As the name suggests, transmission line establishes connection between the signal source and destination by providing a dedicated path for signal passage. As far as communication and electronics engineering is concerned, a transmission line can be defined as a specialized cable designed to carry/transport alternating current of radio frequency, that is, currents with frequency high enough such that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses.
Various implementations of transmission lines are popular among which two-conductor transmission line such as parallel line (ladder line), coaxial cable, stripline, and microstrip are most prevalent. Several literatures refer to waveguide, dielectric waveguide, and even optical fibre as specialized categories of transmission lines, however these lines require different analytical techniques employed in electromagnetics for understanding of its basic working principle.
Majority of the transmission lines are high-voltage three-phase alternating current (AC) type, although single-phase AC is often employed in railway electrification systems. High-voltage direct-current (HVDC) technology is used for greater efficiency at very long distances (typically hundreds of miles), in submarine power cables and in the interchange of power between grids that are not mutually synchronized. Transmission lines are also employed for electricity transmission at high voltage levels (115kV or above) to reduce the energy losses in long-distance transmission. Power is generally transmitted through overhead electric cables. Underground power transmission has an altogether higher expense and associated operational limitations but is sometimes used in urban areas or sensitive locations.
According to the basic electromagnetic theory, at high frequencies (order of few GHz), the wavelength (denoted by ‘λ’) is much smaller as compared to the circuit size (‘l’) or physical length, i.e., λ<< l thus, circumstances emerge where basic circuit theory no more holds good to validate analytical solutions of transmission line theory. Here, laws of electromagnetism are employed which considers transmission line as a distributed network rather than a lumped network, which was the case in circuit theory approach.
As far as commercial usage is concerned, coaxial cables are considered to be the most popular variety. Coaxial cables have a centrally located core wire, concentrically encompassed by a non-conductive material (dielectric or insulator), and then surrounded by a shielding made of braided wires. Finally, the coax is protected by an outer casing made of PVC material. The central conductor carries the RF signal and the outer shield prevents RF signal radiation loss and inter-signal interference between signals propagating inside and outside the core element. Even though the coaxial cable has significantly low radiation losses, there is some resistance suffered by the signal and it eventually fades out while propagating along considerable distances within the core. This fading phenomenon is termed as attenuation requires minimization to the maximum possible extent, maintaining very short cable length and using high quality cables. Since, power loss is non-linear; doubling the cable length would result in considerable loss of transmitted power. Similarly, reduction of cable length would facilitate more power delivered at the destination. Subsequently, the best arrangement is to put the transmitter in close vicinity with the antenna, such that necessity of coaxial cable length interfacing the two would be least.