How to Select Coaxial Cable

Background

High energy transmission have heavy losses because of i) cable dielectric, ii) cable construction, iii) radiation, and iv) interference with adjoining circuits. The coaxial cable construction has been invented to overcome the problem of loss of signal and energy by containing signal and energy within an enclosed space. Transmission through coaxial cables is called an unbalanced transmission because the centre conductor and the shield are not reversible (as opposed to the balanced construction of a twisted pair). Shield can be SPC (silver plated copper) round wire single or double braid, round wire served (helically wrapped), flat foil served (helically wrapped with drain wires), foil and braid combination with drain wires, and other materials. Jackets provide outer cover to keep the cable clean, smooth and to prevent ingress of moisture into shield, as well as for isolation of shield. Materials used are PTFE (our specialty), VFG (Fibre Glass braid covered with lacquer Varnish, over sintered PTFE tape moisture seal), PVC or PU.

Power Transmission

For high power RF transmission applications, the larger diameter cables with higher breakdown voltages rating between centre conductor and outer shield must be selected. (See selection chart)

Characteristic Impedance Selection

Characteristic Impedance (Zo) of standard 50, 75 or 95 ohms are available and customers can specify any other if they have special need.

Characteristic Impedance (Zo) is determined by the ratio of dielectric OD to conductor diameter, hence a large cable and a very small cable can have the same Zo but the smaller cable will have higher losses.

Attenuation

Usually larger diameter cables have lower attenuation.

Dimensions

Modern RF equipment requires miniaturization, which must be balanced against signal loss; thinner cables would have higher attenuation.

Connector system

Termination labour costs are an important consideration in coaxial cable application. The connector affects overall loss and reflection in a system and is an integral part of the overall design.

Working Environment

RF transmission equipment typically operate in hostile working condition eg satellite, radar system etc. PTFE offers unparalleled protection against the such environment.

Additional Information

In early versions of MIL-C-17, the miniature PTFE coaxial cables were specified with PTFE jacket. However, PTFE jackets on coaxial cables are very difficult to process. This is because when the jacket is cured at high temperature, the core dielectric also undergoes expansion and bites into the metal shield. This tends to disturb the delicate ratio between the dielectric OD and the conductor diameter. During the years, we have gained considerable insight into the complex behaviour of coaxial cables with PTFE jackets and can exercise control over the ultimate performance to a great extent. Due to the difficulty with PTFE jackets, MIL specs moved on to melt-extrudible FEP (or other materials) jackets, and also added the requirement of sweep testing of coaxial cables (guaranteeing the behaviour of cable at a large number of frequencies in the specified range). Our listing of coaxial cables groups various items with similar size and properties together for PTFE, VFG (Varnished Fibre Glass) jackets etc.

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