On Sun, 14 Nov 2010 02:25:50 GMT, Owen Duffy wrote:

I meant to elaborate on this a bit more. (Did I hear someone groan?)

If for example, the feedpoint Z of a 0.6 wave vertical over four quarter
wave radials...

I'll bite (or groan as the expectation demands) - why "quarter wave"
radials? A rule of thumb?

* * * Rhetorical questions follow * * * *

Quarter wave in physical length?

Quarter wave in electrical length?

Elevated Quarter wave radials?

If elevated, Quarter wave drooped radials?

* * * Philosophical ponderings follow * * * *

The discussion of radials usually attends ground mounted studies in
the 100M band in the 1930s. Those studies sought to reduce loss while
mimicking a conductive ground of infinite extent. Radiators taller
than Quarter wave were treated to feedpoint loading (such as found in
the current topic, albeit with the possibility of it being elevated
and thus muddying the philosophy here). All such historical (and
current AM band engineering) feedpoint loading presumed, basically, a
non-resonant ground system. As Quarter wave long radials imply
resonance (at least in the first read), this would suggest that,
perhaps, this "tuning" should be further examined in light of
feedpoint loading. The conclusion, to my mind, would be that
significant reduction in feedpoint loading could be accomplished by
tailoring radial length (much less drooping that is already part of
the lore).

At first blush, it would seem that the radials would be shorter than
Quarter wave (forgive me for not first confirming this astonishing
leap of faith).

Of course, there is every chance some reactance will remain to be
"tuned" away (returning us once again to loading) - if the mismatch is
deemed significant.

If such is the case, and returning to the original design, what
problem is the Quarter wave length radial rule of thumb responding to?

* * * * Alternative analysis * * * *

Or to put it into the light of other antenna topological discussions,
and in this regard the off-center fed dipole. Here we have an
off-center feed (we rarely go on to describe all such installations as
"vertical dipoles"). We can fully expect that, as such, we are
transforming the expected 70±j0 Ohms into some other value. Quite
frequently in an OCF design, it is much higher - and variable by the
degree of offset. However, for a fixed frequency, this is better
understood and can be anticipated. The proximity to ground and the
geometry (the radials certainly disturb the shape of an OCF dipole,
even if vertical) further change things, but conceptually the monopole
with resonant radials still constitutes an OCF design that is "on
center fed" for the vertical element when it, too, is a Quarter wave
in length.

For many prospective feed points along the length of the OCF dipole,
the only consideration needed is for a ratio transformation, not
tuning. This is usually resolved in a BalUn. Hence "loading" is
removed from the picture through careful consideration of the whole
antenna, the degree of offset, and not through arbitrary assignment of
Quarter wave length radials to all vertical designs.

* * * * Conclusion * * * * *

The concept of a loading coil where its length of wire "replaces" the
missing length of radiator wire is a commonplace for technologists. It
serves the discussion quite well at that level.

The value of this length of wire's inductance is going to vary by
significant value for the many coil form variables available to the
technician. Hence the exactness of this "replacement" is questionable
on the face of it at the engineering level of discussion. This
equivalence "replacement" is forced further into unresolved exactness
if we move the same coil up into the radiator (without changing the
radiator's length).

The same could be said with the treatment of Quarter wave length
radials, which, after all, are a special and not general solution.

73's
Richard Clark, KB7QHC