Types of Transmission Line

 

Transmission line is a cable that transfer Electromagnetic wave from source to destination. Conventional electrical cables are adequate for carrying low frequency alternating current (AC), such as mains electricity, which reverses direction 100 to 120 times per second, and audio transmissions.  however, Electrical cables cannot be utilized to carry radio wave flux over around 30 kHz since the energy starts to radiate off the cable as radio signals, resulting in power dissipation. Radio frequency currents also generally reflect from cable irregularities like connections and joints and flow back across the cable towards to source. Typically transmission lines are distinguished by having uniform cross sectional dimensions over entire length, resulting in a standard impedance, known as the characteristic impedance, to minimize reflections. Parallel lines (ladder lines, twisted pairs), coaxial cable, and planar transmission lines such as stripline and microstrip are examples of transmission lines. The shorter the wavelength of electromagnetic waves travelling through a certain wire or medium, the higher their frequency. Transmission lines are required when the wavelength of the transmitted frequency is so small that the length of the cable becomes a key element of the wavelength.

Transmission lines are classified into two types: cables and waveguides. For frequencies above HF, RF cables are nearly entirely coaxial cables (referred to as coax, derived from 'of common axis). A core wire is surrounded by a non-conductive substance (dielectric or insulation), which is subsequently surrounding by an enveloping shielding, which is typically composed of braided wires. The dielectric separates the core and the shielding. Finally, the coax is shielded by an exterior shielding, which is often made of PVC. The RF signal is carried by the inner conductor, while the outer shield prevents the RF signal from propagating into the sky and prevents outside signals from interfering with the signal carried by the core. Above 2 GHz, the wavelength is shorter enough even enabling practical  and efficient energy transmission via several methods. A waveguide is a conducting tube that transmits energy in the form of electromagnetic waves. The tube works as a boundary, keeping the waves in the confined region at range. Outside of the guide, the skin effect prevents any electromagnetic impacts from being evident. Electromagnetic fields are carried through the waveguide via reflections off its inner walls, which are ideal conductors. The strength of the fields is highest in the center all along X axis and must be zero at the outer walls since any field parallel to the sidewalls at the surfaces would allow infinite current flows in a perfect conductor.Waveguides cannot transmit RF in this manner.

Proper cable can be chosen according to the application. First rule for cable implementation is always try to use short transmission line to avoid power losses. RG-58 should not be for microwave.RG-213 is only for High frequency radio. On the other hand, waveguide are in two shapes rectangular or circular. The important parameter in communication omain is reflection coefficient and impedance matching, that is required in both the cases.

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