Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without changing their characteristic impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line.

Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The transmission of energy in the line occurs totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. The inner conductor can be made of braid and the outer conductor can be made of corrugated tube for greater flexibility, but this comes at the cost of increased ohmic losses and lower phase velocity. The outer conductor can also be made of (in order of increasing leakage) wound foil, woven tape, or braid.

In radio-frequency applications up to a few gigahertz, the wave propagates only in the transverse electric magnetic (TEM) mode, which means that the electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter is roughly inversely proportional to the cutoff frequency.

Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. Unfortunately, all dielectrics have loss associated with them, which causes most coaxial lines to have more loss than open wire lines. In typical applications the loss in polyethylene is comparable to the ohmic loss at 1 GHz and the loss in PTFE is comparable to ohmic losses at 10 GHz. Most cables have a solid dielectric; others have a foam dielectric which contains as much air as possible to reduce the losses. Foam coax will have about 15% less attenuation but can absorb moisture — especially at its many surfaces — in humid environments, increasing the loss. Stars or spokes are even better, but more expensive. Furthermore the lower dielectric constant of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency, lowering ohmic losses.

Important parameters
 

• The characteristic impedance in ohms (Ω) is calculated from the ratio of the inner and outer diameters and the dielectric constant. Assuming the dielectric properties of the material inside the cable do not vary appreciably over the operating range of the cable, this impedance is frequency independent.

• Capacitance, in farads per metre.

• Resistance, in ohms per metre.

• Attenuation or loss, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. In designing a system, engineers must consider not only the loss in the actual cable itself, but also the insertion loss in the connectors.

• Outside diameter, which dictates which connectors must be used to terminate the cable.

• Velocity of propagation, which depends on the type of dielectric.

• Cutoff frequency
  
   

Standards

Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables.

A series of standard types of coaxial cable were specified for military uses, in the form "RG-#" or "RG-#/U" (RG from Radio Guide, /U indicates multiple uses). They go back to World War II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214," are given for military cables and manufacturer's catalog numbers for civilian applications. However, the RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of the old RG-series cables.

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