Roy Lewallen wrote:
What information are you looking for, capture area or effective height?

Since I wasn't even aware that effective height applied to rod
antennas (or exactly what effective height means) I guess I was
thinking of capture area.

Capture area determines how many watts you'll get into a conjugately
matched load connected to the antenna.

That's it.

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?

The two aren't
directly related. For more information about the two topics, do a
groups.google.com search for postings I've made on those topics in this
newsgroup.

Thanks.

As I've posted here quite a number of times before, the capture area of
a lossless infinitesimally short dipole is very nearly as great as that
of a half wave dipole, in their most favored directions. (The difference
is about 10%, and it's due to the slight pattern shape difference caused
by different current distributions). So except for loss the capture area
of a ferrite rod antenna is within 10% of that of a dipole. But loss in
a ferrite rod antenna will reduce the capture area considerably.

So if a very small rod antenna had a lossless core that could handle
any flux level, and was wound with superconductor, it could couple
into the same volume of space as a 1/2 wave dipole? Amazing.

If
you're interested in knowing how much power you can get from a ferrite
rod, then, what you need to know is its efficiency, which is a function
of wire length, number of turns, and the antenna feedpoint impedance. I
don't have the time right now to work it out for you.

The effective height of a ferrite rod antenna is approximately:

(2 * pi * mueff * N * A) / lambda

where

mueff = effective relative permeability of the rod (mainly a function
of rod length)
N = number of turns
A = rod cross sectional area
lambda = wavelength

I can apply this formula directory to what I am experimenting with,
except that I have to approximate mueff. I am making the rod by
stacking ferrite beads, with various gaps between them. Can I
approximate mueff by taking the ratio of coil inductance with and
without the rod?

And, what if the rod area is not constant all along the rod? Since my
rods are assembled from pieces, I have a lot of freedom in this direction.