Feeder cable.

Purpose.
The feeder cable is designed to transport a signal from one place to another, often between a transmitter and antenna.
The perfect feeder cable would have no loss, no radiation and no signal pick up on the cable. All the signal would be delivered from one end to the other.

Types.
There are two main types of feeder cable in use, coaxial and parallel conductor. The coaxial type is the more commonly used type, and is the type used for CB radio installations.
Coaxial cable also comes in various types, according to it's characteristics.

Characteristics.
RF cable has several characteristics, and there are many different types to allow for different requirements.
Impedance is a preset characteristic, set by design and independent of length or frequency.
Loss is a measure of how much of your signal will be lost in the cable. The amount of loss will depend upon frequency, and is per stated length - more cable equals more loss.
Velocity factor is a measure of how fast the signal will travel along the coax. When in free space, radio waves travel at the speed of light. In coaxial cable, they will travel slower.
Diameter. Low loss cables may often be thicker.

Selection.
When selecting the cable to use, you need to consider the application.
Impedance must match that of the transmitter and antenna.
Loss, being per unit length, will be a question of how much you are using. It is not worth the extra cost and width for small runs, you will not notice the difference.
Velocity factor is fairly standard between many types of coaxial, and is not really important in most installations.
Diameter is only really important if you may have problems installing thick, stiff coaxial as opposed to thin, more flexible coaxial. Otherwise, pay more attention to impedance and loss.

Length.
Under normal conditions, if all is well, the length of coax makes no real difference. If the length of coax appears to change readings, there is a fault on the system somewhere. This issue is covered in more details on following pages in this section.

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: