Differentiate the advantages and disadvantages between the balanced and unbalanced transmission line?

Answer 1

There actually isn't any advantage or disadvantages of using a balanced verse unbalanced transmission line. Selection of transmission line is based upon several factors: 1) required transmission line impedance between the source and the receiver (load), 2) application of the transmission line (RF, audio, digital data, etc.), and 3) transmit/receive port design. Factor 1, 2, and 3 examples: Radio frequency energy transfer from a (HF, VHF, UHF) transmitter to an antenna requires a transmission line impedance that equals the output source impedance and the antenna's input impedance. The transmitter output source impedance is typically 50 - 75 ohms and the antenna input impedance is usually the same. Real life examples are amateur radio, commercial broadcast radio and television, citizen band radio, etc. Therefore, a 50 or 75 ohm transmission line is needed to couple the RF energy between transmitter and antenna. Coaxial cables are used to transfer RF transmitted energy from the transmitter to a receiver, and coaxial cables are unbalanced transmission lines. Another even more familiar example is your Cable Television connection. Cable television industries use 75 ohm coaxial cable, therefore, the television's cable input port had to designed by the manufacturer to accept an unbalanced 75 ohm coaxial cable. Hence, the television's input impedance is 75 ohms. In the case of transmission of digital data, CAT 5 cables are used because of their relatively low loss and excellent rejection of mutual inductive coupling of external noise and signals onto the cable. This is achieved by the use of twisted pair cable with external shielding. CAT 5 cable is a balanced transmission line. CAT 5 cable characteristic impedance is 100 ohms. The digital data transmitter and receivers may have been designed to accommodate CAT 5 transmission medium in this case, hence, the output source impedance and receiver input impedance is 100 ohms. By the way, coaxial cable has excellent rejection of external RF signals and other potential sources of noise because of its design as well. The coaxial cable inner conductor is surrounded by a braided shield along the entire length of the cable. Therefore, signal attenuation characteristics and noise rejection capabilities are not dependent upon whether or not the transmission line is balanced or unbalanced. Excellent signal transmission characteristics can be achieved from both balanced and unbalanced transmission lines. Back in the old days when television reception was dependent upon a set of rabbit ears or an external TV antenna mounted on the roof, called a log-periodic antenna, the transmission line connecting the antenna to the TV receiver was a 300 ohm parallel conductor line. The 300 ohm line was cheaper than coax so the television receiver port was designed with a 300 ohm input impedance. That 300 ohm transmission line was a balanced transmission line. Today, televisions come with an unbalanced 75 ohm cable connection. Therefore, if a home owner wants to connect their new TV to their existing roof mounted log-periodic antenna using their balanced 300 ohm cable they would need to install a device called a balun to convert the 300 ohm balanced connection to a 75 ohm unbalanced transmission line connection. These devices are very common and readily available at electronics stores. Audio cables use balanced transmission lines. The audio amplifier output and speaker input ports are designed for balanced connections..