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.

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 be 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.

High power coax cables

For coax cables where high powers are likely to be used, specially constructed cables are needed. If cables using ordinary polyethylene were used, then the might higher temperatures encountered would soon melt and distort the cable. This would then lead to its destruction.

For very high power applications, typically cables are used that possess an air dielectric and employed. The centre conductor is then held in place by a form of coil that runs along the length of the cable.

For medium to high power coax cables a Teflon dielectric can be used. This can withstand high temperatures of typically up to around 160°C.

When considering which cable to use, it should be remembered that as the frequency increases, so the skin effect becomes more pronounced, and coupled with increased losses in the dielectric, this limits the power handling capacity.

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.

By Ian Poole

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