Basics of Coaxial Cables Used in Electronic and Computer Systems

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A coaxial cable consists of four basic parts:

  • Inner conductor (center conductor)
  • Outer conductor (shield)
  • Dielectric, which separates the inner and outer conductors
  • Jacket, which is the outer polymer layer protecting the parts inside



Parts of a Typical Coaxial Cable Photo Credit : ANIXTER CABLES

The following characteristics/properties help to define a coaxial cable as applied in electronic systems and Computer Systems:
1. Characteristic impedance
2. Voltage Standing-Wave Ratio (VSWR)
3. Velocity of Propagation
4. Voltage Rating
5. Operating Temperature

Characteristic Impedance
The characteristic impedance of a coaxial cable is a function of its geometry and materials. Characteristic impedance is independent of length and typically ranges from 35 to 185 ohms. The most common values are 50, 75 and 93 ohms. The characteristic impedance of a cable is not the same as the  impedance of the conductors in a cable, which is dependent on length.
The most efficient transfer of energy from a source to a load occurs when all parts of the system have the same characteristic impedance. To have better performance with coaxial cable, there is need for impedance matching especially critical at higher frequencies, where the consequences of mismatches are more severe.

Voltage Standing - Wave Ratio (VSWR)
The voltage standing-wave ratio (VSWR) is a measure of the standing waves that result from reflections. It expresses the uniformity or quality of a cable’s characteristic impedance. Uniformity is also measured as structural return loss (SRL).

Velocity of Propagation
Velocity of propagation is the speed at which electromagnetic energy travels along the cable. In free space or air, electromagnetic energy travels at the speed of light, which is 186,000 miles per second. In other materials, however, the energy travels slower, depending on the dielectric constant of the material. Velocity of propagation is expressed as a percentage of the speed of light. For example, a velocity of 65 percent means that the energy travels at 120,900 miles per second – or 35 percent slower than in free space. The dielectric (insulation) separating the two conductors determines the velocity of propagation. Although the electromagnetic energy travels in the dielectric, the current associated with the energy travels primarily on the outside of the center conductor and the inside of the outer conductor (shield).

The two conductors bind the energy within the cable. Consequently, the quality of the dielectric is important to efficient, speedy transfer of energy. Speed is important to engineers who must know the transit time of signals for digital transmission.

Voltage Rating
This is the maximum voltage the cable is designed to handle.

Operating Temperature Range
These are the minimum and maximum temperatures at which the cable can operate.

Types of Coaxial Cables
There are many types of coaxial cables but four types are commonly used namely:
1. Flexible Coax
2. Semirigid Coax
3. Triaxial 
4. Dual Coax
There is also Twinaxial (Twinax) Cable used in high-speed, balanced-mode multiplexed transmission in large computer systems.

Flexible Coax
The most common type, flexible coax has a braided outer conductor (shield) of extremely fine wires. While the braid makes the cable flexible, it does not provide complete shielding – energy (RF signals) can leak through the shield via minute gaps in the braid. To combat this, many cables have several layers in the outer conductor. In addition, thin foils are sometimes used to supplement the braid to provide better coverage for greater shielding effectiveness. The greater the coverage, the better the shield

Semirigid Coax
Semirigid coax has a solid, tubular metallic outer conductor, similar to a pipe. This construction gives the cable a very uniform characteristic impedance (low VSWR) and excellent shielding, but at the expense of flexibility.

Triaxial Cable (Triax)
This coax has two outer conductors (shields) separated by a dielectric layer. One outer conductor (shield) serves as a signal ground, while the other serves as earth ground, providing better noise immunity and shielding. One caution: Do not confuse a flexible cable having a multilayer outer shield with triaxial cable.

Dual Coax
This cable contains two individual coaxial cables surrounded by a common outer jacket.
Shown below are the four basic types of coaxial cables commonly used
Common Types of Coaxial Cables - Photo Credit : ANIXTER CABLES

Twinaxial Cable (Twinax)
Twinax has a pair of insulated conductors encased in a common outer conductor (shield). The center conductors may be either twisted or run parallel to one another. In appearance, the cable is often like a shielded twisted pair, but it is held to the tighter tolerances common to fixed-impedance coaxial cable. A common use of twinax is high-speed, balanced-mode multiplexed transmission in large computer systems. Balanced mode means that the signal is carried on both conductors, which provides greater noise immunity.

A Typical Twinaxial Cable - Photo Credit: ANIXTER CABLE