Skin Effect, Characteristic Impdeance

   1.       Skin Effect:

The tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core is skin effect.

                That is, the electric current tends to flow at the skin of the conductor skin effect causes the effective resistance of the conductor to increase with the frequency of the current. Skin effect is due to the eddy currents set up by the A.C. current.

   2.       Wave Guide:

The skin effect phenomenon has led to the development of hollow conductor known as wave guide.

Wave guide is a structure that guides waves. Wave guide can be constructed to carry waves over a wide portion of electromagnetic spectrum, but are especially useful in the microwave ranges. Depending on the frequency, they can be constructed from either conductive or dielectric materials. Wave guides are used for transferring both power and communication signals.

   3.       Characteristic Impedance:

The characteristic impedance of a uniform transmission line, usually written as Zₒ, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI, units of characteristic impedance is ohm.

                “The characteristic impedance of a transmission line, Zₒ is the impedance measured at the input of this line when its length is infinite under these conditions the type of termination at the far end has no effects.”

The characteristic impedance of an iterative circuit consisting of series and shunt elements is given by

                                               

Z = Series impedance per section = R +jWL  Ω/m

Y = Shunt admittance per section = G + jWC  S/m

                                                    

At radio frequencies the resistive components of equivalent circuit become insignificant and the Zₒ reduces to,

                                                     

It shows that this characteristic impedance is resistive at radio frequency.

 

   4.       characteristic impedance for Parallel Wire Line:

For Parallel wire line the characteristic impedance is as under;

                                                Zₒ=276log(2D/d) Ω                                            

   5.       characteristic impedance for Coaxial Line:

the characteristic impedance for coaxial line is as under:

                                               

Where K is dielectric Constant of insulation.