Feeding at a current maximum

I mentioned in my (revised) article W5DXP’s current maximum calculator that lots of ham subscribe to the strategy of feeding a dipole / open wire feeder combination at current maximum.

Why is that?


On a transmission line (TL) with standing waves, the points of current maximum are points of voltage minimum and locating an ATU near a voltage minimum reduces the risk of flashover in the ATU or balun. A flashover in a near instantaneous event, and it may or may not cause permanent damage, but is best avoided.


However, common T-match ATUs usually have higher internal losses on low impedance loads (as encountered near a current maximum) and that may cause damage, albeit more slowly, and which is likely to be permanent.

Voltage baluns have higher loss when high differential voltage is impressed upon them, so if you are committed to a voltage balun, or an ATU with an internal voltage balun, you should avoid high voltage operation. (The combination of a 4:1 voltage balun and current maximum may exacerbate the ATU losses discussed in the previous paragraph.)

Current baluns work differently and are not subject efficiency problems at high differential voltage.

An approach to avoiding flashover

By design

Feeding at a point of current maximum is an extreme way to reduce feed point voltage, it probably reduces it more than needed and typically at the expense of efficiency (especially on the lower bands).

(Duffy 2011) set out a method of identifying the impedances and TL line lengths that are at risk of ATU flashover. Let us apply that to the scenario discussed in W5DXP’s current maximum calculator.

Let us take the maximum voltage withstand of our ATU to be 4.3kV (peak), and apply a 3dB safety margin, so our safe peak working voltage is 3kV.

If we run a maximum of 1kW, we can calculate the minimum G component of the load admittance to be G=P/Erms^2=1000/(3000^2/2)=2.22e-4S for 1kW and 3kV safe peak working voltage.

Screenshot - 05_03_2015 , 09_48_01 002a

Above is the Smith chart from W5DXP’s current maximum calculator with the area where G<2.22e-4S shaded green. It can be seen that everywhere on the TL, G is less than the critical value for 3kV peak, so there is no voltage restriction on location of the ATU.

It is often the case that behaviour on the lower bands is more critical than the higher bands.

Screenshot - 05_03_2015 , 05_28_55

Above, the same dipole at 3.6MHz. In this case, the constant VSWR circle for the load impedance of 83+j0Ω falls entirely within the critical G circle for 1kW and 3kV safe peak working voltage, so there is no flashover issue.

At lower power, less voltage is encountered but lower power rated tuners have lower voltage withstand. The green shaded areas G<2.22e-4S on these charts is also applicable to 100W with safe peak working voltage of 950V.

By measurement

ATU voltage verification explains how to calculate expected differential voltage at the ATU based on measurement of impedance looking into the TL.