On Dec 27, 1:58*pm, K7ITM wrote:
On Dec 27, 9:35*am, ve2pid wrote:

I am looking for a formula giving the inductance of a solid disk for a
known diam/material/thickness...

Any idea?

Thanks for your hels de Pierre VE2PID

What, exactly, do you mean by "the inductance of a solid disk"? *How
do you connect to the disk? *Are your connection points on opposite
ends of a diameter across the disk, or along its axis from the center
of one face to the center of the face on the opposite side -- or
perhaps some other connection? *With a wire (formed into any shape),
it's reasonable to assume it's the inductance measured between the two
ends of the wire. *For a disk, it's not obvious to me.

Cheers,
Tom

I would think if you measure the inductance from the center of one
side of the disk to the cnter of the other side of it would be like a
large number of hair pins in parallel.
I tried building a collinear antenna around this idea with the
vertical part of the antenna a solid piece of CU tubing with the disk
place at .5wl points. The problem was that the disc act as capacity
hat.


Jimmie

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