On Thu, 04 Nov 2004 18:58:22 GMT, Richard Clark
wrote:

|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.

True.

|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

True.

|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.

I don't believe that Devoldere incorrectly infers this.
|
|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.

Devoldere definitely debunks this idea.
|
|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.

Loading with finite Q inductors surely introduces additional loss.
Nevertheless, the "real" radiation resistance can be increased beyond
that of the unloaded antenna. Hansen reports this in a paper
available at:

http://users.triconet.org/wesandlinda/HansenPaper.pdf

Or, model a short lossless monopole over perfect ground and determine
the feedpoint R. In this case, R is totally due to radiation loss,
i.e. "radiation resistance." Add a lossless loading inductance
somewhere in the middle and see what happens to R.

Regards,

Wes