Because of several reasons, the coaxial lines are not so much constant
impedance as one might aspect. The correct approach is then to measure
the length needed (phisically and electrically) and not something else.
Often, two pieces cut from the same line have different lenghts for the same
electrical lenght. This is more evident as the frequency is raised and leads
to mistakes if a very different frequency is applied to determine lenghts
because some line parameters became more frequency dependent than
others.
Second, what's the matter to use only odd multiples of half wavelenghts if
one could satisfy ?
Althought probably appreciated by cable manufacturers this is a nonsense.
A full wavelenght behaves infact exactly like an half wavelenght (one half
ahead the other) reproducing the input impedance to the other edge,
independently by cable impedance.
Any half wavelenght multiple (odd or not) is perfectly suitable for that
purposes.
As it's very intuitive, instead, it's the quarter wawelenght who needs an
half wavelenght to behave again as a quarterwave line and Since one half
wave plus a quarter wave is equal to 3/4 wave, and one quarter
plus one full wave is 5/4, here comes the need of odd multiples, but
attention, with quarters, not with halfwavelenghts.
Last but not least.....
The half wavelenght has the tendency to give smoother impedance responses
leading to uncertaintinity, expecially when the nominal line impedance is
that of the antenna.
To best determine its lenght is generally preferred to threat it as a
shorted or opened quarterwave impedance transformer checking for resonance
or short on half of the used frequency, or as 3 quarterwaves at 1.5 of the
frequency where the half wavelenght is needed.
I hope now the story it's more clear and practical.
73,
Mauri I4JMY
-----Messaggio originale-----
Da: Robert L. Shohet <kq2m@mags.net>
A: towertalk@contesting.com <towertalk@contesting.com>
Data: lunedì 24 maggio 1999 4.08
Oggetto: [TowerTalk] Measuring 1/2 Wavelength coaxial lines with an MFJ-259
>
>I am in the process of making up feedlines for some monobanders that I
>am putting up and have come across a vexing problem with the MFJ-259.
>
>Here's what happens...
>
>I measure an odd multiple of 1/2 wavelength for let's say 21.250.
>
>At the 3rd harmonic, instead of seeing an swr "dip" at 63.75, I might
>see the dip at 64.25 instead (3rd harmonic of 21.417). At the 5th
>harmonic instead of seeing the "dip" at 106.25 Mhz, it might dip at
>107.30. (5th harmonic of 21.460)
>
>Why does this happen? Shouldn't the 3rd and 5th harmonics be exactly 3x
>and 5x the original frequency?
>
>More importantly, which freq. do I use as the correct measurement for
>the 1/2 wave coaxial line, the 1st, 3rd or 5th harmonic or some
>combination?
>
>
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