What Are Coaxial Cables?

Coaxial cables are transmission lines consisting of a central inner conductor surrounded by a cylindrical dielectric and an outer conducting shield, all sharing the same geometric axis. This structure guides electromagnetic energy along the cable while confining the fields entirely between the inner conductor and the shield, preventing both signal leakage and interference from external sources. Coaxial cables are a fundamental component of radio frequency (RF) systems, connecting transmitters, receivers, test instruments, and antennas across frequency ranges from DC to tens of gigahertz.

The design draws from transmission line theory and electromagnetic field theory, with the geometry of the two conductors setting the cable's characteristic impedance and its propagation behavior. The most widely standardized impedances are 50 ohms, used in most RF and microwave applications, and 75 ohms, used in broadcast video and cable television distribution.

Structure and Characteristic Impedance

The characteristic impedance of a coaxial cable is determined by the ratio of the outer conductor's inner radius to the inner conductor's outer radius, and by the relative permittivity of the dielectric material between them. A lower-loss dielectric, such as foamed polyethylene, reduces the cable's attenuation per unit length and also sets the velocity factor, the ratio of the signal's propagation speed to the speed of light in vacuum. The Tutorial Web introduction to RF coaxial cables explains how these physical parameters govern impedance, capacitance, and inductance per unit length in standard designs. Cables designed for high-power or high-frequency service use thicker center conductors, lower-loss dielectrics, and precision outer conductors to minimize resistive and dielectric losses.

Signal Propagation and Shielding

Coaxial cables support the transverse electromagnetic (TEM) propagation mode, in which both the electric and magnetic fields are perpendicular to the direction of propagation. The TEM mode has no cutoff frequency, which allows coaxial cables to operate from DC upward without the low-frequency limitations of waveguides. Because the fields are confined between the conductors, the cable provides inherent shielding against external electromagnetic interference while preventing the cable itself from radiating. The shielding effectiveness depends on the outer conductor's coverage and conductivity; braided shields offer flexibility but leave small gaps, while solid-wall or semi-rigid outer conductors achieve the best isolation. IEEE Xplore research on shielding effectiveness in coaxial cables quantifies how shield construction affects rejection of external fields at microwave frequencies.

Relationship to Transmission Lines and Waveguides

Coaxial cables are a subset of the broader class of transmission lines, which includes twin-lead, stripline, and microstrip structures used in printed circuit boards. Unlike rectangular waveguides, which require the signal frequency to exceed a cutoff determined by the guide dimensions, coaxial cables carry signals at all frequencies from DC to the point where higher-order TE and TM modes begin to propagate. This multimode onset sets the practical upper frequency limit of a given cable design, typically a few tens of gigahertz for precision laboratory cables. The All About Circuits reference on transmission lines covers the distributed-element model that describes coaxial and other transmission lines as networks of per-unit-length resistance, inductance, capacitance, and conductance.

Applications

Coaxial cables are used across a wide range of RF, microwave, and signal distribution systems, including:

• Antenna feed lines in radio and television broadcasting
• Cable television and satellite signal distribution networks
• Radar and electronic warfare systems connecting transmitters to antennas
• RF test and measurement equipment interconnections in laboratories
• Medical imaging systems such as ultrasound probes