5.2.2. Coaxial

Introduction

Coaxial cable, often called coax, is another common copper medium used in networking and communication. While twisted pair dominates modern local networks, coax still appears in broadband internet, television distribution, and some specialized data links. In this chapter you will see what makes coaxial cable different, its structure, where it is used, and what advantages and limitations it has compared to other copper options.

Structure of a Coaxial Cable

Coaxial cable gets its name from its geometry. "Coaxial" means that the inner conductor and the outer conductive shield share the same axis. This geometry is the key to how coax behaves electrically.

A typical coaxial cable has four main layers, starting from the center:

1. A central conductor
2. An inner insulator (dielectric)
3. An outer conductor (shield)
4. An outer protective jacket

In more detail:

The central conductor is usually a solid copper wire or sometimes copper clad steel. This carries the signal. Around it is the dielectric, an insulating material such as plastic or foam that keeps the center conductor separated from the outer conductor and fixes the distance between them. The outer conductor is commonly a braided copper shield, sometimes combined with a foil shield, that surrounds the dielectric. It serves both as a reference conductor for the signal and as a shield against external interference. Finally, the outer jacket is an insulating protective layer that guards the cable against physical damage, moisture, and sunlight.

The precise spacing between the inner conductor and the shield, and the type of dielectric material, determine key electrical properties such as characteristic impedance and signal velocity. This controlled geometry is what distinguishes coax from simple two wire cabling.

Signal Transmission in Coax

In coaxial cable, the signal is not only flowing inside the copper wire. The electromagnetic field associated with the signal exists mainly in the space between the center conductor and the outer shield, inside the dielectric.

The inner conductor and the outer shield form a transmission line with a defined characteristic impedance, usually written as $Z_0$. For most network related coax types, $Z_0$ is either $50 \,\Omega$ or $75 \,\Omega$.

If the impedance is matched correctly, most of the signal power is transferred from source to cable to load, with minimal reflections. If there is a mismatch, some of the signal reflects back toward the source. Reflections can cause standing waves, signal degradation, and data errors at high frequencies.

The coaxial geometry has an important advantage. The outer shield surrounds the inner conductor and is usually connected to ground. This arrangement tends to confine the electromagnetic field inside the cable, which reduces radiation from the cable and also reduces the effect of external interference entering the cable. This is a major reason why coax is used where low noise and controlled behavior at high frequencies are important.

Common Coax Types in Networking and TV

There are many coax types, but two impedance values are especially important: 50 $\Omega$ and 75 $\Omega$. Each is used in different applications.

The "RG" codes are historical designations and do not by themselves guarantee properties, because modern manufacturers may vary materials and performance. However, they are still widely used in practice to refer to general cable types.

Historically, some Ethernet standards used 50 $\Omega$ coax, such as 10BASE5 (thick Ethernet) and 10BASE2 (thin Ethernet). These used a bus topology with terminators at each end, and devices tapped into the cable along its length. Modern Ethernet uses twisted pair and fiber instead, but you may still encounter coax in older installations or documentation.

In modern access networks, 75 $\Omega$ coax is strongly associated with cable television and cable internet. A common example is RG 6 coax running from a wall outlet to a cable modem or set top box. Larger distribution runs, for example between street cabinets or along building risers, may use thicker 75 $\Omega$ coax types like RG 11 that support longer distances with less signal loss.

Connectors and Termination

Proper connectors and termination are crucial for coaxial cable performance. A connector must preserve the coaxial structure as closely as possible, keep the impedance consistent, and provide solid mechanical attachment.

Some common connector types include:

BNC connectors are used in test equipment, some older Ethernet, and some radio frequency links. They provide a bayonet style twist lock. F type connectors are widely used in cable TV and cable internet equipment. They are normally 75 $\Omega$ and are found on wall plates, splitters, and consumer devices like modems. N type connectors are larger, weather resistant connectors often used outdoors for radio, Wi Fi antennas, and some microwave links.

