Balanced feedline - lengths to avoid (question)

Two years ago, a very interesting discussion of balanced feedline length for 44ft doublets occurred within this thread:

I was just going over it while building a new 44ft doublet and I’m suddenly confused by DL6GCA saying you must avoid feedline lengths that are a multiple of electrical half-wavelength. This doesn’t readily make sense to me. A half wavelength feedline simply delivers the same impedance at the antenna feedpoint to the end of the feedine. I would think instead that what you actually want to avoid are feedline lengths that transform/change the feedpoint impedance to a very high value at the antenna tuner terminals (source), thus presenting a load that might be impossible to match. This would require something like a Smith chart calculation for each frequency and feedline length to check impedance at the source, not a simple calculation of electrical length of feedline.

Do I have this right or wrong? Comments appreciated.

Thanks & 73,
Barry N1EU

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Hello Barry

In the thread you linked to, I pointed out that the length of the feeder line can be included in the total length of the system.

The link there led to an excel sheet where you can determine the overall lengths… good and bad

And in fact some ham do feederline loops with different taps to get to optimal resonance… (No tuner, 2 edition)

73 Armin


Thanks Armin, I now realize I had a misunderstanding on the spreadsheet. Column C is the TOTAL electrical length of radiator PLUS feedline. I previously thought it was the length of just the feedline. It makes obvious sense now that the TOTAL length should not be a multiple of 0.5 wavelengths! (duh)

Thanks again & 73,
Barry N1EU


Barry, I’m sure you know that.

A multiple of half the wavelength creates a very high impedance at the end of the line of approx. 3kOhm. Your tuner will not be able to transform this impedance.

This impedance is also achieved with the EFHW, but the 64:1 transformer does the job.

73 Chris

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