What is a coaxial cable?

Coaxial cable is a type of cable which has an inner conductor and is surrounded by a shield. These shields are concentric and conducting, with the two separated by an insulating material or dielectric. A protective outer sheath or jacket is present in many of the coaxial cables. Thus, the inner conductor and the outer conductor shield sharing a geometric axis is termed as "co-axial". In transmission lines, these coaxial cables are used to carry high-frequency electrical signals with a low amount of power loss. Coaxial cables are used in applications such as broadband internet, high-speed computer data buses, telephone trunk lines, networking cables, cable television signals, etc. To function efficiently as a transmission line, the dimensions of the cable and the coaxial cable connectors are controlled to give a precise conductor spacing that is constant and so it differs from shielded cables.

Application of coaxial cable

For radio frequency signals, coaxial cables are used as a transmission line. Coaxial cables are used to connect feedlines with the antenna's radio transmitters and receivers, distribution of cable television signals, and computer network connections. The electromagnetic field carrying the signal is only present between the inner and outer spaces of the conductor in an ideal coaxial cable and this is the advantage of coaxial cable over another radio cable. Without the power loss, which occurs in other types of power transmission lines, the coaxial cable can be installed in the next of the metal objects such as glitters. From external interference of the electromagnetic field, the coaxial cable protects the signal.

Description of coaxial cable

A coaxial cable consists of an inner conductor which is surrounded by an insulating layer. This layer is typically one to two layers of woven metallic tape which helps to conduct electric signals. An outer insulating jacket protects the coaxial cable. A voltage carrying signal is applied to the center conductor. The outside of the shield is kept always at ground potential. With little leakage outside the shield, the signal's magnetic and electric fields are restricted to the dielectric and this is the advantage of a coaxial design with differential mode. Multiple shield coaxial cables are larger in diameter and have less leakage. The common applications of coaxial cables are RF, microwave transmission, video, CATV distribution, etc. The radii of the inner and outer conductor and the dielectric constant of the inner insulator help to determine the characteristic impedance of the cable. A uniform cable characteristic impedance is important to minimize loss where the cable length is comparable with the wavelength of the transmitted signal in radio frequency systems. To ensure minimum standing wave ratio and maximum power transfer, the load and source impedance are chosen to match with the impedance of the cable.

Construction of coaxial cable

The choices of coaxial cable design affect frequency, physical size, power handling capacities, attenuation, strength, flexibility, and cost. Stranded is more flexible among solid or stranded inner conductors. The inner conductor may be silver-plated to get a better high-frequency environment. The insulator which surrounds the inner conductor may be foam plastics, solid plastics, or air with spacers that support the inner wire. Some of the electrical properties of the cable are determined by the properties of dielectric insulators. Braided copper wires forming the shield are used in many types of conventional coaxial cables. Since the braid cannot be flat, the inner dimension of the shield varies slightly. The braid is silver plated sometimes and some cables have a double layer shield for better shield performance. Covered by a wire braid, it is more common to have a thin foil shield and the shield might be just two braids. The high-quality tri-shield coaxial cable has an inner conductor wire made of copper. The tri-shield cables have a diameter of 1.02 mm which ensures low signal loss and durability. The quad-shield is the type of shield where there are four layers of foil and braid alternatively. Some cables may invest in a quad-shield. Some types of shield designs sacrifice flexibility and focus more on better performance. Some shield types are solid metal tubes and those cannot be bent as the shield will kink which will cause losses in the cable. The inner conductor is supported by spiral strands of polythene in some low-loss coaxial cables such as RG-62 cable type. This is done so that there is an air space present in between the conductor and inside of the jacket. To transmit connecting indoor television and observe camera data the RG6 cables are used. RG-6 cables are applied for baseband near signal transmission distance. Within considerable signal transmission distance, the RG-11 coaxial cables are used for wide broadband. RG-11 coaxial cables are used in application as the main cable line in cable television and transmitting observing camera data. The low loss cables and ultra-low loss cables as compared to RG coaxial cables have far better shielding properties which help to achieve a low loss of attenuation at higher frequencies.

Connectors

The connectors are connected at the end of the coaxial cable usually. Coaxial connectors have the same impedance as the attached cable. These connectors are typically plated with metals that have high conductivity such as tarnish-resistant gold or silver. When using a coaxial cable the signal loss of the connector and the cable must be taken into consideration because this will affect the signal strength.

A coaxial RF connector is a type of electrical connector. There are many types of RF connectors such as C-Type coaxial cable connectors, BNC connectors, Dezifix connectors, F-Type connectors, Motorola connectors, FME connectors, G-Type connector, N-Type connector, etc. For instance, the F-type connectors are basically coaxial RF connectors that are used for over-the-air cable television, terrestrial television, and satellite television usually with RG-6/U cable or with RG-59/U cable. The BNC connector stands for "Bayonet Neil-Concelman. It is a quick-connect or disconnect radio frequency connector which is used for coaxial cables. The Bayonet Neil-Concelman features two bayonet lugs on the female connector. With the help of a quarter-turn of the coupling nut, the mating is fully achieved.

Types of coaxial cables

• Hard line coaxial cable: Hard line coaxial cables are constructed with the help of materials such as silver, copper, aluminum, or steel. They are typically larger in diameter than other forms.
• Flexible coaxial cable: To suit the need and geometry of the application this type of cable can flex and move as needed.
• Semi-rigid coaxial cable: With a dielectric of PTFE they make use of a solid copper outer sheath. The dielectric properties enhance high-frequency performances.
• Formable coaxial cables: These types of cables are alternatives to semi-rigid cables. They are also known as conformable cables.
• Rigid line coaxial cable: Using a PTFE support or disk insulators, the two concentrically mounted copper tubes are supported at fixed intervals across the cable length in rigid line coaxial cables.
• Other types of coaxial cables are Twinaxial, Triaxial, Micro coaxial, High-frequency coaxial cables, etc.

Context and Applications

This topic is significant in the professional exam for undergraduate, graduate, and post-graduate courses.

• Bachelors in Electrical Engineering
• Bachelors in Electronics and Telecommunication
• Masters in Electrical Engineering
• Masters in Electronics and Telecommunication

Practice Problems

1. What type of signals do the coaxial cables carry in transmission lines?

1. High-frequency signals
2. Low-frequency signals
3. Ultra high-frequency signals
4. Ultra low-frequency signals

Answer: Option a

Explanation: In transmission lines, coaxial cables are used to carry high-frequency electrical signals with a low amount of power loss.

2. Where the connectors are connected to the cable?

1. Front
2. End
3. Back
4. None

Answer: Option b

Explanation: The connectors are connected at the end of the coaxial cable.

3. What is the diameter of the tri-shield coaxial cable?

1. 5 mm
2. 1.02 mm
3. 2.02 mm
4. 3.05 mm

Answer: Option b

Explanation: The tri-shield cables have a diameter of 1.02 mm which ensures low signal loss and durability.

4. What is the impedance of coaxial cable connectors?

1. Same as that of the cable
2. Different from that of the cable
3. Both a and b
4. None of the above

Answer: Option a

Explanation: Coaxial connectors have the same impedance as the attached cable.

5. Which kind of cable has the highest diameter?

1. Hard line coaxial cable
2. Flexible coaxial cable
3. Formable coaxial cable
4. Rigid line coaxial cable

Answer: Option a

Explanation: The hard line cables have the highest diameter among the others.

Related Concepts

• Balanced line
• Radio frequency power transmission
• BNC connector
• LEMO connector