RF coax cable power rating
- an overview of the maximum power levels that an RF coax cable may be able
to carry.
Although for receiver applications the level of power that
coax cable can handle is not an issue, when it comes to medium or high power
transmitters, the power handling capacity of the coax cable becomes very
important. If the incorrect type of coax cable is used, it can result in a
failure of the cable, and possible damage to the transmitter.
For most applications where the power is applied
continuously, the limiting factor arises from the heat loss within the cable. If
the power in the RF cable is to be pulsed, then it is necessary to check that
the operating voltage is not exceeded.
RF coax cable power loss
The major root cause for the limit in power handling capacity
of an RF cable is the level of heat caused by the power losses occurring in the
cable. If the temperature of the coax cable rises too high, the cable my become
deformed and be permanently damaged.
For the resistive losses in the coax cable, it is found that
most of the heat is generated in the centre conductor. Additionally any heat
generated as a result of dielectric losses will be dissipated within the
dielectric. It is therefore the construction of the dielectric that is of key
importance in determining the power handling capability of the coax cable. Its
maximum operating temperature, and its heat transfer coefficient both have a
major effect.
It can be seen that the lower the losses of the cable the
smaller the temperature rise, and the greater the power handling capability is
for the cable. As a broad rule of thumb, lower loss cables will have a higher
power rating than higher loss RF cables.
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RF coax cable derating
Although a power rating may be given for a particular coax
cable, it is often necessary to de-rate it to cater for non-optimal operating
conditions.
The temperature of the environment is one factor. If the coax
cable is operating in a high temperature environment, it will not be able to
dissipate as much heat, and therefore the operating temperature will rise. Even
at the highest foreseeable environmental operating temperature, the RF cable
must be able to remain within its maximum internal temperature. Accordingly a
de-rating factor is normally applied if the coax cable is to used at high
temperatures.
If the coax cable is operated under conditions where the VSWR
is high, the cable rating needs to be reduced. The reason for this is that when
there is a high level of VSWR, there are positions of high and low current along
the coax cable. These may be such that they cause the power dissipation to rise
significantly in some areas causing higher levels of power to be dissipated
locally.
Altitude also has an effect, although at significant heights.
If the cable is to be operated at altitude and hence under reduced pressure, the
any cooling will be less effective. Therefore the temperature rise within the
cable will be greater.
Summary
Although the power handling capability of RF coax cable may
not be an issue for many installations, when using medium or high power
transmitters the power rating or handling capability of RF coax cable needs to
be carefully considered.
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