gravity wrote:

one thing i'm not clear on is coil diameter and air core. every dollar
spent on a metal core could be utilized for more turns for the antenna. and
i'm not sure if a big coil would have better aperture and gain than a small
diameter.

one thing i'm noticing is that there are basically two types of antennas.
one type is built by amateurs and is basically guesswork or based on prior
designs. the other type is built by professionals, often doing research in
earthquakes.

A high permeability rod intercepts flux from a volume of space a
little less than a sphere with a diameter equal to the rod length. An
air core (short length to diameter ratio) coil intercepts flux from a
volume of space a bit larger than a sphere with a diameter equal to
the diameter of the coil. The air core coil has the advantage of
being relatively light and cheap. The iron core rod takes up a little
less space, and allows the coil to be electro statically shielded a
lot easier. It also produces the the same inductance with a much
shorter length of wire, so, for the same gauge wire, the coil
resistance is lower. With good winding technique, I think the stray
capacitance can also be lower. If the coil is wound on a tube, you
can also try different cores with little effort.