A coaxial cable carries electrical signals through a central copper wire while using a surrounding metal shield to block interference and keep the signal contained. It’s a deceptively simple design: four concentric layers, each with a specific job, working together to move TV, internet, and radio signals across long distances with minimal loss. Here’s what’s actually happening inside the cable.
The Four Layers and What Each Does
If you slice a coaxial cable in cross-section, you’ll see four rings nested inside each other like a bullseye. At the center is a solid or stranded copper wire (sometimes copper-plated steel) that carries the actual signal. Surrounding that core is a thick plastic insulating layer called the dielectric, which keeps the center conductor physically separated from the next layer and controls the cable’s electrical properties.
Outside the dielectric sits a woven metallic braid, sometimes combined with metallic tape, forming the shield. Some cables use up to four layers of shielding for extra protection. Finally, the outermost layer is a plastic jacket that protects everything from physical damage, moisture, and UV exposure. The key to understanding how coax works is recognizing that the signal doesn’t just travel down the center wire. It travels in the electromagnetic field between the center conductor and the outer shield, with both conductors working as a pair.
How the Shield Blocks Interference
The outer metallic braid acts as a Faraday cage, a continuous conductive enclosure that blocks electromagnetic fields from crossing in either direction. External interference from nearby electronics, radio towers, or power lines can’t penetrate the shield to corrupt the signal inside. Equally important, the signal inside the cable can’t leak out and cause interference with other equipment.
This two-way blocking is what makes coax fundamentally different from a simple pair of wires. Unshielded wires act like antennas, picking up stray signals from the environment and radiating their own energy outward. Coax keeps everything contained. That’s why it became the standard for carrying television signals into homes and connecting antennas to radios: the signal arrives clean, even when the cable runs past dozens of potential interference sources.
Why Signal Quality Drops Over Distance
No cable is perfect. As a signal travels along a coaxial cable, it gradually weakens, a process called attenuation. Two main factors drive this loss, and they behave differently depending on the frequency of the signal.
The first factor is resistance in the conductors themselves. At higher frequencies, electrical current doesn’t flow evenly through the entire cross-section of the wire. Instead, it concentrates near the surface, a phenomenon called the skin effect. This effectively shrinks the usable area of the conductor, increasing resistance and wasting more energy as heat. The higher the frequency, the thinner the layer of metal carrying the current, and the greater the loss. Silver-plated conductors improve this slightly, since silver conducts better at the surface.
The second factor is the dielectric material absorbing energy. The plastic insulation between the conductors isn’t a perfect insulator. At higher frequencies, it absorbs a small fraction of the signal’s energy. Cables designed for maximum performance use foam or even air as the dielectric, since air absorbs almost nothing. The tradeoff between conductor loss and dielectric loss is frequency-dependent: conductor losses dominate at low to mid frequencies, while dielectric losses take over at very high frequencies.
The practical result is straightforward. Longer cables lose more signal, higher frequencies lose signal faster, and thicker cables with better materials lose less. This is why your cable installer might use a signal amplifier for long runs, or why a thicker cable is specified for distances over a certain length.
50 Ohm vs. 75 Ohm: Two Standards for Different Jobs
You’ll often see coaxial cables rated at either 50 ohms or 75 ohms. This number, called characteristic impedance, is determined by the physical dimensions of the cable and the dielectric material. It’s not resistance you can measure with a simple meter. It describes how the cable handles the relationship between voltage and current as a signal passes through.
The 75-ohm standard is what you’ll find in home cable TV, satellite, and antenna connections. It’s optimized for low signal loss, making it ideal for carrying weak signals from an antenna or distributing video across a building. There’s also a convenient coincidence: a standard half-wave dipole antenna has a natural impedance of about 73 ohms, so 75-ohm cable is a near-perfect electrical match for the most common antenna designs.
The 50-ohm standard dominates in radio transmission, cellular equipment, Wi-Fi infrastructure, and lab instruments. It offers a better compromise between power handling and voltage capacity, which matters when you’re pushing a transmitter’s output through the cable rather than just receiving a faint signal. RF engineers default to 50-ohm cable for most active transmission work.
Matching the cable’s impedance to the equipment on both ends is critical. A mismatch causes part of the signal to bounce back toward the source instead of reaching the destination, wasting energy and potentially distorting the signal.
Common Cable Types
The “RG” numbering system dates back to military specifications, but the designations are still used to distinguish cable sizes and performance levels.
- RG-6 is the modern standard for home cable TV, satellite, and broadband internet. It uses an 18-gauge center conductor, handles high-frequency signals well, and maintains signal quality over longer runs. If a cable installer is wiring your house today, this is almost certainly what they’re using.
- RG-59 has a thinner 22-gauge center conductor and was the previous home standard. It works fine for short runs and lower-frequency applications like analog security cameras, but it loses too much signal over long distances to be practical for modern broadband or satellite use.
- RG-11 is a thicker, stiffer cable used for long outdoor runs or trunk lines. Its larger diameter means lower signal loss, but it’s harder to bend and route through walls, so it’s rarely used inside buildings.
Connectors: How the Cable Meets Your Equipment
The connector at the end of a coaxial cable maintains the same shielded, concentric geometry as the cable itself. A poorly designed connection would create a gap in the Faraday cage, letting interference in and signal out.
F-type connectors are the threaded fittings on virtually every home cable TV, internet, and satellite connection. You screw them on by hand, and the center conductor of the cable itself often serves as the connector’s center pin. BNC connectors use a quick twist-and-lock bayonet mechanism and are common in professional video, test equipment, and some security camera systems where cables need to be connected and disconnected frequently. SMA connectors are smaller, fully threaded fittings used in higher-frequency applications like Wi-Fi equipment, cellular antennas, and GPS devices, where their compact size and reliable connection at microwave frequencies matter most.
Each connector type is designed for a specific frequency range and cable size. Using the wrong connector, or installing the right one poorly, can degrade signal quality just as much as using a cheaper cable.