Coax cables and connectors are used in a wide range of applications with various purposes.
One of the biggest strengths of the coaxial construction is its excellent insertion loss. This is mainly achieved through a constant, controlled impedance. The impedance is controlled by providing an environment where the distance between the signal (inner conductor) and the ground (outer shield) remains constant. Combined with the use of a high performance, low dielectric insulation material such as PTFE, coax yields a solution with excellent insertion loss. Currently, there are various coax solutions for frequencies up to 110 GHz, so there are solutions for nearly all applications.
Another strength of coax is its isolation, which comes in three main forms: crosstalk between adjacent signals within a system, transmitted EMI (emissions) that exit to the outside world, and received EMI (susceptibility) which enter from the outside world. In all cases, the coax topology yields excellent isolation by fully encapsulating the signal conductor inside the outer shield. In theory, this construction provides a perfect shield to the outside world. In reality, there is always some leakage depending on the effectiveness of the outer shield. Various shielding options are available depending on the level of required shielding effectiveness. A single braid yields about 50-60dB, while a braid and foil yields around 90dB. For very sensitive applications, additional braids/foils are used to further improve the isolation.
Lastly, another strength of coax is its versatility. Coax is generally understood to be used for analog signaling, but some video applications also use coax.
One of the biggest downsides to coax is its size. Though coax contacts are available in many sizes, they are larger than other topologies such as discrete wire or twisted pair. This is due to both the impedance requirement (which dictates the size of the insulator) and the shielding requirements (which require a 360⁰ shield around each signal). In other words, the strengths of the coax often make its use challenging. To get an understanding of size, Omnetics Nano Coax contacts are spaced at 0.125”, and only a single row is available. By contrast, standard Micro pins are spaced at 0.050”, Nano contacts are spaced at 0.025”, and both can be dual row. Even if differential pairs are used – with two lines used for each signal – the overall size per signal is significantly smaller than coax.
Coaxial contacts are also challenging to assemble. Proper assembly has a significant impact on the performance and must be done with care. As frequencies increase, the precision of the assembly must increase accordingly in order to achieve a high level of performance.
Coaxial cables and connectors are used in all sorts of industries and applications. This three-part series provided an in-depth look at coax, including impedance, the purpose of coax contacts, and their strengths and challenges. The goal of the series is to provide a deeper understanding of the uses for coax and assist engineers in selecting the correct coax connector with confidence.