Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #6

Sixth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

This article documents measurement of impedance.

Impedance measurement


The antenna system is a G5RV with tuned feeders (9m of home made 450Ω open wire). The tuned feeders terminate on the balun described in this series, and it is located on the outside of the antenna feed entrance panel shown above.

Impedance at the panel


Impedance was measured on the inside of the panel over the range 1-40MHz using an AIMuhf antenna analyser. Because of experience with the many shortcomings of the AIM system, the measurements were verified using a Rigexpert AA-600.


Above is a screenshot from AIM910b software.

That is well and good, but a bit academic as the dataset cannot be used with the AIM910b software to refer it to another point (eg the ATU end of a fly lead) after capture.

Impedance at the ATU

Clip 148

Above is the impedance as measured at the panel re-plotted in Excel.

Clip 150

Above is calculation of the impedance looking into a 0.5m flylead of RG142, ie the impedance seen by the ATU. The line loss figure is for the flylead alone.

The spreadsheet allows convenient re-calculation of a range of flylead scenarios, eg length and cable type.

Clip 152

The spreadsheet also allows convenient calculation of the peak voltage at 100W out at the ATU output terminal above. at 80m and above, the voltage is well below 1000Vpk and well within the capacity of most 300W class T match ATUs (though 300W might challenge those ATUs in this scenario). The curve can be multiplied by four for 1600W and it can be seen that an ATU would need to withstand around 3000Vpk at some frequencies.