If baluns bug you…

A recent article (Howarth nd) entitled “If baluns bug you, do yourself a doublet with balanced feed. Antenna matching unit for balanced feedpoint antennas.” described an ATU for two wire line.

The article discusses related matters.

Terms

Let’s discuss / define some terms for clarity.

Balanced feedpoint antennas

Hams speak of balance as a binary property, eg an antenna is balanced or it is unbalanced, nothing in between.

In fact perfection in balance or symmetry is rarely achieved in real wire dipole antenna systems at HF. Physical and electrical symmetry is often compromised by nearby built structures, vegetation, non-uniform soil characteristics etc.

If you have not measured a three terminal equivalent impedance of an antenna system, asserting that it is balanced is hand waving without evidence, wishful thinking.

The subject article title states that this ATU is for “balanced feedpoint antennas”, you might ask whether it is relevant to real world antennas that might look symmetric but on measurement are not so symmetric.

Balance

The term balance is used widely in ham radio without declaring a meaning, that is so in the subject article.

In the context of dipole antennas fed with two wire line, the desired balance condition is usually that at all points along the feedline, the current in one conductor is equal to the current in the other conductor but opposite in direction so that at a distant point, the net magnetising moment is approximately zero, ie that the distant field effects of the two currents approximately balance.

This is also expressed as zero common mode current, and has benefits of reducing radiation contribution from the feed line itself when transmitting and noise pickup by the feed line when receiving. For more discussion see Differential and common mode components of current in a two wire transmission line.

Minimisation of feedline common mode current is a common and rational objective.

Balanced line

The subject article discusses “balanced line” as a solution to some problems.

The term balanced line is used frequently in ham radio as if balance was simply a property of line construction, but the benefits accrue from balanced currents and that is not controlled or constrained by the line construction. Two wire lines can carry common mode current just as coaxial lines can do, just the mechanism is a little different.

The use of a two wire line does not assure current balance, and referring to the construction as a “balanced line” is naive and misleading, specious even.

Balun

In ham radio, the term balun is used in so many different ways, and some meanings exclude what seems like quite valid applications of a device. Hams often call upon the construction or implementation to declare whether a device is a balun or not. An online expert recently stated And remember!!!: A transformer is NOT a balun, it’s just a transformer, no more and no less. It should be followed by a properly chosen balun.

Let me suggest that a more general meaning of balun is a device that permits, facilitates or even encourages some different state of balance on either side of the device. That state of balance might be current balance as discussed above, or it could be voltage balance (equal voltages but opposite phase at two terminals), or it could be some state of imbalance that is not clearly current or voltage balance. The device characterisation should be done at its terminals treating it as a black box, ie without knowledge or regard to the internal implementation.

An ATU might also function as a balun by virtue of a discrete / dedicated balun component (eg the Ruthroff 1:4 balun incorporated in many commercial ATU designs) or by virtue of its characteristic of some more complex circuitry (eg link coupled tuned tank common in ATUs of older designs.)

To deny that an ATU might also function as a balun is a misguided exercise in semantics, again focusing on implementation rather that functional characteristics.

Voltage Balun, Current Balun

It is convenient to identify two broad types of balun that are design objectives for different types of applications, see Definition: Current Balun, Voltage Balun.

Note that a particular balun may be a good Current Balun or a good Voltage Balun or neither. A good Current Balun cannot be a good Voltage Balun and vice versa, these are mutually exclusive.

Common mode current

If current balance, ie acceptably low common mode current is the design / optimisation objective, then aren’t direct measurements of common mode current the most relevant indicator of achievement?

The scarcity of reports of such measurement might leave one wondering why no direct measurements when it is often so easy.

Analysis

Above is a figure from (Howarth nd) annotated to show the common mode component of current (only) at key points of the simplified schematic.

The circuit permits common mode current flow, the question is just how easily or not it permits it, that is, what is the common mode impedance Zcm?

To the extent that a different state of balance can exist on the antenna side compared to transceiver side, this circuit incorporates a balun function.

Whilst good Voltage Baluns (those with very low Zcm) drive balanced currents into perfectly symmetric (ie balanced) loads, good Current Baluns (those with very high Zcm) drive balanced currents into ALL loads. Current baluns have certain advantage with HF wire antennas that measurement shows are not usually close to electrically symmetric.

Common mode impedance for this ATU will depend on the various settings and frequency. It is unlikely to be very high (say Zcm|>2000Ω) under operating conditions.

A more important question is what is the measured common mode current at various points on the feed line, at various frequencies, and with a range of similar end-user implementations of the same ATU?

There is a certain amount of what is old is good again, which has quite an acceptance in ham radio. (Duffy 2015) discusses “balanced ATUs” and analyses several popular configurations that are held to have superior characteristics, some of those old designs faded away for good reason.

For the birds?

At the outset, bird damage to coax is the reason given for migration to two wire line.

Birds are pretty ubiquitous, and coax is used successfully very widely. It seems most of us have found solutions to allow use of coax where it suits without enduring short coax life due to bird attack.

There are good reasons to use two wire line for certain applications, and parts of the antenna system remain vulnerable to damage from birds and other creatures and may warrant protective measures.

Conclusions

The subject article appears in Amateur Radio, the membership journal of the Wireless Institute of Australia, it is included in memberships so a somewhat captive readership. Articles selected for publication have to have wide appeal to its members and that probably means not challenging their existing beliefs, to provide entertaining / satisfying pseudo technical articles that do not make the readers uncomfortable. You might say a good serve of traditional ham wisdom is the road to success.

The subject article quite fits that description, with plenty of traditional ham wisdom, some hand waving and doubtful claims eg the line losses are very nearly nil.

The subject article does not set out clear design objectives, so in a sense it does not fail to deliver on the unstated objectives.

No quantitative measurement data is given to demonstrate achievement of balance whatever that is taken to mean.

References

Last update: 7th November, 2024, 8:09 AM