True balanced tuner

A recent long running thread on QRZ entitled “True balanced auto-tuner” was sure to tease out some pretty wooly thinking… the word “true” was enough to signal the outcome.

There are only three words in the title, we can dismiss “true” as a harbinger of wooly thinking, and though people will argue the toss on the appropriateness of the term “auto-tuner’, most people share an understanding of the meaning. “Balanced” is another problem altogether.

After thirty odd posts, there has been no definition or discussion of the term balanced, or its advantages or disadvantages.

One of the recommendations by several posters is the old is new again solution, the once popular link coupled tuner and the work of W5ZQ featured in one of those recommendations.

W5ZQ and WW8J

Topology

W5ZQ describes a tuner inspired by WW8J. W5ZQ extended the design and provides a writeup on optimising balance.

Above is W5ZQ’s partial circuit. In the article he describes and shows:

  • adjustment of the grounding point of the output tank; and
  • current meters which presumably attach to J2 and J3.

Key to analysis of the topology is that the centre of the output inductor is grounded. This results in the circuit tending towards equal but opposite phase voltages on the output terminals.

His article shows his method of determining balance, he uses two oscilloscope voltage probes which he calibrates to match each other, and then with one probe attached to each feed line he can observe the magnitude and phase of one wrt the other, and he shows that balance is achieved where the voltage on one feed line wire is equal in magnitude and opposite in phase to that of the other feed line wire. He adjusts the ground point of the output tank to achieve exact balance.

What is balance, and why pursue it?

Though both of these authors describe a “balanced tuner”, only WW8J hints the purpose of balance when he gives the following explanation.

Each feed line exhibits an out of phase condition as well as differences in amplitude. In addition to a host of other problems, this condition will allow RF at higher power levels, to radiate from the feed line onto other conductive surfaces, usually resulting in various RFI conditions.

In fact feed line radiation is due to the difference in current (amplitude and phase) in the feed line wires so to minimise feed line radiation, the objective is equal but opposite phase currents in the two wires.

WZ5Q gives a similar demonstration of achieving perfect balance, but without explanation of the objective.

If the voltages are equal magnitude but opposite phase as WW8J and WW5Q achieve, the currents are equal magnitude and opposite phase ONLY if the load is absolutely symmetric and neither demonstrated that to be the case. In any event it is easier and simpler to directly measure the net current imbalance.

WW5Q’s construction incorporates two current meters which presumably attach to J2 and J3 and they would show the magnitude of the current in each output connection, but not the phase difference. This does not prove balance of the system, nor indicate the extent of imbalance (eg the meters could read the same magnitude and yet the phase could be different so they would be unbalanced).

Measuring current balance

Had WW5Q used only one current transformer but passed both antenna wires through it in the same direction, the attached meter would read the magnitude of current imbalance, or the magnitude of Common Mode Current.

Hams talk endlessly about antenna feed line balance, and the means of adapting a transceiver coaxial output to two wire feed line, but as in the thread mentioned and both of the referenced articles, no measurements of current balance are given, yet it is so easy to measure.

“True balanced”

The qualifier “True balanced” usually hints that the user is more concerned with unvalidated implementation topology than proven measurement of current balance.

Watch out for asteroids!