ATU voltage verification

I described a method for designing antenna systems to avoid excessive voltages in baluns and ATUs at (Duffy 2011) .

This article reports post implementation measurements of an antenna system designed using that method and using a G5RV Inverted V with tuned feeder and ATR-30 ATU with integral 1:1 current balun. The tuned feeder is a home-made line section of 2mm diameter copper conductors spaced 50mm, and 9m in length. An additional 0.5m of 135Ω line connects from the antenna entrance panel to the ATU.

The ATU has an external link from the T network output to the balun input, which makes for a convenient point for measurement of the load impedance seen by the T network. The link was opened and impedance measured from 3 to 30MHz using a Rigexpert AA-600 with very short connecting wires.

The measurement results allow calculation of load admittance and peak voltage as (2*PEP/Gload)^0.5.

Clip 147

Above is a plot of peak voltage at the balun input. Voltage remains comfortably withing the capacity of the ATR-30 on all amateur bands. Note that although the differential voltage is high on some bands, it does not drive loss in the current balun in the way that would happen with a voltage balun (used in most ATUs with integral balun).

The voltage encountered at 100W PEP would be half that shown in the graph, at 1600W it would be double (and still comfortably within the voltage withstand of the ATR-30).