Coaxial Cable Power
This application note covers power han- dling 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.
CONNECTOR CENTER CONDUCTOR
AVERAGE (CW) POWER
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
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.
CONNECTORS AS A LIMITING FACTOR
Heat generation in a connector is analyzed
by examining the center conductor diameter
“a” of the connector involved (see Table 1).
Generally, if the diameter of the center conductor of the cable is approximately the same
AND PETER WALTZ
Cobham Antenna Systems,
Microwave Components, Exeter, NH