What Is a Coaxial Cable? Four Layers and One Impedance
Quick Answer: A coaxial cable carries a signal on a centre conductor surrounded by a dielectric, a metallic shield and an outer jacket, all concentric on the same axis. The geometry sets the characteristic impedance, most commonly 75 ohm for video and audio, and that impedance must stay constant through every connector.
The word coaxial describes the construction: the conductor and the shield share an axis. That geometry gives the cable a controlled impedance and makes it relatively immune to interference, which is why the same basic design has carried television, radio and digital audio for decades.
The Four Layers
Each layer has one job, and substituting a cheaper material in any of them changes the cable’s behaviour at high frequency.
Centre conductor
Solid copper or copper-clad steel carries the signal. Solid copper is better for long runs and for bending that stays put; stranded copper is more flexible for patch use.
Dielectric
Foamed polyethylene or solid PTFE holds the conductor at a fixed distance from the shield. The dielectric constant and the diameter set the impedance, which is why a crushed cable changes behaviour.
Shield and jacket
Aluminium foil plus braid provides screening; the jacket protects against moisture and abrasion. Shield coverage is usually quoted as a percentage, and higher coverage means better rejection of interference.
Why 75 Ohm Is the Common Value
Characteristic impedance is a property of the geometry, not a resistance you can measure with a multimeter. The ratio of shield diameter to conductor diameter sets it.
75 ohm for video and digital audio
Television, satellite, cable and S/PDIF digital audio all use 75 ohm. The standard was chosen as a compromise between loss and power handling for broadband signals.
50 ohm for radio and test
Two-way radio, laboratory instruments and antenna systems use 50 ohm. Mixing 50 and 75 ohm parts creates a mismatch and reflections, which show up as loss rather than as a visible fault.
Common Coaxial Types
The table covers the types most often specified for video, audio and short data links.
| Type | Impedance | Centre conductor | Typical use |
|---|---|---|---|
| RG6 | 75 ohm | 18 AWG copper-clad steel | Satellite, cable TV, digital audio |
| RG59 | 75 ohm | 20 AWG copper-clad steel | CCTV, short baseband runs |
| RG11 | 75 ohm | 14 AWG copper-clad steel | Long drops, lower loss |
| RG58 | 50 ohm | 19 AWG stranded | Radio, test leads |
| RG213 | 50 ohm | 13 AWG stranded | Two-way radio, higher power |
Where Coax Still Wins
Coax survives because it is robust, cheap and easy to terminate in the field. A single cable can carry a broadband signal over tens of metres without active electronics.
Digital audio at 75 ohm
S/PDIF coaxial uses the same 75 ohm construction and RCA or BNC connectors. A video-grade coax usually works, provided the impedance stays 75 ohm through the connectors.
Legacy video distribution
Analogue camera systems and RF distribution still rely on coax because the installed base is enormous and the cable outlasts the electronics.
Installation Rules That Preserve Impedance
Keep bends gentle, avoid crushing the dielectric with cable ties, and use connectors matched to the cable diameter rather than universal fittings. Every transition – a barrel, a wall plate, a push-on connector – is a potential impedance discontinuity. For digital signals, a single well-terminated run outperforms a longer run with two joints, even when the total length is the same.
Two Coax Runs
Coax is still the cheapest way to move a broadband signal across a building.
A satellite drop from roof to living room
RG6 at 75 ohm with compression F connectors handles the run. The shield must be bonded at both ends, or the receiver loses transponders in rain.
A digital audio feed from a streamer to a DAC
A 75 ohm coaxial cable with RCA plugs carries S/PDIF cleanly at 1 to 2 m. A video-grade coax works if the connectors maintain the impedance.
Frequently Asked Questions
What is the difference between coaxial and RCA cable?
RCA describes the connector; coaxial describes the cable construction. A digital audio RCA cable is usually a 75 ohm coaxial cable with RCA plugs on both ends.
Can I use 50 ohm coax for TV?
It will usually produce a picture at short lengths, but the mismatch causes reflections and loss. Use 75 ohm cable for television and digital audio.
Why is coax shielded?
The shield confines the signal and blocks external interference. Without it, the centre conductor acts as an antenna and the cable becomes unusable at high frequency.
Does coax carry power?
Not for general purposes, but satellite and antenna systems can send DC power up the cable to a remote amplifier or LNB. That is a system feature, not a property of the cable.
How far can a coaxial cable run?
It depends on the signal and the cable. Baseband video over RG59 is usually limited to a few hundred metres, while RF distribution over RG11 reaches further because the signal is modulated onto a carrier.
How this guide was built: Built from coaxial transmission-line practice: impedance geometry, shield coverage, attenuation by cable size and termination effects. Disclosure: No manufacturer names or prices are given. Attenuation and reach vary by cable specification; confirm with the datasheet.