Electronics components, Magnetism, RF circuits, Telecomunications

Coaxial cable: how it works?

How the coaxial cable works and why it’s so used in signal transmission in telecommunications? It’s this post subject.

Coaxial cable structure

Source: Pinterest.

The core conductor is usually made of copper and transmit high frequency data. The metallic mesh is a shield, usually made of copper or aluminum. Whose function is protect central conductor from electromagnetic interference (EMI) and also transmit signals. Between the mesh and central conductor, there is the insulator, a plastic dielectric to avoid short-circuit. Coaxial cables also have a aluminum foil between the mesh and the insulator to increase protection against EMI. Finally, PVC outer to protect the wire.

coaxial cable
The core can also many twisted wires with a steel cable in the center. Source: Bolton Technical.

How it works?

When receiving current, both central conductor and mesh generate electric fields. These fields are calculated using Ampère law. To understand this law, it’s necessary to know advanced mathematics.

fields inside coaxial cable
The electric field is generated by potential difference between inner and outer conductors, forming a capacitance. H is produced by the current on conductor. Source: Wikimedia.
direction of magnetic field
Convention of any vector or field direction. You can think like an arrow. Source: RF Cafe.

Applying Ampère law, the magnetic field outside the cable is zero. What is Poynting vector? It’s the vector \vec{S}, which describes the energy transmission rate by area unit, caused by an electromagnetic wave. It’s calculated with the equation below.

\vec{S}=\frac{1}{\mu _{o}}\vec{E}\times \vec{B}

  • \mu _{o} is the vacuum magnetic permeability, whose value is 1,26\cdot 10^{-6}H/m.
  • \vec{B} is the vector of magnetic flux density.

In addition to protection against short-circuit, insulator keep constant the distance between the conductors, avoiding magnetic field distortion inside.

Vantages and disadvantages of coaxial cable

The advantages to use coaxial cable over other transmission lines:

  • Can transmit long distances with minimal losses.
  • Longer durability than common wires and twisted pair.
  • Because it doesn’t have an external magnetic field, don’t causes interference in sensitive electronic devices.
  • Can be used in broadband applications, as high definition video.
  • Cheaper than optic fiber.

However, the disadvantages are:

  • A failure on the cable can compromise the entire network.
  • For long distance installations, becomes more expensive than twisted pair cable, due to thickness and rigidity.
  • Network subject to congestion.
  • Security: the cable can be broke to put splitter or T joint.
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