# Transmission lines (feeders)

Feeders can be balanced (twin line) or unbalanced (coaxial). The characteristic impedance of a balanced feeder is determined by the diameter of the wires and their spacing. The characteristic impedance of an unbalanced feeder is determined by the diameter of the inner conductor, the inner diameter of the outer sheath, and the relative permittivity of the dielectric. A feeder which is perfectly matched to an antenna will indicate resistance only. A dipole is a balanced antenna and should be fed by a balanced feeder.

The velocity of propagation in free space is given by

Hence the relationship,

The velocity of propagation in a medium is less and is obtained by multiplying v by the velocity factor.

A feeder on open-circuit or short-circuit is completely mismatched. This is the basis of a stub, which can be used to attenuate an unwanted signal. Quarter-wave lengths of feeder can be used for matching dissimilar characteristic impedances.

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