It would be the length from the feedpoint to the ground; you also need
to factor in the Common-Mode velocity factor of the line - probably in
the range 0.95 to 0.99.
If I model that feedline length connected to a half-wave 20m dipole,
assume a velocity factor of about 0.97 and a ground connection
resistance of 10 Ohms, I get 40% of the leg current flowing back down
the shield as CM current.
If you are trying to estimate whether or not the conducted CM impedance
is high or low at the feedpoint, remember you need to do it for all
bands on a multiband antenna!
On 01/01/2012 21:30, Tony wrote:
> The total length of feed line to the rig is about 85 feet. It's
> grounded at 70 feet where it then goes into the shack using a 15 foot
> jumper. Which do you take into account as being a multiple of
> half-wave? The total length of the coax or the length between the
> antenna and ground? Tony
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