What is Coaxial Cable?

Coaxial cable (coaxial) is a type of electrical cable with a specific construction, designed to carry high-frequency signals with minimal losses and protection against external interference. Its unique coaxial geometry - where all components share a common axis - is key to its performance.

Main Components:

• Inner Conductor (Core): Central wire, typically solid copper or copper-clad steel, which carries the high-frequency signal.
• Insulator (Dielectric): Layer of insulating material surrounding the core, electrically separating it from the shield. The dielectric properties (e.g., polyethylene, Teflon) are crucial for the cable's electrical parameters.
• Outer Conductor (Shield): Metallic layer, often in the form of a braid of thin copper wires or aluminum foil, surrounding the dielectric. Serves as the second conductor of the circuit and protects the core from external Electromagnetic Interference (EMI).
• Protective Jacket: The outermost layer, typically made of PVC, providing mechanical protection and resistance to environmental conditions.

How It Works: TEM Wave

The coaxial structure acts like a waveguide, confining the electromagnetic energy of the signal between the inner and outer conductors. This energy propagates mainly as Transverse Electromagnetic Wave (TEM).

For a TEM wave in coaxial cable:

• Lines of Electric Field $(E)$ radiate from the inner conductor to the outer shield.
• Lines of Magnetic Field $(H)$ form concentric circles around the inner conductor.
• Both fields are perpendicular to each other and to the signal propagation direction along the cable axis $(z)$.

This field confinement is what gives coaxial cable excellent shielding properties and low interference emissions.

Primary Electrical Parameters (RLCG)

The behavior of a coaxial line is fully described by four distributed primary parameters, given per unit length (e.g., per meter or kilometer).

• Resistance $(R)$: Represents ohmic losses in the inner and outer conductors. Increases with frequency due to skin effect.
• Inductance $(L)$: Results from magnetic field energy stored around and within the conductors. Consists of frequency-dependent internal part and geometry-dependent dominant external part.
• Capacitance $(C)$: Results from electric field energy stored in the dielectric between conductors. Is determined by cable geometry (diameter ratio) and dielectric permittivity ($\epsilon_r$). Formula: $C = \frac{2 \pi \epsilon_0 \epsilon_r}{\ln(\frac{D}{d})}$
• Conductance $(G)$: Represents energy losses in the dielectric insulator due to leakage current. Increases with frequency and is proportional to the dielectric loss tangent of the dielectric.

Secondary Wave Parameters

Although the RLCG parameters fully describe the line, in practice it's more convenient to work with secondary (wave) parameters that directly describe how the signal propagates.

Characteristic Impedance $Z_c$

This is probably the most important parameter. It is the impedance that the transmission line presents to a propagating wave. To ensure maximum power transfer and no signal reflections, the impedances of source, line, connectors, and load must be matched.

Standard impedances for coaxial cables are 50 Ω (used in RF test equipment, data networks) and 75 Ω (used in video and television systems). These values are an engineering compromise: minimum attenuation occurs around 77 Ω, while maximum power handling occurs around 30 Ω.

Propagation Constant $\gamma$

This complex number describes how the amplitude and phase of a signal change during propagation along the line. Formula: $\gamma = \sqrt{(R + j\omega L)(G + j\omega C)}$. It consists of two parts:

• Attenuation Constant $\alpha$: Real part of $\gamma$, measured in nepers/m or dB/m. Describes exponential decay of signal amplitude with distance.
• Phase Constant $\beta$: Imaginary part of $\gamma$, measured in radians/m. Describes the rate of phase change of the signal with distance. It is related to the wavelength in the cable: $\beta = \frac{2\pi}{\lambda_{\text{cable}}}$.

Shielding Effectiveness

The main advantage of coaxial cable is its excellent shielding. The effectiveness of this shield is quantified using several measures:

• Transfer Impedance $Z_t$: Measure of how current flowing on the inner side of the shield "leaks" through, inducing voltage on the outer side of the shield. Measured in $\Omega/\text{m}$, and lower value means better shielding.
• Shielding Effectiveness (SE): Measured in decibels (dB), it is the ratio of external electromagnetic field intensity to the field intensity that penetrates through the shield to the inner conductor. Higher value in dB means better shielding.

  Formula: $SE_{\text{[dB]}} = 20 \cdot \log_{10} \left( \frac{E_{\text{incident}}}{E_{\text{internal}}} \right)$

Popular Applications

Coaxial cables are used in a wide range of applications, including:

• Cable television distribution (CATV)
• Satellite television installations (connecting dish to receiver)
• Antenna feed lines (e.g., in amateur radio, cellular base stations)
• Legacy computer networks (e.g., 10Base-2 "Thinnet" Ethernet)
• High-frequency test and measurement equipment
• Video connections (e.g., composite, SDI)