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
- Sunlight
- 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.
Summary
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.
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