Trask Balun

Transmission Line

There appears to be a controversy or disagreement between Chris Trask and myself surrounding the use of the words "transmission line balun" or "transmission line transformer". The following is my viewpoint.

Most engineers familiar with transmission lines understand two-conductor transmission lines are fed differentially at one end by a source, and ideally have a load placed differentially across the far end. A 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 at a very small distance is caused for two reasons:

	All outgoing currents on one line are matched by equal level and exactly opposite currents on each conductor at any point along the line. In other words, we have no magnetic field extending outside the area of the line.
	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). In other words, we can have no electric field extending outside the area of the line.

As a matter of fact the only difference separating balanced from unbalanced systems, including antenna feedpoints, is the voltage from the feedpoint to the imaginary reference point representing the world around the source, feedline, or feedpoint. All systems, in the case where the feedline does not radiate, have equal and opposite currents at every point including at the source, along the transmission line, and at the load.

There is a name for the operational mode of a transmission line, it is called the transverse electromagnetic mode or TEM mode. Trask calls this a transverse electric magnetic mode. Here is what he says at http://www.home.earthlink.net/~christrask/TraskTLTTutorial.pdf :

Actually that definition causes problems.

TEM means Transverse Electromagnetic, not electric magnetic. The difference may seem subtle, but it is important to how we perceive the system functions. Electric magnetic implies a circuit component mode like a transformer, while TEM implies wave propagation through a transmission line.

Page 206 section 7.10 in the second edition of the classic engineering text by Jordan and Balmain "Electromagnetic Waves and Radiating Systems" describes TEM mode excitation of concentric (coaxial) or parallel conductors. (You won't find anything in that section allowing anything but differential voltage exciting one end of the transmission line.)

Here is a copy of the engineering text I'm referencing:

To further quote that section:

"Although the TEM transmission-line wave is but one special case of guided wave propagation, it is so important practically, that it is usually treated as "ordinary transmission line theory" quite early in the training of the electrical engineer."

In other words it is common basic training for EE's and widely understood transmission lines employ TEM (transverse electromagnetic) mode, not electric and magnetic coupling or energy transfer.

Conventional Transformer

A few people use the term "transmission line" when describing a conventional transformer with parallel windings. Parallel winding styles are not new. Parallel wound (or interleaved winding) transformers are actually quite old technology. As far as I know the original use was in transformers, where the goal was to minimize flux leakage and increase high frequency response. I have several early Hi Fidelity audio transformers with interleaved windings, and in 1983 the AL12 series power transformer was converted from a conventional to interleaved winding style in order to improve regulation.

In the AL-l2 series power transformer, an 1800 VA CCS rated Hypersil core was used to supply 2600 Vac to a bridge rectifier. The rectifier was followed by a capacitor input filtering system. In an effort to reduce full-load sag in the supply, it was necessary to reduce clipping of the sine wave during the charging period of the capacitors. The interleaved winding reduced leakage reactance slightly at high frequencies, and provided a 1-2% increase in regulation. This was relatively old technology, not something new.

Besides the increase in flux coupling, a parallel winding also adds direct capacitive coupling between windings.

Here is a schematic showing the path of increased capacitance when we build a conventional transformer using interleaved or bifilar winding :

Fig 2

In the above case all energy transfer is through flux coupling or capacitance. There is no direct wire connection from load to source, fitting the definition from the Communications Standard Dictionary:

"an electrical device that can convert a voltage, current, or impedance to a higher or lower level based on turns ratio."

Transformers do not function as transmission lines. Transformers are defined by the use of Lenz's Law to generate an opposing current in the secondary. A transmission line has equal and opposite currents caused by TEM mode excitation. This is because a wave is launched differentially from one end of the conductor pair. A transformer has a single or multiple conductors fed end-to-end (winding start-to-finish) by the source. The secondary has an opposing current just like the transmission line does, but this opposing current is a result of flux coupling (Lenz's Law) and not differential (TEM) excitation.

In a transformer, Lenz's Law requires a loaded secondary to produce a current opposing the primary current. This secondary current produces a magnetomotive force, and the primary draws extra current that increase primary flux levels enough to counteract the secondary magnetomotive force.

This flux interaction between primary and secondary is why transformers have core losses and core materials interact with electrical characteristic through the system.

In a normal transmission line mode, the configuration would be:

Fig 3

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 transmission line, which by definition requires a 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.

Figures 1 and 3 above are transmission lines, but connections like 2 are not. Figure 2 fits the description of a transformer, and even appear to be a transformer when drawn on paper! It does not use TEM mode for energy transmission, it follows Lenz's Law.