by the cable’s dimensions and its dielectric material, and it has nothing to do with the DC resistance an ohmmeter reads.
That produces the single most common misunderstanding in coax work. Put a meter across the end of a 75 ohm cable and you will not see 75 ohms — you will see either an open circuit or the loop resistance of the center conductor and shield, which is a fraction of an ohm per meter. The 75 ohm figure describes how the cable behaves to a traveling wave, which is why it only means anything at the frequencies the cable is used at.
If you need to identify an unknown cable, the practical tests are the printed legend on the jacket and the dielectric diameter, not a resistance measurement.
Why the industry settled on two values
Both figures are engineering compromises, and they were optimized for different jobs.
For radio systems, the aim is to move power efficiently and to handle high power without breakdown. The compromise that balances those against loss lands near 50 ohms, which is why transmitters, antennas, test instruments and the cables that feed them are 50 ohm.
For signal distribution — broadcast video, television, satellite, CCTV — the aim is different. The signal is low power and loss per unit length is what matters most, and the value that minimizes loss for an air-spaced or foam cable lands near 75 ohms. So the video and television world standardized there, and every F connector, every distribution amplifier and every television input is built around it.
The point worth carrying forward: neither value is better. They are different design targets, and equipment is built to match one of them.
What a mismatch does
Where two impedances meet, part of the signal reflects instead of passing through. In a system with several discontinuities — a wrong cable, an unmatched connector, an un-terminated splitter port — the reflections add up into a standing wave pattern along the cable.
The symptoms depend on the signal.
With analog video, reflections arrive slightly after the main signal and appear as a ghost or a smeared edge. With digital services, reflections raise the error rate, and the failure is more abrupt: a picture that works perfectly until it drops out entirely. With a return-path or bidirectional service such as satellite internet or MoCA, a mismatch hurts in both directions, and the upstream path is often the more sensitive of the two.
Using 50 ohm cable in a 75 ohm video system is the mismatch people actually run into, usually because a reel of RG58 was close to hand. It does not damage anything. It raises loss and reflections, degrades the picture, and is worth undoing if the run is not working.
The connector trap
The BNC connector is the one place where the impedance is genuinely hard to see, because 50 ohm and 75 ohm BNC plugs and sockets look the same, fit each other, and mate without complaint.
They differ internally — the 75 ohm version uses a smaller center contact and a differently shaped dielectric — and a 50 ohm plug on a 75 ohm socket introduces the mismatch the impedance was supposed to avoid.
Three practical rules follow. Buy BNC for the application, specifying the impedance. Do not swap BNC between a video system and a radio system. And when a video installation misbehaves for no obvious reason, check the BNCs before suspecting the cable. Connector selection is covered in more detail here.
F connectors do not have this problem: they are a 75 ohm family, used with 75 ohm cable.
Equipment that expects one or the other
| Equipment | Impedance |
|---|---|
| Television, set-top box, satellite receiver | 75 ohm |
| CCTV camera and DVR, analog and HD analog | 75 ohm |
| Video distribution amplifier, splitter | 75 ohm |
| Coaxial S/PDIF digital audio | 75 ohm |
| Two-way radio, scanner, antenna feed | 50 ohm |
| Signal generator, spectrum analyzer, oscilloscope input | 50 ohm, normally |
Reading the equipment’s socket before buying cable settles the question in seconds, and it is the step most often skipped.
Scenarios
A house with a satellite dish and four outlets. All 75 ohm: RG6 from the dish, a 75 ohm splitter, F connectors throughout.
A ham radio operator adding a run to a new antenna. 50 ohm cable and N or PL-259 connectors. Nothing from the television cabinet belongs in that run.
A rack with a 50 ohm test instrument and a 75 ohm video router. Two separate cable stocks, and connectors that cannot be confused. Mixing them in one drawer is how a wrong cable ends up in a system.
A CCTV system where the picture has ghosting. Check the cable impedance matching first, then the connectors, then whether a splitter port is left open. An un-terminated port is a reflection source.
FAQ
Can I use 50 ohm cable for television? It will carry the signal, with higher loss and reflections. It does not damage the equipment, and it is the wrong cable for the job.
Can I use 75 ohm cable for a radio antenna? No. Radio equipment is 50 ohm, and the mismatch degrades both transmitted power and received signal.
How do I tell 50 ohm and 75 ohm cable apart? Read the jacket legend. Failing that, the dielectric diameter and the center conductor size differ, and a 50 ohm cable is usually thicker for the same family.
Will an ohmmeter tell me the impedance? No. It reads DC resistance, which is unrelated to characteristic impedance.
Do 50 and 75 ohm BNC connectors fit each other? Yes, and that is the problem — they look and fit alike but are not the same part.
Does impedance matter on a short run? Less than on a long one, because there is less cable to accumulate reflections. It still matters at the connectors and at any splitter.