Coax cable environmental resistance
- an overview of the environmental elements associated with coaxial cables and the precautions to take when using them.
Coax cable, or as it is sometimes called RF cable can be expensive, but it is also required to withstand some harsh environmental conditions. In view of its cost, care needs to be taken otherwise the performance of the coax cable will deteriorate and the RF cable will require replacement. By taking suitable precautions, the life of the coax cable can be maximised and the periodic replacement costs for RF cables can be reduced.
There are many factors that affect coax cables to greater or lesser degrees:
- Humidity and water vapour
- Corrosive vapours and liquids
Effect of humidity and water vapour on coax cables
One of the biggest enemies for coaxial cable is that of water vapour. If it enters a coax cable then it can significantly degrade its performance, requiring the cable to be replaced. Moisture causes two main effects that give rise to an increase in the level of attenuation or loss in the cable. The first is an increase in resistive loss arising from oxidation of the braid that gives rise to an increase in the resistance of the braid or outer conductor in the coax cable. The second is an increase in the loss arising in the dielectric. Water absorbed into the dielectric heats up when power is passed along the coax cable. This heat is as a result of power loss in the cable.
Water vapour, or even water itself can enter the coax cable through a number of ways:
- Through the termination of the coax cable (i.e. through connector or other termination method)
- Through pin-holes in the jacket
- By water vapour transmission through the jacket.
1. Moisture entry through coax cable termination The most obvious method of humidity entering a coax cable is through the termination. One very good example is the small termination box provided with many TV antennas. When used externally these termination enclosures provide little protection against the elements and the coax will quickly deteriorate. Even when a connector is used to terminate the coax cable there will be problems if the coax is used externally. Very few connectors are weather proofed, and even if they are supposedly weatherproof, then it is wise to take additional precautions.
Normally the best method is to use self-amalgamating tape. This tape comes in the form of a roll and appears like thick PVC tape but it has a thin paper backing on one side to keep each layer separate and prevents it amalgamating with itself before use. It is used in a similar way to insulating tape. The backing strip is peeled off and then it is wrapped around whatever it is to be waterproofed - in this case a coax cable termination - overlapping each winding by about 50% of its width to ensure a good seal. When applying the tape keep it stretched so that it is applied under tension. Also it is best to start from the thinner end of the job, i.e. where the diameter of whatever it is being applied to is smallest. Where there is a connector on a cable, start on the cable and work towards the connector. Also when winding ensure that there are no holes of voids in which water could condense or enter. Keep the self-amalgamating tape in intimate contact with whatever it is to be waterproofed.
2. Moisture entry though pin holes in coax cable jacket Most coax cables have some small holes in their jacket along their length. It is therefore quite possible that moisture will enter through any imperfection in the coax cable jacket. If the pin holes are located externally where they can be affected by the weather then moisture will enter. Unfortunately it is very easy for small abrasions to occur during the installation of a cable and these can include small pin holes right through the jacket. Great care must therefore be taken when installing a cable, and in particular when the coax cable is passed trough a wall or other barrier.
3. Water vapour transmission through the coax cable jacket All materials exhibit a finite vapour transmission rate. Accordingly if a coax cable is constantly in contact with moisture, then this will permeate through the jacket. In view of this coax cable should never be buried directly in the ground. Either use some external protection such as a waterproof pipe, and ensure that no water enters it so that small patches of water form in it. Alternatively use the "bury direct" cables that are available. It is also found in airborne applications that the large temperature extremes encountered cause water condensation in the coax cables. This moisture can collect in low areas of the cable causing local areas of corrosion. One method of overcoming this is to fill any voids in the aircraft where coax cable are carried with non-hardening moisture-proof compound.
Effect of sunlight on coax cables
Sunlight has an effect on many substances, and the same is true of coax cable jackets or sheaths. It is particularly the ultra-violet light that causes the degradation to the cables. To increase the life of coax cables, manufacturers use high molecular weight polythene. Polyvinylchloride (PVC) jackets exhibit less than half the life expectancy of the high molecular weight polythene.
Effect of corrosive vapours on coax cables
Using a coax cable in the vicinity of corrosive liquids and vapours can reduce the life of a cable faster than if it was used externally. Salt water is a common problem on sea going vessels, and chemical vapours may be present on other installations requiring coax cables. Although the rigours of the weather can be very tough, some vapours and liquids can speed the deterioration of the coax cable even faster. The use of tin or silver coatings can provide some additional protection but this is not permanent. However it is recommended that specially environmentally hardened cables be used where extreme conditions are anticipated.
Coax cables are normally quite tolerant to being used in a variety of conditions. However to ensure the longest operational life it is best to ensure that they are not exposed to environmental conditions that would cause their performance to deteriorate. If they are then it is necessary to adopt a few precautions to ensure that the coax cable life is maintained for as long as possible.
By Ian Poole
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