Coaxial cable.

Coaxial.
The purpose of coaxial cable is to contain the signals within the cable, delivering as much as possible at the other end. Coaxial is made up of 4 parts, as shown below:


Under normal conditions, the signals on the inner and outer conductors are equal but opposite. These cancel out, carrying the signal inside the cable without radiating or picking up signals.

Characteristic impedance.
The characteristic impedance is the value of impedance which, if matched by the load and source, will be presented at the source end of the coaxial. To the source, the coaxial will seem invisible - all it will see is the load.
The characteristic impedance of coaxial cable can be calculated from the conductor diameters and the dielectric constant of the insulating material:
Actual impedance.
Coaxial cable will only present it's characteristic impedance under certain conditions: If none of the above apply, then the impedance presented by the coax will be different from it's characteristic impedance. Coax length issues are covered in more depth on another page in this section.

Loss.
Although the ideal is to deliver all the signal to the other end, all practical cables will lose some of the signal on the way. There are several ways that cable can cause loss of signal: Resistive losses are due to the performance of the conductors used in the cable. Due to skin effect, for a specific cable, these losses will increase with frequency. Increasing conductor diameter will reduce these losses.
Different types of insulation will have a different dielectric constant, changing the capacitance and losses within the coaxial cable. These losses also increase with frequency, but not as fast as resistive losses.
Most coaxial should radiate very little signal, but cheap coaxial may be much worse, as the manufacturer skimps on the braided shield to save money. To make matters worse, there is no longer a definitive standard for RG type coaxial cables. This means that you can no longer rely on a standard specification chart, the manufacturer of a cable may have their own specification for a specific RG type cable.

Velocity factor.
The velocity factor, also known as the velocity of propagation, is the speed at which the signal travels down the coaxial - relative to that of free space.
Being relative to a set speed, and slower than the reference, it will be a fractional figure.
The velocity factor is directly related to the insulation dielectric constant:

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