Cables

RF cables are, for frequencies higher than HF, almost exclusively coaxial cables (or "coax" for short, derived from the words "of common axis"). Coax cables have a core wire, surrounded by a non-conductive material (which is called dielectric or insulation), and then surrounded by an encompassing shielding which is often made of braided wires. The dielectric keeps the core and the shielding apart. Finally, the coax is protected by an outer shielding which will generally be a PVC material.
The inner conductor carries the RF signal and the outer shield is there to keep the RF signal from radiating to the atmosphere and to stop outside signals from interfering with the signal carried by the core. Another interesting fact is that the electrical signal always travels along the outer layer of the central conductor: the larger the central conductor, the better signal will flow. This is called the “skin effect”.

Even though the coaxial construction is good at containing the signal on the core wire, there is some resistance to the electrical flow: as the signal travels down the core, it will fade away. This fading is known as attenuation, and is measured in dB/m. The rate of attenuation is a function of the signal frequency and the physical construction of the cable itself., and a table of these values can be found in the next chapter. Obviously, we need to minimize the cable attenuation as much as possible, keeping the cable very short and using high quality cables.