RadioBanter - View Single Post - Current in loading coil, EZNEC

November 4th 04, 06:58 PM

On Thu, 4 Nov 2004 08:08:00 -0600, (Richard
Harrison) wrote:

Devoldere says the full size 1/4-wave vertical has a radiation
resistance of 36.6 ohms. His 50% length base loaded example has a
radiation resistance of 6.28 ohms. His top loaded example has a
radiation resistance of 18.3 ohms. His center loaded example has a
radiation resistance of 22.1 ohms

Hi Richard,

This material has all the hallmarks of pencil whipping. The radiation
resistance of an antenna is NOT necessarily the same as its drivepoint
impedance at resonance. Without expressing the size of the radiator
in each of the examples above, I am forced to consider that the reason
for such loading examples is that the structure is significantly
smaller than a quarterwave. I say this principally due to the
inference of one line:
His 50% length base loaded example
If we are speaking of a 1/8th wave tall radiator under different
loading conditions, then it follows that the "radiation resistance" is
incorrectly applied to drivepoint Z with a forced resonance due to
loading.

Similar pencil whipping occurs when discussion centers on folded
antennas that purport to raise "radiation resistance" when in fact
they are simply raising drive Z. The argument in that vein generally
plods on that even so, efficiency is raised. Then the argument is
dashed in that the loss resistance is ALSO raised by the same
mechanism and the efficiency either suffers by comparison, or at best
breaks even.

This is the bane of loading, it introduces new sources of loss in
comparison to the native "radiation resistance" that is unaltered by
their inclusion.

73's
Richard Clark, KB7QHC