The Core – Skin Effect

The benefits of highly stranded cores

An electromagnetic field is an intrinsic part of electrical ‘flow’. In other words, whenever current flows in a conductor there will be a magnetic field all round that conductor.

Electromagnetic fields are easily established in free space and in insulating materials, but much slower to establish in conducting metals. When an electrical current passes through a conductor, the first place it starts to flow is in the easiest place for it to set up its electrical field: on the surface of the conductor right next to the insulation.

Once the surface is conducting, the flow goes deeper and deeper into the conductor as the magnetic field becomes established in the conducting material.

This establishing of electrical current flow on the outer edge of the conductor is true of all electrical circuits and is called the ‘skin effect’. In high frequency circuits the currents change so rapidly that no current gets the time to get established in the body of the conductor, as a result all the energy at higher radio frequencies travel just around the surface (in the ‘skin’) of the conductor.

Conductors with a lot of skin (for example highly stranded core or large diameter copper tubes), have much better high frequency performance and therefore much better phase coherent audio – and that’s important for line array systems.

To get the benefits it is essential to use many strands. A few strands will not significantly increase the total conductor surface area. Also with few strands there is a stronger magnetic field between the strands. This causes distortion due to vibration between the strands. Using many strands distributes these magnetic fields and dilutes this effect rapidly. Kelsey’s SPL cable is highly stranded with up to 56 strand conductors (dependant on cable size).

The illustration shown here is a 40cm diameter conductor used to feed a high frequency transmission mast. To improve its performance the tubes are also filled with nitrogen gas, the best known dielectric material. This cable shows all the ‘ideal’ theory that we try to encapsulate in practical ways into Kelsey cable design.

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