Outside the traditional parameters such as insertion loss, return loss and VSWR found on datasheets, additional design and construction methods can enhance a coaxial cable for more accurate and consistent performance or a longer operational lifetime. Many applications call for precise construction to ensure repeatable and reliable performance. This article dives into some of these requirements, the detrimental effects of poor cable construction and fabrication methods to enhance a coaxial cable’s performance.
INSERTION LOSS
Dielectric Choice
Figure 1 Typical RF/microwave cable assembly construction.
Air is a perfect dielectric, allowing signals to pass at nearly the speed of light, relatively unimpeded. This, however, is not practical in commercial coax construction, as air provides no structural support to uniformly separate the inner and outer conductors, so another dielectric is used (see Figure 1).
The main intrinsic sources of loss within a coax are:
- Resistive losses in the inner and outer conductors.
- Loss tangent and conduction current in the dielectric.
While the first source is hard to avoid, addressing the latter has more options. The relative permittivity or dielectric constant of the insulating material contributes to the overall attenuation of the coax, given by the equation
where Ld is the loss due to the dielectric, f the frequency, tanδ the loss tangent, εr the dielectric constant and c the speed of light. The dielectric constant of high density polyethylene (HDPE) is 2.34, low density PE is 2.28 and foamed PE drops to 1.6. Introducing air into the dielectric nearly halves the dielectric constant and greatly reduces the loss tangent. Solid dielectrics exhibit the highest insertion loss, low density dielectrics have moderate loss and expanded or microporous dielectrics have the least loss.
Still, using a solid dielectric provides benefits of homogeneity and isotropy, while lower density materials typically have a less consistent dielectric constant along the length of the cable. For heterogeneous, anisotropic systems such as microporous structures, the dielectric constant is heavily dependent on the shape of the cells within the structure,1 and the expanded dielectrics are also the least temperature sensitive, with stable loss and phase.
Wider Coax
Although a dimensionally smaller coax generally operates mode-free to higher frequencies, smaller diameter coax is often chosen because of its reduced mass and greater flexibility. The reason for this increased flexibility is shown by the bend stress equation:
where σ is the bend stress, E the elastic modulus, y the distance from the neutral axis and R the radius of the bend. The bend stress increases linearly from the neutral axis, so a thicker coax experiences more strain at the farthest point from the neutral axis than does a thinner cable.
A thicker cable, however, has less resistive loss due to the greater amount of material in the metallic conductors - yielding less overall loss. Equation 3 shows the loss per unit length is inversely proportional to the diameter of the inner and outer conductors.
where LR is the resistive loss of the conductors, d and D the diameters of the inner and outer conductors, σin and σout the conductivity of the inner and outer conductors and μin and μout the permeability of the inner and outer conductors. Many low loss cables are generally thicker than their RG counterparts and can be used for large communications installations, such as cellular. For cellular installations, passive intermodulation distortion (PIM) is also a major consideration.
