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From: David Jefferies
Date: 17 Apr 2002
Time: 13:59:16
Remote Name: 195.92.168.166
Bill,
Right. The coax is a TEM line with only transverse field components. That means that the Zo = sqrt(L/C) where L is the inductance per unit length and C is the capacitance per unit length. The dielectric perturbs C but not L. You adjust the ratio of outer/inner diameters to compensate which is why the bead is undercut. It can all be calculated very precisely, and is used in (eg) BNC connectors.
For the antenna the magnetic field lines are still running around the antenna rods as an axis of rotation, but the E field is not entirely radial. Therefore you may be right in large part, but one would expect some residual effect on the length. Altering the fatness of the rods, or the effective fatness, will alter the radiation resistance I expect.
Perhaps this is why we are not seeing an effect on the resonant length.
Thanks muchly for the steer.
regards
David G6GPR