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:
Inner conductor. May be either solid wire or
made of strands.
Outer conductor. Braided copper wire, allowing
the cable to be flexible.
Dielectric. Creates a set spacing between the
two conductors.
Covering. Protects the cable from moisture and
damage to the braided outer conductor.
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:
D = Diameter of outer conductor.
d = Diameter of inner conductor.
Zo = Characteristic impedance.
= Dielectric constant.
Coaxial cable will only present it's characteristic impedance under certain
conditions:
1. Both source and load match this characteristic impedance.
2. The coax has infinite length (incredibly unlikely).
3. The coaxial length is a multiple of 1/2 wavelengths at the frequency in use.
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.
Dielectric losses.
Radiated signal losses.
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:
Vp = Velocity of propagation.
= Dielectric constant.
