John Popelish wrote:
Roy Lewallen wrote:
John Popelish wrote:

Roy Lewallen wrote:
. . .

Effective height determines how many volts you'll get from an open
circuited antenna.


Does that include an antenna that has been brought to resonance with
an appropriate capacitive load?


No. "Open circuited" means that there's nothing connected across the
feedpoint.

But I can design the coil to be self resonant or not, just by adjusting
the surface area of the wire, or the spacing. It is non intuitive that
if I peak the coil this way, and obtain more voltage, it is a different
case than if I peak the coil with cable capacitance, or an additional
capacitor. I guess I really don't comprehend the point of this value.

Sorry, I don't understand what you're "peaking" and how. I believe that
the formula I gave assumes that the coil is well below self resonance
where the shunt capacitance is negligible. It won't be valid near self
resonance, if that's what you mean.

. . .

Well, now I can calculate the effective height of my antennas, even
though I am not sure what it has to do with height.

Do a groups.google.com search of this newsgroup for my postings
containing "effective height" or "effective length" -- I posted quite a
bit about it not long ago.

. . .

I am also experimenting with designs that do not necessarily have a
small, coil, close to the rod. (My interest in the discussion of
extended coils is showing.) One of the possibilities that shows a
significant increase in tuned Q is an hour glass shaped coil (small
diameter in the center, but sweeping to a larger diameter at the ends).
I have been asked to try putting a rod through the center of a flat
spiral coil. It seems to me that, at some extreme, the above formula
will fail, because it assumes that essentially all the signal energy
exiting the coil was collected by the rod, and that the signal the coil
would collect by itself would be insignificant. But if my coils get
large enough, they become loop antennas in their own right, and the rod,
though it may have a significant length and area, is only a part of what
is happening. In other words, the mueff can get pretty small, even
though the rod has significant dimensions. I guess, what I am asking
are what assumptions about coil dimensions (that are not explicitly
referenced in the formula) are being made in the above formula?

I'm sorry, I don't know. The reference it came from doesn't say. It in
turn references Laurent, H.J. and Carvalho, C.A.B., "Ferrite Antennas
for AM Broadcast Receivers", an application note from Bendix
Corporation. That app note, if you can find it, might or might not tell.
I'm sure it's not valid anywhere near self resonance, though.

Your concept of signal energy getting collected separately by the rod
and the coil, or exiting the coil and being collected by the rod doesn't
fit at all with what I know of the behavior of electromagnetic fields,
so I won't even try to comment on that aspect or the conclusions
resulting from it. Maybe it makes sense to Cecil or art -- they seem to
view things in a different way.

Roy Lewallen, W7EL