For some applications the coax cable power rating or power specification is of great importance.
The coax power rating will depend on a variety of factors, but in general the higher the power rating, the larger the diameter of the coax. As low loss coax tends to have a wide diameter, higher power rating is often combined with a low loss.
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 main factor that generally limits the power handling capacity of a coax cable is the level of heat that is dissipated as a result of power losses. If the temperature of the coax cable rises too high, it may become deformed and be permanently damaged.
There are two main areas of heat loss within the coax cable:
- Conductor resistive losses: It is found that most of the heat is generated in the centre conductor. This is because the centre conductor has less area to carry the current. For many frequencies, the skin effect will come into play, and even if the centre conductor is multi-stranded, the overall surface area will mean that it dissipates more than the out. The outer is often a braid which will have a large surface area, and over if it is solid, the surface area will be much more than that of the centre conductor.
- Dielectric losses: 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 from all causes, 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 power de-rating
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.
- Environmental temperature: 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.
- High VSWR conditions: 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 coax cable power dissipation to rise significantly in some areas causing higher levels of local heating.
- Operation at altitude: Altitude also has an effect on the coax cable power handling capability, although this really only comes in to play 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.
Although these are some of the main factors that may cause a coax cable power handling to be de-rated, a general awareness of the operating conditions is always beneficial. It also helps to have a good margin in place to ensure that even under fault conditions, the cable will be able to withstand any additional stresses that may occur.
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.
The coax power rating is a very important consideration when using radio transmitters which have a medium or high power capability. It is necessary to ensure that there is sufficient margin to ensure that even with a slightly higher than normal VSWR, then the coax cable will not be damaged handling the required power level.