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Microwave Coaxial line basics


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This article describes microwave coaxial line basics and mention types of coaxial lines. It also mention characteristic impedance of coaxial line.

Coaxial line is one of the type of transmission line which is used to transport RF energy from one port or part to the other port/part of the system. The standard coaxial cable is used as microwave coaxial line. This form of line usually will have two conductors shaped cylindrical around the common axis. They both are separated by the dielectric material. At lower frequencies polyethylene form is used as dielectric and at higher frequencies teflon material is used.

Types of coaxial Lines

There are various forms of coaxial line depending upon the conductor structure and shielding method employed. The coaxial line types include standard coaxial cable as described above as well gas filled coaxial line,articulated coaxial line and double shielded coaxial cable.

Flexible coaxial cable is used in TV broadcast receiving antenna and its outer conductor is made of foil or braid.

At microwave frequency outer conductor is rigid and dielectric will be solid. In a gas filled type of coaxial cable, center conductor is made of thin ceramic insulator,teflon is also used. The dry nitrogen can be used as dielectric material.

In an articulated coaxial line inner insulator is ridged around the inner conductor.Around that shield conductor and around that protective insulating sheath is covered.

In a double shielded coaxial line usually will have two layers of protection by providing inner and outer shield layers. This protects the signal from EMI and also any radiation from the cable to affect the nearby systems.

Characteristic Impedance of Coaxial Line

Characteristic Impedance of basic coaxial cable can be determined using following equation.
Zo = 138/sqrt(K) * Log(D/d) Ohm
Where,
K is the dielectric constant of the insulator between inner and outer conductors. D is the diameter of outer conductor and d is the diameter of the inner conductor.

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