by Raymond Schwartz and Peter Waltz, Cobham Defense Systems, Microwave Components, Exeter, NH
This application note covers power handling capability of coaxial cables. The matrix of average power over frequency provided for each example cable type is to be used as a guideline.
Peak and average power
There are two potential failure modes in cables used to transmit high peak power. One is voltage breakdown; the other is overheating. The major concern associated with application of peak power is breakdown due to high potential. By themselves, the cable and the connectors may break down under high voltage due to peak power. However, the cable-to-connector junction is the one location on the cable assembly most sensitive to high potential breakdown. Prudent design of overlapping dielectrics and proper selection of connector type, combined with actual high potential or severe requirements testing, ensures that breakdown will not occur. Another consideration in pulsed systems is overheating due to CW power.
Average (CW) power handling capability
The major effect of average power in cable assemblies is the generation of heat from power dissipation and the resultant temperature rise. Many factors are involved in determining this effect for a particular cable assembly, but a short discussion may help distinguish the many facets of the problem.
In all cases, the limit of CW power level is reached when the hottest surface temperature (measured anywhere on the cable assembly) has reached a predetermined temperature, Tmax. For most high performance high power cable assemblies, Tmax is on the order of 400°F (204°C). This temperature is chosen based on explosive atmosphere mil spec requirements and also because higher temperature starts to soften the dielectric used in most cables. The temperature Tmax usually occurs near or on the connector nearest the source. For different types of cables, the tolerance temperature unit that a component within that cable will withstand determines Tmax. Expressed differently, one may allow Tmax to increase up to the limit of initial damage to the most sensitive component within the cable.
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