On Thu, 03 Dec 2009 16:51:05 -0500, Registered User
wrote:
On Tue, 01 Dec 2009 16:42:00 -0800, Richard Clark
wrote:
On Tue, 01 Dec 2009 16:22:08 -0500, Registered User
wrote:
I'm under the impression the current flow
is identical whether metal rods or wire mesh is used in the antenna's
construction.
A discone does not exhibit any quality of shielding, so it wanders off
in that regard.
Maybe I'm confused and can't distinguish between Art's all-band mesh
antennas and his mesh Faraday shields.
I can understand the confusion. To return to your question above,
there is NOTHING about the discone that falls under the topic of
Faraday shield.
I was questioning Art's statement
-quote-
When you feed a time varying current to the mesh it is best to view it
in small parts, say a square in the mesh. The hole is a static field
alongside the applied current flows.
- end quote -
The idea of examining the characteristics of a single square of mesh
seems impractical. The impact of adjacent squares should be accounted
for otherwise the single square is a loop.
I discussed both the single mesh opening, and the total contribution
of all mesh openings. To respond to your last statement, yes, the
single square is a loop. A very, very inefficient coupler of energy.
Either way I've learned as current varies the fields it produces will
vary. If the fields vary they're not static. Too simplistic? What am I
missing?
Static comes in two flavors. One means "not moving." The other means
high potential (which can be "not moving" AND, ironically, "moving").
Such is the legacy of electrostatic potential covering DC to Gamma.
The difference between rods, number of rods, thickness of rods, and
mesh all speak to bandwidth. 2, 3, or 4 rods will not be remarkable.
16 rods will closely approximate a cone of sheet metal (as would a
grid of similar spacing). The same can be said of the
rod/rods/mesh/sheet in the upper section approximating a solid disk.
IIUC the current flows around the cone of a discone regardless of
solid, sheet or mesh construction. This appears to be contrary to the
quote above where current flows around each individual hole in the
mesh.
Well, language can be a barrier here when you say "around the cone."
Go to:
http://www.antenna-theory.com/antenn...rture/slot.php
I don't endorse this page, but it gives you a beginning for slot
antennas and especially the construction mimicry between them and a
dipole. Note in figure two that a "source" appears across the two. In
the case of the slot antenna, this source would give rise to a
circulating current. If you were to approach this with knowing the
current alone, it would follow that the source is "apparent." In
other words, the two models (or experiences) are equivalent.
Do not confuse this circulating current with a larger, general current
UNLESS that more general one can spawn the circulating one. I wrote
to this already and to put it shortly, it is physical length vs. wave
length dependant.
Again, all these "appearances" are a strict function of wavelength to
physical length and spacing relationships.
I've built several discones over the years and understand these
relationships. How well is subject to conjecture hi.
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm
will illustrate how varying the flare of the skirt shifts the
operating properties of the discone.
73's
Richard Clark, KB7QHC