KA0KA’s Youtube test of ATU balance

K0KA made a quite polished presentation published as a Youtube video explaining how to measure balance performance of an antenna system and an ATU.

He did not define what he means by balance… but it will become apparent.

Midway through his video, he measures the ‘balance’ of a Palstar AT5K (though possibly modified) by connecting a symmetric load consisting of two 470Ω 2% resistors in series and connecting the junction to the ground terminal on the ATU.

Above, from KA0KA’s video, his test load. The oscilloscope probes can be seen.

The oscilloscope channel gains are carefully adjusted to be equal.

KA0KA shows two oscilloscope measurements.
Continue reading KA0KA’s Youtube test of ATU balance

VU3SQM directional wattmeter build – #1

VU3SQM offers an interesting directional coupler based on a Sontheimer coupler, and using AD8307 power sensing for a nominally HF coupler. I must say that I am not a fan of Sontheimer couplers… but that is what the board uses.

This article lays out a preliminary design review to assist in selection of appropriate toroids, and ordering of the needed parts.


Above, the top side of a PCB. Continue reading VU3SQM directional wattmeter build – #1

An explanation of W5DXP’s ‘line extender device’

A correspondent wrote seeking explanation of W5DXP’s no-tuner tuner which purports to obtain a near match by adjusting the length of the transmission line using relays or switches of some kind.

The particular device that is of interest is one using a single double pole knife switch as a three position On-Off-On switch.

The accompanying explanations states that this “is a way to use a single DPDT knife switch to obtain one, two, or three feet of ladder-line depending on the position of the switch”. Continue reading An explanation of W5DXP’s ‘line extender device’

An RF choke for a 1.8-30MHz coax power injector – LF1260 core

This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.

Design criteria are:

  • Insertion VSWR of the RFC in shunt with 50+j0Ω < 1.1;
  • Dissipation < 2% of a 100W transmitter.

The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.


Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enameled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core

End Fed Half Wave matching transformer – 80-20m

A prototype broadband transformer for a End Fed Half Wave operated at fundamental and first, second, and third harmonic is presented.

The transformer comprises a 32t of 0.65mm enameled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. The winding layout is unconventional, most articles describing a similar transformer seem to have their root in a single design.
Continue reading End Fed Half Wave matching transformer – 80-20m

Is a ham transmitter conjugate matched to its load?

Following on from KL7AJ on the Conjugate Match Theorem, KL7AJ on the Conjugate Match Theorem – analytical solution asked the question Is a ham transmitter conjugate matched to its load?

The answer speaks to the relevance of Walt Maxwell’s Conjugate Mirror proposition to ham stations. Continue reading Is a ham transmitter conjugate matched to its load?

KL7AJ on the Conjugate Match Theorem – analytical solution – Winsmith

KL7AJ on the Conjugate Match Theorem asked the question Should we have expected this outcome?

Let us solve a very similar problem analytically where measurement errors do not contribute to the outcome.

Taking the load impedance to be the same 10.1+j0.2Ω, and calculating for a T match similar to the MFJ-949E (assuming L=26µH, QL=200, and ideal capacitors) we can find a near perfect match.

The capacitors are 177.2 and 92.93pF for the match.

Now turning the network around by swapping the capacitors and changing the load to 50+j0Ω. Continue reading KL7AJ on the Conjugate Match Theorem – analytical solution – Winsmith

KL7AJ on the Conjugate Match Theorem

KL7AJ proposed a little test for his readers on QRZ:

One of the most useful (and sometimes astonishing) principles in radio is the Conjugate Match theorem. In the simplest terms, what this says is that the maximum power will be transferred between a source (like a transmitter) and a load (like an antenna), when the source impedance is the COMPLEX CONJUGATE of the load impedance (or vice versa).
Here’s a neat little experiment to prove the conjugate match theorem. You need four basic ingredients: an antenna analyzer like the MFJ259 (or an actual impedance bridge, if you know how to use one). A good low loss antenna tuner. A good 50 ohm resistor. And a good 200 ohm resistor. And some appropriate connecting hardware, namely some short bits of coax.

Step 1) connect the 50 ohm resistor to the OUTPUT of the antenna tuner. Connect the antenna analyzer to the INPUT of the antenna tuner.

Step 2) Adjust the antenna tuner to get precisely 50 ohms, zero reactance on the antenna analyzer. This step simply confirms everything is working.

Step 3) Replace the 50 ohm resistor with the 200 ohm resistor. Readjust the antenna tuner to get 50 ohms, zero reactance on the antenna analyzer. Do not disturb the antenna tuner adjustments after this point.

Step 4) Remove the 200 ohm resistor and insert the antenna analyzer in its place (at the OUTPUT of the antenna tuner).

Step 5) Insert the 50 ohm resistor at the INPUT of the antenna tuner.

Step 6) Take a careful reading of the antenna analyzer. (What do you think it will say?)

10 points for anyone who will correctly explain why this works.

Some clarifications

Jacobi maximum power transfer theorem

Jacobi published his maximum power transfer theorem in 1840. It states that maximum power is transferred from a (Thevenin) source to a load when the load resistance is equal to the (Thevenin equivalent) source resistance.

It was later adapted to apply to AC circuits with sinusoidal excitation, maximum power is transferred from a (Thevenin) source to a load when the load impedance is the complex conjugate of the (Thevenin equivalent) source impedance.

Walt Maxwell’s Conjugate Mirror

(Maxwell 2001 24.5) states

To expand on this definition, conjugate match means that if in one direction from a junction the impedance has the dimensions R + jX, then in the opposite direction the impedance will have the dimensions R − jX. Further paraphrasing of the theorem, when a conjugate match is accomplished at any of the junctions in the system, any reactance appearing at any junction is canceled by an equal and opposite reactance, which also includes any reactance appearing in the load, such as a non-resonant antenna. This reactance cancellation results in a net system reactance of zero, establishing resonance in the entire system. In this resonant condition the source delivers its maximum available power to the load. …(1)

Note that it states that if a conjugate match is established an any junction, then a conjugate match occurs in any (all) other junctions, simultaneously a conjugate match exists everywhere. Continue reading KL7AJ on the Conjugate Match Theorem

True balanced tuner

A recent long running thread on QRZ entitled “True balanced auto-tuner” was sure to tease out some pretty woolly 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 woolly 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


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. Continue reading True balanced tuner