In coaxial cable construction, the correct impedance is controlled by the inner and outer conductor dimensions and the dielectric material separating the two conductors.
A dielectric material exhibits achieved Velocity of Propagation (VP ) relative to the speed of light and a Dielectric Constant based on the material selected and the amount of air (or not) created within the construction.
Typically, cables with a solid dielectric will have a Low VP, and cables with a foamed or taped dielectric material—which allows for the introduction of more air—will have a higher VP.
In delivering Phase Matched assemblies, the selected cable type or construction must be carefully screened to ensure a consistent VP between cable batches.
This allows the electrical length tolerance to be achieved within mechanical length tolerance, an important consideration if cable assemblies are to be deployed in phase sensitive applications such as phased array radar.
In practice, cables are pre-conditioned for mechanical and thermal stability to minimize transmission line changes, keeping electrical length deviation to a minimum. With this achieved, cable assemblies can be manufactured using either Absolute Matching or Relative Matching.
