RADIO FREQUENCY LINES OR LOW LOSS LINES

A LOW LOSS TRANSMISSION LINE IS ONE FOR WHICH

R<<шl

AND                      G<<шC

Thus                     Z=R+jшL- jшL

And                     Y=G+jшC – jшC

(a)    Characteristic impedance,

Z₀=R+jшL/G+jшC

= jшL/jшC

Z₀= L/C

Z₀ is pure resistance for low-loss line.

(b)   Propagation constant

Ỳ= (R+jшL)(G+jшC)

= (jшL)(jшC)

Ỳ=jш LC

As                          Ỳ=α+jβ

By comparing real and imaginary parts of above two equations, we get

Attenuation constant, α=0

And phase shift constant, β=ш LC

Let us also discuss another concept called BALUN

BALUN consists of two words bal(short form of balanced) and up (short form of unbalanced).It is a form of a quarter wave transformer matching balancedline  to an unbalanced transmission circuit.this problem arises when an ungrounded antenna is being fed by a coaxial cable.The cable from the dipole antenna offers reasonably high output impedance which does not match with the input impedance of the TV receiver.A transformer is thus required that matches the output impedance a parallel wire cable with input impedance of the TV set.

This transformer is known as balun and has got few turns of winding on a ferrite core. Balun is also called as Bazooka.