Wirral & District Amateur Radio Club

Cables Part 1

 


There are hundreds of different types and sizes of wire and cable. Although any piece of wire may be used to join point A to point B, some cables are much better at certain jobs than others.

Generally, a wire will contain a single conductor, usually insulated by a plastic sheath so that it will not short out to nearby metal objects, other wires, or allow anyone to be electrocuted. Cables usually contain more than one conducting wire, insulated from each other and then covered by an overall protective and insulating sheath. Broadly speaking, wires and cables break down into four different areas:


This usually carries power from one place to another, either inside equipment or between power sockets and equipment.


These usually contain at least one core wire, covered in an insulating sheath, which is then further covered by an over-all grounded metal shielding layer that protects the inner wire from acting as an aerial and picking up interference from nearby power cables or other equipment. The protective shield layer is, in turn, covered by at least one protective and insulating plastic sheath.


RF cables are constructed in a very similar way to single core audio cables, but are specially constructed to allow high frequency signals to connect from one place to another as efficiently as possible without picking up any other radio or interference signals. Any piece of open wire connected to RF sensitive equipment will act as an aerial, both receiving unwanted signals from the world around it, and sometime transmitting signals that may cause interference to nearby equipment. Use of the correct RF cable will prevent this happening.


These are made up of a number of insulated wires that allow many data signals to be connected from one digital circuit to another. If the data has to travel from one piece of equipment to another, the wires are sometimes wrapped up in an overall shield or screen layer which prevents the data signals from being radiated to other sensitive equipment nearby.
The first thing you need to do is to work out which of these basic types will be most suitable for the job you want your wiring to do. From the descriptions above, this should be fairly easy, but then the more difficult part begins.


The answer to this is yes, you probably can make the cable as long as you need to, provided you think carefully about the job the cable will be doing, and where it is going to be used.
Once you know what sort of cable you need, you will probably find that there are many different wires or cables that would seem to do the job. However, not all cables will do the job as well as others. Letís go through our main four types again but look at the reasons you might choose one cable or wire over another.


The main considerations here are the amount of current to be carried by the cable, and whether it needs to be very flexible, or is fixed in position once wired up.

Cables and wires have a maximum current rating and this should never be exceeded. In fact you should choose a wire or cable with a rating of about 30% higher than the maximum current you expect to flow in the circuit.

If you donít know the current that will be drawn by your circuit, you can either measure it using a multimeter or calculate it if the power consumed by the circuit is known (rated in Watts). It is simple to work out the current that will flow in a supply cable if you know the voltage supplied and the power used.

Simply divide the power rating of the equipment by the voltage supplied to it. For example, a 1kW bar fire supplied with 230 Volts will draw 4.35 Amps (1000/230 = 4.35).

For mains electrical use, cables are supplied in just a few standard current ratings so, once you know the current drawn by the device to be wired, you simply need to look at the appropriate range of cables and pick a cable with a rating comfortably higher than the figure you have calculated.

Cables between mains sockets and equipment generally need to be flexible, and multi stranded round cables are usually the best choice here. The flat three core single strand mains wiring (twin and earth) used within walls and conduits to distribute mains around a building are inflexible and prone to fatigue and break if used in a position where they are allowed to flex.

The choice of mains distribution cables in a house or other building is, in any case, covered by the IEE Wiring Regulations, which set out the different current ratings of cables to be used for the various types of ring main and spur wiring.

Within equipment, the same considerations exist. You need to work out how much current will flow through internal wiring, then decide whether the wiring will be fixed in place, or whether a multi-stranded, more flexible wire would be more suitable, often the case if occasional movement of circuit boards may be necessary for service or modification.

Power supply and switching contact wiring, amplifier outputs and so on will be carrying the highest currents, and you need to choose thicker wires to carry these without heating. Thinner wires offer more resistance than thick ones, meaning that power will be lost in the wiring if too thin a wire is used. The longer you need a lead to be, the more resistance it will present to the circuitry it is connected to. This means that signal or power will be lost in the lead, but you can minimise this by choosing a cable with a lower resistance, usually a thicker cable.

This is particularly important where the voltages involved are fairly small, and proportionally more voltage drop occurs across a long lead. For example, the use of thin wiring to supply an amplifier in the back of a car from a fuse under the bonnet may result in a drop of several volts along the length of the lead when the amplifier is being used. This reduces the power supply available at the amplifier to perhaps 9 Volts, resulting in a poor quality distorted sound at anything above medium volume.

So, use a thicker wire for less loss and better performance and efficiency, especially over long lengths.


Having decided that you need a screened audio cable (except for loudspeaker outputs) you simply need make a suitable choice from the many cables on offer.
The simplest audio cables a
re the thin lapped core cables, suitable for connections between audio or other low level signal boards inside equipment, or very short external runs in non hi-fi applications. They are not suitable for longer runs or for high quality audio connections.

Cables with braided screens are good for general purpose audio use. They allow a high quality signal transfer over short to medium runs and are available in single and twin versions, for mono or stereo connections.

At the top of the range come various special cables for the very best signal transfer and minimum interference and hum pick up. High grade cables include oxygen free copper, special additional insulating sheaths, silver plated conductors for minimum connection resistance and surface resistance.

Where capacitance per metre is quoted, lower capacitance leads will have less of a loading effect on high frequency signals and are suitable for longer runs.

If you need to make up long leads for line level connection of, for example, a video recorder and a Dolby surround amplifier, then use the best cable you can afford for the job. If on the other hand you just need a point to point run in a piece of equipment, an inexpensive thin lapped cable will be fine.

   
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