Co-ax Cable
Bill's article on co-ax cable describing the different types, the differences and which to choose:
http://www.wrightsaerials.tv/coaxcablequalityhmdim.htm>
Summary: use CAI certified cables for better results all round.
Co-ax in More Detail
Since a wire run through a pipe will not stay central of its own accord some means of keeping it in place is necessary. This is accomplished by filling the space with some sort of insulator. The insulation may be solid, a foam or an extruded shape with radial ribs holding the central core in place. At the high frequencies used by UHF TV and even more so by Satellite downlinks the insulation material causes attenuation of the wanted signals, and the choice of insulation material and the way in which it is formed are inevitably a compromise between maintaining the structure of the cable and reducing its losses.
The outer, cylindrical, conductor of the cable was traditionally a braid of thin copper wires. At the frequencies used by FM Radio and the older black-and-white 405-line TV standards even a fairly sparse braid 'appeared' electrically to be a solid cylinder inasmuch as radio signals would not leak into our out of the cable. At the higher frequencies used by UHF (colour, 625 line) TV and Satellite gaps in the braid lead to more leakage of signal into and out of the cable so better quality TV cables, and all cables suitable for satellite feeds, also employ a metal (or metallised) foil layer under the braid.
Characteristic Impedance
A non-intuitive property of transmission lines such as coax is that to electrical signals they appear to have a particular resistance which is nothing to do with any power that is lost in the cable. To understand this it may help to consider what happens to an electrical impulse applied to one end of a cable: it will travel along the cable at a finite speed so there will be a few nano- or micro-seconds before it reaches the other end and finds out what is connected there. During this time the ratio of voltage to current of the impulse travelling along the cable is determined by the capacitance and inductance of the cable, which happens to 'look' like a particular value of resistance for a particular type of cable. If what is actually connected to the other end of the cable does not match this 'characteristic' resistance then, like ripples in water reflected against the concrete edge of a pond, the electrical signal will be reflected back down the cable. This usually results in loss of signal power and is therefore undesirable, so it is important to use the correct impedance cable for a particular application.