Is there an optimum length of ladder line for use with a dipole and ATU?
Ladder line is often operated at high VSWR, even extreme VSWR, resulting is the load impedance seen by the ATU being very sensitive to line length, and in turn the capacity of the ATU to match the load efficiently is affected, especially on the lower bands.
A very common configuration for manual ATUs is the high pass T-match. This discussion will focus on performance of a typical T-match ATU on 3.5MHz to explain some of the effects.
To support the discussion, a model of a typical T-match ATU was constructed, and its performance calculated for the load presented by a 400Ω line at various source end VSWRs (80, 40, 20, 10, 5) and (electrical) line lengths in 1° increments. The model ATU has stepless variable capacitors of 10-220pF (Q=2000) and a switched inductor of 0.5µH to 32µH in 12 geometric steps of 1.4x (Q=100), and an ideal 1:1 current balun.
Above is a plot of the model data. If a data point is not plotted, it is because it was outside the range of the ATU. The steps are a result of using a switched inductor.
Traditional advice is to adjust the feedline length so that the ATU is at variously, a (differential) voltage maximum, a current maximum, either, an odd number of eighth waves, and any convenient length. None of these lead to the lowest losses at high VSWR.
Placing the ATU at a voltage maximum:
So, there is a mix of good and bad. Best to avoid if you are using a voltage balun, and could be difficult to match for extreme VSWR.
Placing the ATU at a current maximum:
Again, not really an optimal operating point.
Is there a better option?
At low VSWR(400), say less than 10, the length doesn’t matter very much. Losses are relatively low, and load impedance should be within an ATU’s range at any electrical length.
For 400Ω line lengths around 135° longer than a voltage maximum, the losses are low and not very sensitive to small changes in length, even for extreme VSWR, and the impedance presented should be well within the range of modest ATUs.
Note that lengthening a feedline to obtain lower ATU loss has its own loss costs, and a system perspective is necessary.
(If the extreme VSWRs plotted seem too high, keep in mind that the load end VSWR(400) of a G5RV on 80m is around 60.)
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