You're thinking in terms of the wrong "return path". The current is
only indirectly affected by the dielectric. The current creates a
magnetic field around the wire, but the difference in voltage along the
wire is an electric field (obviously created by the current distribution
along the wire) that is affected by the dielectric. The "return path" of
interest here is simply the effect on the current caused by the
difference in the electric field. It's all intertwined, is affected by
the dielectric, and has almost nothing to do with ground as far as the
dielectric goes. That description may not be 100% rigorous, but it's
close and it's how I picture it all in my mind.
EZNEC is now a free download and is trivially easy to learn for what you
want to know.
I don't intend to model it myself since it would be so easy for you to
do it yourself, but I'd bet that you'll find that ground proximity has a
greater effect on the resonant length of the wire than the insulation
does ... and as you implied the actual distance to the effective ground
is typically unknown.
Unless you get really lucky you're going to be doing some trimming anyway.
73,
Dave AB7E
On 9/20/2022 10:46 PM, Edward McCann wrote:
Thanks for the observation.
I’ve not yet heard from anyone whether and how EZNEC incorporates its
modification mathematically to account for dielectric insulation, although I am quite
familiar with the math suggested by Richmond, Lee and Popovic. But that was only
through 1984 or so. The line has been dead since.
Maybe someone will run a couple EZENECs and determine Vf of insulated wire in a
meaningful manner.
The obvious return path is through the ground, not the dielectric, just as it
is the vertical, hat or no hat, with the theoretical ideal case being the
hard-to-find perfect ground into which the image can be realized. Of course, as
the bumper sticker says, stuff happens, and that perfect ground is unobtanium.
Thanks again for your interest and reply.
73
Ed McCann
AG6CX
On Sep 20, 2022, at 7:06 PM, David Gilbert <ab7echo@gmail.com> wrote:
There is indeed a "return path" for a single wire. The wire capacitively
couples back upon itself, and the dielectric affects that capacitance. That may sound
strange, but think of a capacitive hat loading at the top of a vertical or at the end of
a dipole. The wire itself is just a different geometry and therefore has less effect,
but it's the same principle.
EZNEC+ is based upon NEC2 and simply allows more wire segments than some other
versions. I can't say whether or not NEC2 handles dielectrics, or whether W7EL
added a calculation on his own, but if you define a dielectric in EZNEC+ it
will result in a shorter wire for the same effect as a bare wire. That at
least fits the physics of the situation.
Dave AB7E
On 9/20/2022 6:36 PM, Edward McCann wrote:
I’ve been looking for several decades for a technical analysis of the Vf of a single
bare and insulated wire, hearing nothing but lore from just about everywhere, unbelievable
proposals and formulas from some, exhortations from academics that such a search is
pissin’ in the wind, since there is no return path, as there is in twin lead or in
coax.
73
Ed McCann
AG6CX
Sausalito
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listi
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|