The ratio of voltage to current at each point on an infinitely long transmission line is equal to the characteristic impedance, denoted by the symbol Z. express. Characteristic impedance of coaxial cables

　　Z. =[ 138/√ ε r] x log (D/d) ohms. Usually Z= 50 ohms/or 75 ohms

　　In the formula, D is the inner diameter of the copper mesh of the outer conductor of the coaxial cable;

　　D is the outer diameter of its core wire;

　　ε r is the relative dielectric constant of the insulating medium between conductors.

　　It is not difficult to see from the above equation that the characteristic impedance of the feeder is related to the conductor diameter, conductor spacing, and dielectric constant of the medium between conductors, and is independent of the length of the feeder, operating frequency, and the impedance of the load connected to the feeder terminal.

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