On Dec 28, 8:37*am, ve2pid wrote:
On 28 déc, 09:52, Cecil Moore wrote:

John Smith wrote:
*... perhaps Cecil is aware of something?

I'm wondering if he means "reactance" instead of
inductance. The reactance to ground is capacitive
for a solid disk.
--
73, Cecil *http://www.w5dxp.com

Hello to all,

The context is this: We are discuting about capacitance hats.. It
seems that the name is confusing, since it adds inductance instead of
capacitance to an antenna.

So, since a solid disk adds inductance (the resonnant freq of the
antenna drops), we are looking for a formula to evaluate that
inductance (and then inductive reactance)..

BTW, why don't we use the name 'inductive hat'...??

Remember, the current where a capacitive hat is located in the antenna
(at the end of an element) is relatively low. Adding inductance there
has relatively little effect. On the other hand, that's just where
even small amounts of capacitance have a fairly large effect.

Consider too that if the environment is symmetric, such as with a
vertical over a flat, uniform (or radially symmetrical) ground, with a
top had that's also radially symmetrical, the current in that top hat
will be radial and symmetrical, and will for all practical purposes
cancel out with respect to radiation from the top hat itself.

Look at the change in current distribution along the antenna
conductor, and compare the situation with and without top hats. It
should become immediately obvious what the advantage of a top hat is.
I would certainly call the effect "capacitive" and not "inductive."

It may also be instructive to look up design notes about VLF
transmitting antennas. Very large non-radiating "top hat" structures
are commonly used in them to get reasonable efficiency. I think
you'll be able to find examples of this sort of design from as far
back as a hundred years ago. The folks I've talked with about the
design of one such local antenna system are quite clear about the top
wires' role in changing the current distribution in the vertical
element, but not contributing directly to the radiation in any
significant way.

Adios,
Tom