Although part of the audio range, loudspeaker
wiring is very different and needs a little consideration. Little
loudpeakers in equipment may be wired up using almost any wire,
as may intercom type circuits and so on.
Hi-fi loudspeaker cabling and In-Car Entertainment
speaker wiring is much more critical. It is very easy to drop
a significant part of the output of an audio amplifier over
a medium to long run of inadequate but inexpensive speaker cable.
Many people automatically use a thin figure of eight “Zip” wire
for speaker connections but for any amplifier delivering over
a few Watts, a thicker and more substantial cable is needed
for a quality reproduction of sound without burning part of
your output as heat and having to have the volume higher to
achieve the same volume at lower quality.
Look at heavy duty and Hi-fi loudspeaker
cables or, if the increased bulk of those is a problem, there
are several special flat variants that will go under the carpet
easily. Silver plated and oxygen free copper are also available
and provide a very clean transfer of power to your loudspeakers.
The first thing to consider with an RF cable is impedance.
Impedance is a combination of DC resistance as well as capacitive
and inductive effects in the cable. It may be considered as
the effective resistance of a cable to an AC signal.
Most RF equipment is designed to work with
a cable of a specific impedance. TV’s, video recorders and satellite
equipment are designed to use 75 Ohm cables, whereas radio transmitters
and some receivers are designed to use 50 Ohm cable. It is important
to select a cable with the correct impedance for the job, otherwise
a bad match will be made between the equipment and signal will
be lost, resulting in poor reception. In the case of transmitters,
it is possible that this will cause the output stages to heat
up in use, or even burn out.
The other associated figure to look at
when choosing an RF cable is attenuation. Attenuation is a measure
of how much signal you will lose over a given length of cable
at a stated frequency, expressed in Decibels. The explanation
of both impedance and attenuation is beyond the scope of this
guide, but suffice it to say that a cable of the correct impedance
for the application, with the lowest attenuation figure at the
frequency range to be used, will give the very best results,
although the voltage rating of the cable is also important at
very high transmitter output levels.
Impedance is also important for some professional audio
uses, such as the use of balanced microphones in studio or stage
situations. Check the impedance rating of the equipment and
use the correct cable and connectors for the job.
Networking connections that use the BNC connector are
designed to use 50 Ohm RF cable to interconnect computer equipment.
Newer networking connections tend to be made using special multi-way
twisted pair cables that help to reduce both radiation of the
very sharp edged RF data signals as well as reducing the likelihood
of pick up of interference from other electrical equiment.
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There are various standards for these
data cables (Category 5 for example) and it is always best to
check the instructions supplied with each unit before choosing
a cable.
If you intend to do a lot of cabling, there
are a number of special cable stripping tools that will make
the job a lot easier.
Many connectors need soldering onto the cables and
sharp cutters, a craft knife and small scewdrivers for any screw-fixed
connectors will be needed.
A multimeter or continuity tester is ideal for reassuring
you that your completed cable and connector assembly really
does connect as you think it does, with no open or short circuits
that could cause damage or other problems later.
Consider the use of conduits or trunking
to box up, hide and protect long cable runs. Wiring inside equipment
may be secured by using cable ties, lacing cord or spiral wrapping.
There is a large range of different cable clips that will also
help tidy your wiring and keep it in place after installation.
If your cables or wires have many different
terminations, you may wish to use coloured or numbered sleeving
at each end of each wire to aid later fault finding or rewiring.
Heat shrink sleeving will allow you to join cables in places
where no connectors are required, and then shrink the sleeving
over the individual joins to remake the insulating layer, or
just provide an additional protective layer for a cable in a
vulnerable position.
Cable glands and grommets will allow you to make neat
entry to boxes or through panels, and will prevent the cable
chafing on metal edges.
It is never ideal to run a cable across
the floor but, if you have no alternative, consider using a
cable protector, which is a heavy rubber strip the cable lies
protected inside on the floor, shaped to minimise tripping over
it.
It seems fairly obvious that the ends of your cable
will somehow need to be securely attached to the equipment,
either by directly soldering each conductor to a terminal or
tag, or by screw or clamp terminals, or via a suitable set of
connectors. Check before choosing connectors that their current
and voltage rating is sufficient and safe for the job in hand.
RF cables must be properly connected and joined by
suitable RF connectors of the same impedance. Use of the wrong
connector, or a hard wired soldered or screw terminal joint,
may result in a break in the smooth transfer of RF energy that
may result in the signal being reflected back up the cable instead
of passing through it.
Note that, especially for RF, a soldered
connector is usually far better than a screwed one, and that
TV aerial and distribution wiring should always be soldered
for best long term results.
We hope that this guide to cables has helped you to
think about what you need from your cables and wiring, and that
you now feel ready to make your choice. |
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