For legacy bus style networks that used coax, termination resistors were required at each end of the bus. These resistors had the same value as the cable’s characteristic impedance, for example 50 $\Omega$. Their purpose was to absorb the signal energy and prevent reflections. In modern home coax systems for TV and cable internet, the termination is usually provided by the devices and splitters in the system, but the same principle still applies. Open or badly terminated coax segments can cause reflections that degrade signal quality.

When connectors are attached poorly, for example if the braid is not properly folded back or if stray wires touch the center conductor, both impedance and shielding are affected. This can lead to local reflections and higher susceptibility to noise, even if the rest of the cable run is good.

Advantages of Coaxial Cable

Coaxial cable offers several practical advantages in certain situations, especially when compared to simple two wire cabling of similar thickness.

One key advantage is good shielding. The outer conductor reduces the pickup of external electromagnetic interference and also limits how much the cable radiates. This makes coax useful in noisy environments and for radio frequency signals.

Another advantage is predictable high frequency performance. Because the geometry and dielectric are controlled, the characteristic impedance and signal velocity are stable. This predictability is important for broadband television, DOCSIS cable internet, and radio systems where wide frequency ranges are used.

Coax can also support longer runs at higher frequencies than unshielded twisted pair of similar size, before attenuation becomes excessive. While exact distances depend on cable type and frequency, coax is often chosen when a signal must travel many tens or hundreds of meters without active repeaters.

Mechanically, coax tends to be robust and can be routed relatively easily through buildings. Different jacket types allow indoor, outdoor, and direct burial installations.

Limitations and Drawbacks

Despite its strengths, coaxial cable has notable limitations in modern networking.

The cable is thicker and less flexible than standard twisted pair, especially for the larger low loss types. This makes dense terminations and tight patch panels more difficult. The connectors are larger and usually slower to install. Creating clean, reliable terminations often requires specific stripping tools and some practice.

From a network design point of view, coax has mostly been replaced in local area networks by twisted pair and fiber. Legacy coax Ethernet relied on a shared medium bus topology where all devices tapped into the same cable segment. Collisions were common and any physical problem along the cable segment could affect many devices.

Scalability and flexibility are also limited. Adding a device to a legacy coax bus could require interrupting the cable run and installing extra connectors or taps. In contrast, twisted pair Ethernet with switches allows simple star connections where adding or moving a device is as easy as changing a patch cable.

Another factor is cost. While coax itself is not always more expensive per meter than high quality twisted pair, the overall system cost, including connectors, splitters, and labor for specialized termination, can be higher, especially if many end points are needed.

Finally, coax remains a copper medium, so it does not provide the extremely long distances and immunity to electromagnetic interference that optical fiber can achieve. In many backbones and high capacity links, fiber is preferred.

Coax in Modern Broadband Networks

Even though coax is rare in new Ethernet LAN installations, it is still important in broadband access networks. Many cable internet providers use a hybrid fiber coaxial system, sometimes called HFC.

In an HFC architecture, optical fiber runs from the provider’s core network to a neighborhood node. From that node, signals are converted and distributed to individual homes or buildings over 75 $\Omega$ coaxial cables. The same coax that carries television channels also carries data signals using DOCSIS technology.

Within a building, splitter devices divide the coax feed into branches that serve different rooms or units. Each endpoint device, for example a cable modem, connects to the coax with an F type connector. The shared nature of the coax segment is managed at the provider level with careful frequency planning and channel assignments.

From the perspective of an end user home network, coaxial cable often appears as the link from the wall outlet to the modem. Behind the modem, twisted pair Ethernet and Wi Fi typically provide local connectivity. This is a common example of different physical media working together in one overall network.

Summary

Coaxial cable is a structured copper medium with a central conductor, surrounding dielectric, outer shield, and protective jacket. Its coaxial geometry provides controlled impedance and good shielding, which make it effective for high frequency and broadband signals. While coax has largely disappeared from new Ethernet LAN cabling, it remains central in cable television, cable internet, radio systems, and some specialized links. Understanding its structure, impedance, and termination helps you recognize where coax fits among the various transmission media in modern networks.

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