Transmission Line

There appears to be some controversy, disagreement, or lack of understanding surrounding the term "transmission line". While most engineers familiar with transmission lines understand two-conductor transmission lines are fed differentially at one end by a source and have a termination placed differentially across the far end, a few seem to disagree.

Let's go through this and see if we can sort transmission line mode operation from other forms of energy transfer parallel or concentric conductors can sustain. 

Conventional Use of Transmission Line

 We all know a traditional transmission line system appears like this:

Fig 1.

In each of these cases, the line will not radiate or contain substantial electric or magnetic fields external to the line area. The lack of external fields, even at a very small distances, is rooted in two conditions:

    1.) All outgoing currents on one conductor are matched by equal level and exactly opposite phase currents on a return conductor at any given point along the line. This causes an exactly equal and opposite magnetic field along each conductor. The opposing magnetic fields caused by equal currents flowing opposite directions cancel magnetic fields outside the general area of the two conductors.

     2.) All voltages from each conductor of the line to the outside environment surrounding the line are either contained within a closed shield, or are exactly equal and opposite an imaginary neutral reference point representing the environment around the line (balanced lines). We always have a constant differential voltage across the line (between the conductors) and that voltage changes only with standing wave ratio as we move along the line.

    3.) The vector product of differential current flowing in conductors and voltage between line conductors at any point along the line always equals the power transmitted in transmission line mode.

(Number two and three above are very important. They indicate a TEM wave. To understand it think about how your rig connects to your feedline. Everyone knows the alternating current coming from our radios has voltage across the output jack. At any instant of time when energy is being transferred to the load  ((except when zero is being crossed)) the voltage polarity of the two conductors is of opposite signs. Except for zero crossing or when the transmitter is off the potential difference is always there, and the vector product of voltage across the line and current flowing through the line always equals applied power.) 

The conditions above are required to support energy flow through a transmission line. That mode is called TEM mode, or transverse electromagnetic mode. All two conductor transmission lines, either coaxial or balanced, transfer energy down the line by TEM mode.

Here is what Edward Jordan and Keith Balmain say about TEM mode operation of transmission lines in the classic Prentice-Hall Electrical Engineering Series textbook "Electromagnetic Waves and Radiating Systems":

To make a long story short, classic transmission line theory (called "ordinary transmission line theory" in the text above) requires the wave to be launched from one end of the two parallel conductors forming our transmission line. If we do not do that, we simply have two parallel conductors magnetically and electrically coupled. Energy will not be confined to the "transmission line", and can radiate out into the surroundings freely. This is a very important distinction when dealing with feedlines and antennas! In transmission line mode (transverse electromagnetic mode) we can sleeve the line with ferrites and properties inside the line do not change. The electrical length does not change, losses do not change, and the frequency response is very wide as it is with any transmission line of similar design. This occurs because energy in a two-conductor transmission line is transferred via TEM mode; fields are confined to the general area between the conductors. Things outside that energy path, such as ferrite beads, metallic conduit, and other conductors or cables do not affect TEM mode or transmission line mode energy flowing through the transmission line. Except for very low levels caused by slight flaws in the lines, signals don't leak in and signals don't leak out when we have a transmission line operated in transmission line mode.  

Balance

 Unbalanced lines (coaxial cables) actually have equal and opposite currents in the shield and center conductor at any place along the transmission line. So do balanced lines. This leaves us with a question. What makes one line or system balanced and the other unbalanced? Currents behave the same way and are always balanced in a properly working transmission line, its source, or its load regardless of line type, so what's the difference?

The thing separating balanced from unbalanced lines or systems, including antenna feedpoints, is voltage from each conductor or terminal with respect to a physical or imaginary reference point representing the world around the source, feedline, or load.  In the case where the feedline does not radiate both systems have equal and opposite currents at every point. These points include the source, the entire length of transmission line, and the load. It is the voltage that actually makes sources or loads "balanced" or "unbalanced", and the containment of fields inside a shield that causes a coaxial transmission line to be considered "unbalanced".

Common Mode Excitation

We can make a transmission line become a conventional radiating conductor if we apply energy in a non-TEM mode. This can be useful when we wish to use a feedline as an antenna or as a conventional conductor. When we excite a cable like this:

or like this:

The cable freely radiates. Things outside the line influence the line. Adding a ferrite core will add loss and make the line electrically longer. The SWR will change. This is true even when currents are equal in the two conductors, and can even be true when currents are equal and opposite as long as the line was excited from end-to-end!

The key to having a line behave like a transmission line is feeding it differentially at one end, and not applying voltage across the length of one or both conductors.

This is a transmission line as we generally know it, and as dozens of reputable engineering textbooks define it:

The above configuration shows a direct wire connection from source to load, and cannot transform voltage, current, or impedance based on turns ratio. This fits the definition of classic transmission line, which requires TEM mode in coaxial or parallel wire lines. Jordan and Balmain cover this extensively in "Electromagnetic Waves and Radiating Systems" (Guided Waves, p215, 2nd ed). Kraus also covers this in "Antennas" in various sections dealing with transmission lines and wave propagation, as does Terman in his "Radio Engineers Handbook" Circuit Theory chapter under the subheading "Transmission Lines".

Most engineering text I have clearly state parallel conductor transmission lines employ TEM mode of energy transfer. If not, they are not considered transmission lines.

Uses of Parallel or Concentric Conductors in Non-transmission Line Mode

Parallel or concentric conductors (coaxial conductors) in non-TEM mode have two major applications, antennas and transformers. Antennas include (but are not limited to):

(this area under construction)

See Lenz's Law

since 7/1/2005