Conceptually, a coaxial cable consists of circular inner
and outer conductor separated by an insulator (a dielectric
medium, can be air or plastic material like PTFE) and sharing
the same geometrical axis. A cross sectional view of a coaxial
transmission line is shown below.
In reality, the coaxial cable is an electrical cable
with an inner metallic conductor surrounded by a flexible
and tubular insulating layer, which in turn is surrounded
by the outer metallic conductor shield. Oliver Heaviside
invented coaxial cable in 1880. The coaxial structure is
ideally suited to transport microwave signal because of
the confinement of the propagating electric and magnetic
fields (associated with the microwave energy) within the
enclosed space between the inner conductor and the outer
shield. Unlike other transmission line structure, the coaxial
structure does not allow any electromagnetic energy to escape
or radiate outside and also serves the purpose of protecting
the transmission line from outside environment and interference
from other sources. The outer conductor or the shield is
held at an electrical ground potential to ensure that it
does not radiate any microwave energy.
A relevant question to ask at this point is: can an ordinary
wire be used to transport microwave energy? The answer is no,
since an ordinary wire acts as a radiating element like an antenna
leading to power loss.
The propagation of microwave signal through a coaxial cable
assumes that the dimensions of the inner conductor, the outer
conductor and the space in between are held uniform along its
length. If this is not so, any abrupt change in its dimension
causes a discontinuity at that point, which alters the characteristic
impedance of the transmission line leading to signal reflections
towards the signal source and lossy propagation of microwave
energy.
Fig. Construction detail
The manufacturer of the cable ensures that the dimension
remains uniform by embedding the circular inner conductor in
a flexible yet semi-rigid dielectric material like Polytetrafluroethylene
(PTFE), which can be easily bend and handled during its use.
This avoids abrupt kinks or discontinuities, which are the primary
cause of reflections of microwave energy. In fact, a good quality
cable is measured in terms of its construction, uniformity of
its dimension, materials used in construction and the electrical
parameters like return loss, insertion loss and radiation leakages.
As mentioned earlier, a single TEM mode of propagation is
a desirable property of a coaxial cable system. What happens
is that above the cutoff frequency or the highest frequency
of operation that the coaxial cable supports, other modes such
as TE and TM tend to be generated (as in the case of wave guide).
These additional modes in turn interfere with the single dominant
mode (TEM) leading to undesirable results, as far the microwave
signal transmission is concerned. It is to be noted that the
highest frequency or the cutoff frequency is determined by the
dimension of the outer conductor; smaller the outer diameter,
higher the cutoff and vice versa. Compared to an air dielectric
coaxial system, the presence of a solid insulator dielectric
material, like the PTFE, lowers the cutoff frequency and adds
to the insertion loss. For most practical purposes, a coaxial
cable is constructed using a solid dielectric material between
the inner and the outer conductor, trading-off insertion loss
and cutoff frequency with the stability and strength it offers
to the anchoring of the outer conductor on the inner conductor
and maintaining uniform dimension along the length of the cable.
The outer conductor is shielded from EMI (electromagnetic interference)
by a sheath of outer cover called the jacket. The jacket is
usually made of Polyvinyl chloride (PVC) material and comes
in different colours such as black, gray, white and tan.
The entire effort when selecting a coaxial cable assembly
goes in ensuring that the cable exhibits a single TEM mode of
propagation below the cutoff frequency (specified by the outer
conductor dimension), maintaining flexibility along the cable
length, low insertion loss, VSWR close to one, maintaining uniform
inner conductor dimension (complying to one of the industry
standard dimension. When it comes to designing and manufacturing
coax for military and space related applications, there are
other stringent specifications, such harsh environmental compliance,
easy installation and quick field deployability, thermal management
etc.
Popular dielectric materials used for cable fabrication are:
