Types of Radio-frequency Transmission Lines

Author: Edmund A. Laport

Circuit-ally there are two basic types of uniform transmission lines for single-phase operation:

1. Balanced lines, where there are equal and opposite potentials from both sides of the transmission circuit to ground.

2. Unbalanced lines, where one side of the circuit is at high potential and the other side is at ground potential. Structually there are also two basic forms:

1. Open-wire lines, where the conductors are supported in the air above ground.

2. Enclosed lines, where one or more conductors forming the transmission circuit are enclosed by a metallic shield that confines the field within the enclosed space. Both balanced and unbalanced lines are included in this class.

Then there are two basic classes of applications for transmission lines:

1. For guiding electrical energy from a generator to a load circuit. This is the application implied by the use of the alternative term "feeder." The charges delivered by the generator move along the line to the load in a single traveling wave.

2. For storing electrical energy in excess of that dissipated in the load. The charges on the system are moving from the generator to the load, and also in the reverse direction, and form standing waves on the system. In this form lines are used as tuned circuits , as reactors, and as impedance transformers. Since a section of transmission line can be used to obtain any desired reactance at any given frequency, a combination of sections can be used to form networks which act as low-pass, high-pass, and bandpass filters of constant-k, m-derived, or lattice types.