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: