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.

Type of Transmission Lines

Practical types of Transmission lines. As follow.

    1)       Coaxial Cable

    2)       Microstrip

    3)       Stripline

    4)       Balanced Line

a.       Twisted Pair

b.      Quad, star Pair

c.       Twin-lead

d.      Lecher Line

     5)       Unbalanced Line (Single-Wire Line)

     6)       Waveguide

     7)       Optical Fiber

1)     Coaxial cable:

Coaxial lines confine virtually all of the electromagnetic wave to the area inside the cable. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them. In radio-frequency applications up to a few gigahertz, the wave propagates in the transverse electric and magnetic mode (TEM) only, which means that the electric and magnetic fields are both perpendicular to the direction of propagation (the electric field is radial, and the magnetic field is circumferential). However, at frequencies for which the wavelength (in the dielectric) is significantly shorter than the circumference of the cable, transverse electric (TE) and transverse magnetic (TM) waveguide modes can also propagate. When more than one mode can exist, bends and other irregularities in the cable geometry can cause power to be transferred from one mode to another.

The most common use for coaxial cables is for television and other signals with bandwidth of multiple megahertz. In the middle 20th century they carried long distance telephone connections.

2)     Microstrip:

A microstrip circuit uses a thin flat conductor which is parallel to a ground plane. Microstrip can be made by having a strip of copper on one side of a printed circuit board (PCB) or ceramic substrate while the other side is a continuous ground plane. The width of the strip, the thickness of the insulating layer (PCB or ceramic) and the dielectric constant of the insulating layer determine the characteristic impedance. Microstrip is an open structure whereas coaxial cable is a closed structure.

3)     Stripline:

A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line.

4)     Balanced lines:

A balanced line is a transmission line consisting of two conductors of the same type, and equal impedance to ground and other circuits. There are many formats of balanced lines, amongst the most common are twisted pair, star quad and twin-lead.

        I.            Twisted pair:

Twisted pairs are commonly used for terrestrial telephone communications. In such cables, many pairs are grouped together in a single cable, from two to several thousand.

     II.            Quad, star quad:

Quad is four-conductor cable used sometimes for two circuits, as in 4-wire telephony, and other times for a single circuit, called star quad, a balanced circuit for audio signals. All four conductors are twisted together around the cable axis. In the quad format, each pair uses non-adjacent conductors. For star quad, two non-adjacent conductors are terminated together at both ends of the cable, and the other two conductors are also terminated together.

The combined benefits of twisting, differential signalling, and quadrupole pattern give outstanding noise immunity, especially advantageous for low signal level applications such as long microphone cables, even when installed very close to a power cable. The disadvantage is that star quad, in combining two conductors, typically has double the capacitance of similar two-conductor twisted and shielded audio cable. High capacitance causes increasing distortion and greater loss of high frequencies as distance increases.

   III.            Twin-lead:

Twin-lead consists of a pair of conductors held apart by a continuous insulator.

   IV.            Lecher lines:

Lecher lines are a form of parallel conductor that can be used at UHF for creating resonant circuits. They are a convenient practical format that fills the gap between lumped components (used at HF/VHF) and resonant cavities (used at UHF/SHF).

5)     Unbalanced Line (Single-wire line):

Unbalanced lines were formerly much used for telegraph transmission, but this form of communication has now fallen into disuse. Cables are similar to twisted pair in that many cores are bundled into the same cable but only one conductor is provided per circuit and there is no twisting. All the circuits on the same route use a common path for the return current (earth return). There is a power transmission version of single-wire earth return in use in many locations.

6)     Waveguide:

Waveguides are rectangular or circular metallic tubes inside which an electromagnetic wave is propagated and is confined by the tube. Waveguides are not capable of transmitting the transverse electromagnetic mode found in copper lines and must use some other mode. Consequently, they cannot be directly connected to cable and a mechanism for launching the waveguide mode must be provided at the interface.

7)     Optical fiber:

Optical fiber is a solid transparent fiber of glass or polymer that carries an optical signal. Optical fiber is a variety of waveguide. Optical fiber transmission lines form the backbone of modern terrestrial communications networks due to their low cost, low loss, and high signal bandwidth (high data rate).