How to install RF coax cable
- some of the key points to note to help optimse the performance from the
installation of the coax cable.
Coax cable is widely used for a variety of applications from
proving down leads for domestic television and hi-fi systems, to antenna
installations for professional or commercial radio systems, and coax cable is
also used for many computer applications for distributing data over networks.
Fortunately coax cable is very easy to install. It is relatively robust and
comparatively few precautions need to be observed when installing coax cable.
However there are some hints and tips that help ensure that the best performance
will be obtained from the coax cable installation, and that it will last as long
as possible.
The hints and tips to help install coax cable can be grouped
into a number of categories:
- Choosing the right cable
- Weatherproofing
- General installation
- Terminations / connections
Choosing the right coax cable
There is an enormous variety of coax cables on the market,
and at first sight the choice may not appear easy. The first decision to make,
prior to any installation is to choose the required impedance. Domestic hi-fi
and video antenna feeds use 75 ohm coax cable. Professional, CB, and amateur
radio standardise on 50 ohm cable. Once this choice has been made the next
decision will probably be made on the level of signal loss that is acceptable.
Typically the lower the loss, the greater the diameter of the cable, and also
its cost. Typically there are several cables with similar performance figures
and often the decision of the exact type number will depend on the stock
position of suppliers. Once a suitable cable has been found then it can be
purchased and installed.
Weatherproofing
When installing coax cable externally it is very important to
ensure the cable is adequately weatherproofed. This is critical because any
moisture entering the RF coax cable will produce a considerable increase in the
level of loss. If any moisture passes into the dielectric material spacing the
inner and outer conductors, this will impair the performance of the dielectric,
and increase the level of loss. Moisture will also cause the outer braid to
oxidise, and reduce the conductivity between the small conductors making up the
braid.
It is therefore very important to seal the end of the cable
if it is to be used externally, and ensure that no moisture enters. It is also
necessary to ensure that the outer sheath of the cable remains intact and is not
damaged during installation or further use.
An additional method of preventing large amounts of moisture
entering the cable is to loop it up and down. In this way it is more difficult
for water to enter the cable and then move along it. However if some moister
enters the cable it will move into it by capillary action, so it is always best
to ensure that the ends are properly sealed and protected.
General installation tips for RF coax cable
All cables have a bend radius. In order to prevent damage
they should not be bent into curves tighter than this. If RF coax cable is bent
beyond its limit then damage to the inner construction of the cable may result.
In turn this can lead to much higher levels of loss.
In a similar line, care should be taken to ensure that the
cable is not crushed, or likely to be crushed. If the RF cable does suffer
damage in this way, the dimensions of the cable will be changed and it will not
maintain its characteristic impedance. Additionally if the dielectric between
the two concentric conductors in the coax cable is damaged, then there is the
likelihood of an increase in the level of loss.
While on the subject of physical damage to the cable, it is
necessary to ensure that the sheath of the cable remains intact. If it is broken
in any place, then this may allow moisture to enter if it is used externally,
and this will cause oxidation and moisture retention in the dielectric that will
increase the level of loss.
On some occasions it is necessary to bury coaxial cable.
Ideally, normal cable should not be buried directly as this relies purely on the
outer sheath for protection and it is not designed for these conditions. Instead
it can be run through buried conduit manufactured for carrying buried cables.
This has the advantage that it is easy to replace. However ensure that the
conduit does not become water logged. Alternatively solution to using some form
of conduit is to use a form of coax cable known as "bury direct". This is
designed for being buried, and its outer sheath can withstand these conditions.
Terminations / connections
When installing RF coax cable, it is important to terminate
the cable correctly. In most instances the coax cable will be physically
terminated using an RF connector, the electrical termination being either at the
antenna or in the receiver. Accordingly the connections to the connectors must
be made correctly and the right quality RF connectors should be used.
Although connectors for domestic installations are often poor
in terms of their electrical radio frequency performance, there is little
alternative to using them in view of the fact that they have to mate with the RF
connectors on the equipment. For professional applications, RF connectors can be
far better, although it is necessary to ensure that the connectors are suitable
for the frequencies used. Some cheap versions of RF connectors may not meet the
full specification and can thereby impair the performance of the RF coax cable.
It is therefore wide to always buy connectors from reputable sources.
Summary
By correctly installing a RF coax cable it can provide many
years of satisfactory service. However wear, and exposure to the elements will
mean that after some time it may be prudent to replace the RF coax cable. As the
degradation in performance will be slow, it may mean that this is not noticed.
Only when it is ultimately replaced will a major difference be seen.
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