Characteristic Impedance

The average characteristic impedance of a coaxial cable is determined by the ratio of the inner diameter of the outer conductor of the centre conductor, and by the dielectric constant of the insulating material between the conductors.  The most common values are 50, 75 and 95 Ohms.  It is important to note that the actual input impedance at a particular frequency may be quite different from the characteristic, or surge impedance of the cable due to reflection in the line.  The VSWR of a particular length of cable is an indicator of the difference between the actual input impedance of the cable and its average characteristic impedance.

Impedance (VSWR) Uniformity

The VSWR of a cable assembly is the summation of reflections of the connectors, the connector termination technique and the cable.  The VSWR is the summation of random and periodic reflections within the cable.  The VSWR will vary with frequency.

Capacitance & Impedance Stability

The capacitance and impedance of long lengths of cable will exhibit very little change over their operating temperature range (<2%).  Semi flexible foam dielectric cables normally exhibit the least change.  In short cable lengths at frequencies > 1000 MHZ, the VSWR can vary significantly if dielectric movement at the connector interface occurs.

Attenuation / Insertion Loss

In general, this parameter is the extent to which the amplitude of a signal is decreased by its passage through the cable and connectors; more specifically the ratio of the output signal power, voltage or current to the input-signal power, voltage or current.  This ratio is usually expressed in decibels (dB).

Attenuation Uniformity

The attenuation of any cable may not change uniformity as the frequency changes.  Random and periodic impedance variations give rise to random and periodic attenuation responses.  Narrow band attenuation “Spikes” may occur.  If required, cable can be procured in various lengths where a maximum attenuation variation from nominal is specified on a swept basis.

Attenuation Stability

The attenuation of braided cables can increase with time and flexure.  The change with time can be caused by corrosion of the braided shield, by contamination of the primary insulation due to jacket plasticizers, and by moisture penetration through the jacket.  Attenuation degradation is more pronounced at frequencies above 1 GHz.  Cables having bare copper and tinned copper braids exhibit far greater attenuation degradation than do cables having silver plated copper braids.

Velocity of Propagation

The velocity of propagation of a cable is determined primarily by the dielectric constant of the insulating material between the conductors.  This property is usually expressed as a percentage of the velocity of light in free space.  This information is only obtainable from the cable manufacturer.

Electrical Length Stability

Applications such as antenna feed systems require cable assemblies trimmed to an electrical length rather than a physical length.  In these applications, the change of electrical length of a cable with temperature, flexure, tension and other environmental factors are critical.