UK How coaxial cable works
A coaxial cable is a cable built in four concentric layers: a solid or stranded centre conductor, a dielectric insulator around it, a conductive shield of foil, braid or both, and a plastic outer jacket. The name comes from the fact that the conductor and the shield share the same geometric axis. Oliver Heaviside patented the design in 1880, and the basic construction has barely changed since.
The signal travels along the centre conductor and returns along the inside of the shield. Because the electromagnetic field is held between the two, very little energy radiates out of the cable and very little external noise gets in. The spacing between the conductor and the shield, and the material of the dielectric, set the cable’s characteristic impedance, which is what lets coax carry high frequencies cleanly when the cable, connectors and equipment all match.
A coax cable is the same thing under its short name. In everyday use it usually means the round cable behind a TV, satellite box or cable modem, but the same principle runs through CCTV, broadcast video, radio antennas and test equipment.
50 ohm vs 75 ohm coax
Coax comes in two main impedances, and they are not interchangeable even though the cables can look the same. Mixing them causes reflections, which show up as weak signals, ghosting or dropped data.
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75 ohm coax. The video and broadcast standard. It is used for terrestrial and satellite TV, cable broadband, analogue CCTV, HD-TVI and AHD cameras, and SDI video.
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50 ohm coax. The radio-frequency standard. It is used for antenna leads on Wi-Fi, cellular and GNSS radios, two-way radio, test equipment and timing signals, because it handles transmit power with low loss.
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Matching the whole run. Cable, connectors, splitters and the equipment at each end should share one impedance. A 50 ohm patch lead in a 75 ohm SDI run is a common cause of short reach.
Coax cable types
Coax is named by an RG (radio guide) number, a legacy military designation still used to describe a cable’s size and impedance, or by a manufacturer’s low-loss series name. Thicker cables lose less signal per metre but are stiffer and harder to route.
| Cable | Impedance | Typical outer diameter | Relative loss | Common uses |
|---|---|---|---|---|
| RG-59 | 75 Ω | Around 6 mm | Higher | Analogue CCTV, short SD and HD-SDI links |
| RG-6 | 75 Ω | Around 7 mm | Moderate | TV, satellite, cable broadband, 3G to 12G-SDI |
| RG-11 | 75 Ω | Around 10 mm | Lower | Long video and broadband trunk runs |
| RG-58 | 50 Ω | Around 5 mm | Higher | Test leads, short antenna and timing links |
| Low-loss 400 series | 50 Ω | Around 10 mm | Low | Roof and mast antenna runs for cellular and Wi-Fi |
Shielding varies too. Single-shielded cable has one foil or braid layer, while dual-shield (foil plus braid) and quad-shield cables add layers for electrically noisy sites, such as near drives, welders or high-voltage cabling. Outdoor and direct-burial grades add a UV-stable or gel-filled jacket.
Coax connectors
The connector has to match the cable’s diameter and impedance as well as the socket on the equipment. Most coax connectors fall into a handful of families.
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F-type. The threaded 75 ohm connector on TVs, satellite receivers and cable modems.
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BNC. A quarter-turn bayonet connector for video and RF. Our guide to how BNC connectors lock and which impedance to use covers it in detail.
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SMA and RP-SMA. Small threaded 50 ohm connectors for Wi-Fi, cellular and GNSS antennas on routers, gateways and embedded computers.
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N-type and TNC. Larger, weather-resistant threaded 50 ohm connectors used on outdoor antennas and mast-mounted equipment.
Where coax is used in industrial systems
Twisted-pair Ethernet has replaced coax for computer networking, but coax is still the right cable wherever a video or radio signal has to travel without picking up noise.
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Analogue HD CCTV. HD-TVI, AHD and HD-CVI cameras send HD video over existing 75 ohm coax, so sites can upgrade cameras and record them with HD-TVI capture cards without pulling new cable.
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Broadcast and machine vision video. SDI cameras and switchers connect over RG-6 class coax to SDI capture cards in recording, replay and inspection systems.
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Antenna leads. Cellular, Wi-Fi and GNSS antennas connect to industrial IoT gateways through 50 ohm coax, often run from a cabinet to a roof or mast antenna.
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Vehicles and rolling stock. In-vehicle computers use short 50 ohm leads to roof-mounted combination antennas for 4G or 5G, Wi-Fi and GPS.
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Test and timing. Oscilloscopes, signal generators and 10 MHz or 1PPS timing references use 50 ohm coax.
Coax vs Ethernet cable
Coax and Ethernet cable solve different problems. Coax carries a single high-frequency signal through one shielded conductor, which suits video, RF and cable broadband using the DOCSIS standard. Ethernet uses four twisted pairs terminated in RJ45 plugs and carries packet data between computers, switches and cameras, with Power over Ethernet on the same cable.
For new camera networks, IP cameras on Ethernet are now the default. Coax remains the practical choice where the cable is already installed, where runs are long for analogue HD video, or where the signal is radio frequency rather than data.
Quick answers on coaxial cable
Coax cable: common questions
What is a coax cable used for?
Video, broadband and radio signals. Typical uses are TV and satellite feeds, cable internet, CCTV, SDI video, antenna leads for Wi-Fi, cellular and GPS, and laboratory test equipment.
What does a coaxial cable look like?
A round cable with a single centre pin. Cut across it and you see a copper core, a white or clear insulator, a foil or braided shield and a black or white jacket. The end usually carries a threaded or bayonet connector.
Do you need a coax cable for Wi-Fi?
Only if your broadband arrives over cable. A cable modem needs coax from the wall, but Wi-Fi itself is radio. Fibre and DSL services use other cables, and an access point connects to the network over Ethernet.
How do you test a coax cable?
Start with a continuity check. A multimeter should show continuity along the centre conductor and along the shield, and no connection between the two. A coax tester or time-domain reflectometer then locates breaks, shorts and poor connectors.
Is coax better than Ethernet?
For RF and analogue video, yes. For data networks, no. Coax shields a single high-frequency signal well, while Ethernet is the standard for computer networks and PoE devices.
We fit antenna leads, cellular modems and video capture cards in our industrial and in-vehicle computers as standard, and route and test the coax before the system ships. Tell us your antenna positions, cable runs and video sources, and we will specify the cable and connectors to match.