Line inductance:

Short lengths of any type of transmission line behave as inductors, as long as the line is below 90 electrical degrees in length. At 100 MHz, 90 electrical degrees, or 1⁄4

Figure--            Parallel-wire transmission line.

Figure ---         -Coaxial transmission line.

wavelength, in free space is just 75 cm, or a little more than 2 ft. Generally if f is the frequency in megahertz, then 1⁄4 wave length in free space, in centimeters, which can be given by

s  =   7500/f

The length of the quarter-wavelength section of transmission line is shortened from the free space quarter wavelength by effects of the dielectric. Practically, 1⁄4 wave length along the line can be anywhere from about 0.66 of the free space length to about 0.95 of the free-space length.

The factor by which wavelength is shortened is called as velocity factor of the line. This is because shortening of the wavelength is the result of a slowing down of the speed with which the radio signals move in the line, as compared with their speed in space. If the velocity factor of the line can be given by v, then the above formula for the length of the quarter wave line, in centimeters, which becomes

s =    7500v/f

Very short lengths of line a few electrical degrees produce small values of the inductance. As the length approaches 1⁄4 wavelength, the inductance of it increases.

Transmission line inductors behave differently than coils in one important way: the inductance of a transmission-line section changes as the frequency changes. Firstly the inductance will become larger as the frequency increases. At some particular limiting frequency, the inductance goes to infinite.