Coaxial cable anatomy and how they work

May 11, 2019
[[TitleContent]]

As is well known, coaxial cable is a broadband, relatively low loss and highly isolated transmission line technology. The coaxial cable includes two concentric conductive cylinders separated by dielectric spacers. The capacitance and inductance distributed along the coaxial line create a distributed impedance throughout the structure, called the characteristic impedance.

The distributed resistive losses along the coaxial cable result in predictable losses and behavior along the line. These factors enable coaxial cables to be used to transmit electromagnetic (EM) energy, which is much less than the free space propagation and less interference using the antenna.

As a product of a coaxial cable with an electrically conductive outer shield, an additional layer of material can be applied to the exterior of the coaxial cable to increase environmental performance, EM shielding and flexibility. Coaxial cables can be made from braided conductive strands and intelligently layered to create highly flexible and reconfigurable cables that are both lightweight and durable. As long as the concentricity of the conductive cylinder of the coaxial cable is maintained, bending and bending will only have a slight effect on cable performance. To this end, coaxial cables are typically connected to coaxial connectors using a helical mechanism. Use a torque wrench to complete the tightness control.

Some important frequency-dependent behaviors on the same axis define their application potential - skin depth and cutoff frequency. The skin depth describes the phenomenon that a higher frequency signal traveling along a coaxial line occurs. The higher the frequency, the more electrons tend to migrate toward the conductor surface of the coaxial line. The skin effect causes an increase in attenuation and dielectric heating, which results in greater resistance loss along the coaxial line. In order to reduce the loss of the skin effect, a larger diameter coaxial cable can be used.

While improving coaxial technology performance is an obvious solution, increasing the size of the coaxial cable reduces the maximum frequency that the coaxial cable can transmit. When the wavelength of the EM energy exceeds the transverse electromagnetic (TEM) mode and begins to "bounce" along the coaxial line as the transverse electron 11 mode (TE11), the coaxial cable cutoff frequency is generated. The new frequency mode is cumbersome because it propagates at a different speed than the TEM mode and can cause reflections and interference with TEM mode signals propagating through the coaxial cable.

The solution to this problem is to reduce the size of the coaxial cable to increase the cutoff frequency. There are now coaxial cable and coaxial connectors that can reach millimeter wave frequencies - 1.85mm and 1mm coaxial connectors. An important point to note is that as the physical size shrinks to handle higher frequencies, the losses increase and the power handling capability of the coaxial cable decreases. Another challenge in building these very small components is to ensure that the mechanical tolerances are tight enough to reduce electrical significant defects and impedance variations along the line. This becomes an expensive process that leads to relatively sensitive cables.