Whole House Wiring - Understanding Coaxial cable

Coaxial cable might appear to be a simple wire with a solid center conductor, a circumferential outer conductor, and an insulator separating these two conductors. All coax is not created equally and you must know what to use to maximize performance and dependability. Television antenna systems require a 75-ohm cable. The significance of the characteristic impedance comes from concepts involving power transfer in electrical circuits. Specifically, 75 Ω is a good match for a center fed dipole antenna, the type of antenna almost always used for OTA television and radio reception. The characteristic impedance of the cable is determined by the ratio of the size of the center conductor as it relates to the size of the dielectric.

Why coaxial configuration? Coaxial lines confine the electromagnetic wave of the signal to the area inside the cable, between the center conductor and the shield. Because of this, coaxial lines can be bent and moderately twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. In short, coaxial cables allow efficient installation practices. In radio-frequency applications up to a few gigahertz, such as in the antenna distribution discussed here, 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. Above a certain cutoff frequency, transverse electric (TE) and/or transverse magnetic (TM) modes can also propagate, as they do in a waveguide. This means that coaxial cables are designed with an anticipated cutoff frequency. You must use a coaxial cable with both the right characteristic impedance and the right bandwidth for your application.

Coaxial cable is typically denoted with the prefix RG as in RG-6, RG-59, RG-11 and others. RG stands for “radio guide” and comes from military applications first standardized during World War II. The military standard, MIL-HDBK-216, was first published in 1962. Although these designations are now obsolete, the RG-series designations were so entrenched in the industry that they are still used to describe both the general characteristics of the cable and to select matching connectors.

Susceptibility to interference has little relationship to broad cable type designations (e.g. RG-59, RG-6) but is strongly related to the composition and configuration of the cable's shielding. For television applications, with frequencies extending well into the UHF range, a foil shield will provide total coverage and good effectiveness against high-frequency interference. Foil shielding is ordinarily accompanied by a tinned copper or aluminum braid shield, with anywhere from 60 to 95% coverage. The braid is important to shield effectiveness because (1) it is more effective than foil at absorbing low-frequency interference, (2) it provides higher conductivity to ground than foil, and (3) it makes cable termination easier and more reliable.

Which Coaxial Cable Should You Use?

We now know that we need a 75-ohm coaxial cable. We know that a dual shield is better than a braid alone or a foil alone. A quad shield is even more impervious than a dual shield. That points our search for cable to the right neighborhood, but there are still a lot of choices that can be made. Here is a short list of the proper cables that may be considered for a residential installation.