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Old January 6th 06, 01:59 PM posted to rec.radio.amateur.antenna
Cecil Moore
 
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Default Cobra multiband dipole (linear loaded) help pse

Paladin wrote:
I'm still a bit confused though. I still seem to NEED that
feedline LENTH. TOTAL.....
80ft. of L.L. the balun,and then 50ft. of coax. That's 140ft.
total. Works great,though !


When the length of 50 ohm coax to the 50 ohm source
has a large effect, that is usually not good news.
We are usually aiming for a low SWR on that section
of feedline and if we are successful, length doesn't
much matter. Here's what can be happening:

The impedance looking into the ladder-line with an
SWR of 18:1 is 25 ohms because that point is a current
maximum point. The 4:1 balun takes that impedance
down to 6.25 ohms, a lossy condition for the tuner and
an impedance it may not be able to match. 0.15 WL of 50
ohm coax (SWR = 50/6.25 = 8:1) will transform that 6.25
ohms up to 17+j65 ohms, an impedance the tuner can handle.
Have you measured the SWR on the coax on the antenna
side of the tuner?

If one doesn't know what impedance is being seen by the
balun, a 4:1 balun may not be the best choice. It may
even be a bad choice.
--
73, Cecil http://www.qsl.net/w5dxp