A transmission line is a set of conductors or other structures designed to guide electromagnetic waves (energy) from one place to another. In power system engineering, the transmission line is used to carry electrical power from a source to a load with minimal distortion and loss. It serves as a path for electrical waves to be transmitted through them. Transmission lines are crucial components in various electrical and electronic systems, including electric power distribution, telecommunications, radio frequency (RF) systems, cable television signals distribution, computer network connections, and high-speed digital data transmission.   

There are different types of transmission lines, each suitable for specific applications. The types of transmission lines include coaxial cable, planar transmission line, microstrip, stripline, waveguide, and open wire transmission line.

­What is a balanced line?

A balanced line is a transmission line. It consists of two conductors of the same type and same geometry, with equal impedances along their length and to ground and other circuits. The balanced line exhibits a symmetrical relationship between the conductors.

A few examples of balanced lines are:   

• Twin-lead cable - used for radio frequency communications
• Twisted pair- Used for traditional telephone, data communications, and professional audio. Category 5 cable is an example of twisted pair cable.  
• Transmission lines are used in single-phase and three-phase electric power transmission.    

Figure: Examples of balanced lines

Key benefits of balanced line:

Balanced lines are inherently good at rejecting common-mode noise and electromagnetic interference. This is because the lines have the same impedance to the ground. Due to this same impedance to ground characteristic, the interfering fields or common mode noise currents induce voltage with the same magnitude & identical polarity in both conductors. The load or appliance (e.g., differential amplifier) connected at the balanced line end responds only to the difference in voltage between the two conductors. Hence, the voltage induced with the same magnitude & identical polarity by the interfering field or noise current interference will be canceled out when reaches the load. The ability of balanced lines to reject common mode noise and interference makes them ideal for transmitting signals over long distances with signal integrity. 

Drawbacks of balanced transmission line:

Cost: Circuits driving by balanced lines must be balanced to maintain the benefits of balance. This may require transformer coupling (repeating coils) or impedance balancing in each conductor. Hence, implementing a fully balanced system might be more expensive than an unbalanced one. 

Complex design: The design and implementation of balanced systems can be more complex.

Balanced transmission line applications: 

Balanced lines are favored where noise rejection and signal integrity are critical, especially in long-distance communication. They are widely used in applications where signal integrity is important, such as professional audio systems, security camera systems, telecommunications, and networking, as well as in long-distance communications.

Note: Conductors carrying symmetrical signals (the signals that have equal magnitude but opposite polarity on each conductor) are often incorrectly called "balanced". This is known as the differential signaling concept. Balanced lines and differential signaling are often used together, but they are not the same concept. Note that Differential signaling (i.e., symmetrical signals) does not make a line balanced, or the noise rejection in balanced cables does not require differential signaling (i.e., symmetrical signals). i.e., Balanced lines are always balanced irrespective of whether any signal is present and do not rely on symmetrical signals for correct operation.

What is an unbalanced transmission line?

An unbalanced line is a transmission line consisting of a pair of conductors that have unequal impedances along their lengths and to ground and other circuits. The unbalanced line usually consists of a signal conductor and another conductor that is grounded (i.e., at ground potential). The grounded conductor serves as a reference or return path and often takes the form of a ground plane or screen of cable. Also, the ground conductor often and may be common to multiple independent circuits. The conductors of the unbalanced line have different geometry, which exhibits an unsymmetrical relationship between the conductors.

Figure: Unbalanced line examples 

Coaxial cables are a common example of unbalanced transmission lines. The coaxial cable has a central signal conductor surrounded by insulation and a grounded shield.  The coaxial cables are widely used in several RF applications.

Other examples of unbalanced lines are listed below.   

• Planar transmission lines- Flat conductors manufactured onto a substrate, working at microwave frequencies. It is used to interconnect components on printed circuits and integrated circuits. 
• Stripline - a flat conductor sandwiched between two parallel ground planes, and the structure is supported by the dielectric. It can be easily built on a printed circuit board (PCB) and used in several RF applications. 
• Microstrip - a conductor is separated from a ground plane by a dielectric layer (substrate).  It can be easily built on a printed circuit board (PCB) and used to convey microwave-frequency signals and used in several applications. 
• A transmission line used in single-wire earth return for remote location electric power transmission is also an example of an unbalanced line.

Key benefits and applications of unbalanced transmission line:

Simplicity: Unbalanced systems are often simpler to design and implement as compared to balanced line systems.

Cost-Effectiveness: Unbalanced systems can be more cost-effective than balanced line systems.

Drawbacks of unbalanced transmission line:

Susceptibility to Noise: Due to the unequal impedance along the lines and other conductors, the unbalanced lines are more susceptible to noise interferences, which can degrade signal quality. 

Limited Distance: The unbalanced lines are not suitable for longer-distance transmission due to their higher susceptibility to noise interference. They are used to transmit signals over short distances only.

Unbalanced transmission line applications:   

Unbalanced lines are often chosen for shorter distances and when simplicity and cost-effectiveness are primary considerations. The unbalanced lines are used in several applications, including consumer audio equipment, such as home stereos and basic musical instruments, consumer-level products, camera inputs, simple data communication systems, and more.  

Note: The unbalanced lines are not to be confused with single-ended signaling since both are different concepts. The former is a cabling scheme, and the latter is a signaling scheme. However, single-ended signaling is commonly sent via unbalanced lines.

Balanced vs Unbalanced transmission lines:

The following table provides a comparison between the balanced and unbalanced lines.

 How to interface the Balanced and unbalanced lines?

 In RF engineering, Balanced and unbalanced lines can be interfaced by using a device called a balun.