On 08/25/2016 08:23 AM, Richard Fry wrote:
J.B Wood clip: ... Likewise we consider an "electric dipole" to be a
straight conductor of very small length (compared to a wavelength)
carrying uniform current. ________

Just note that while the currents along the two sides of a dipole can
be equal, they can never be uniform. Essentially no r-f current
exists at the far ends of a dipole, no matter how short or long it is
in terms of wavelengths.


It's a theoretical (textbook) construct but finds practical antenna
modeling use in method-of-moments software such as the Numerical
Electromagnetics Code (NEC). The idea is if we take smaller and smaller
sections (say about 1/20 wavelength) of a conductor carrying alternating
current we can consider the current to be uniform in that small
conductor. Of course an actual antenna would consist of a series of
these small conductors each carrying its respective value of uniform
current. Programs like NEC also consider, in addition to conducted
current the capacitive and inductive interactions between all the
segments comprising an antenna model.

Similarly we can build a transmission line using a number of identical
tee or pi sections connected ladder-fashion. The currents and voltages
associated with a section depend on its position along the length of the
line. Sincerely, and 73s from N4GGO,

--
J. B. Wood e-mail:

On 8/25/2016 10:54 AM, J.B. Wood wrote:
On 08/25/2016 08:23 AM, Richard Fry wrote:
J.B Wood clip: ... Likewise we consider an "electric dipole" to be a
straight conductor of very small length (compared to a wavelength)
carrying uniform current. ________

Just note that while the currents along the two sides of a dipole can
be equal, they can never be uniform. Essentially no r-f current
exists at the far ends of a dipole, no matter how short or long it is
in terms of wavelengths.


It's a theoretical (textbook) construct but finds practical antenna
modeling use in method-of-moments software such as the Numerical
Electromagnetics Code (NEC). The idea is if we take smaller and smaller
sections (say about 1/20 wavelength) of a conductor carrying alternating
current we can consider the current to be uniform in that small
conductor. Of course an actual antenna would consist of a series of
these small conductors each carrying its respective value of uniform
current. Programs like NEC also consider, in addition to conducted
current the capacitive and inductive interactions between all the
segments comprising an antenna model.

Similarly we can build a transmission line using a number of identical
tee or pi sections connected ladder-fashion. The currents and voltages
associated with a section depend on its position along the length of the
line. Sincerely, and 73s from N4GGO,


Richard is correct. The current at the feed point diminishes linearly
(on a short dipole) from the feed point to the open end of the antenna
as it must. Look at the current distribution using your NEC modelling
program.

On 08/25/2016 12:07 PM, John S wrote:
Richard is correct. The current at the feed point diminishes linearly
(on a short dipole) from the feed point to the open end of the antenna
as it must. Look at the current distribution using your NEC modelling
program.

No one said he wasn't. Did you read my last post? The uniform currents
in each segment aren't the same value. Of course the end segments would
be minimum. Sincerely,
--
J. B. Wood e-mail:

On 8/25/2016 11:55 AM, J.B. Wood wrote:
On 08/25/2016 12:07 PM, John S wrote:
Richard is correct. The current at the feed point diminishes linearly
(on a short dipole) from the feed point to the open end of the antenna
as it must. Look at the current distribution using your NEC modelling
program.

No one said he wasn't. Did you read my last post? The uniform currents
in each segment aren't the same value. Of course the end segments would
be minimum. Sincerely,

I did read your last post. But you also posted "Likewise we consider an
"electric dipole" to be a straight conductor of very small length
(compared to a wavelength) carrying uniform current."

That is the one Richard and I take exception to. I think your last post
explained your position better with breaking the antenna up into very
small segments each with a uniform current. The currents in each segment
can be considered to be uniform over that segment. However the segment
currents diminish from the feed point to the open end of the element.
I'm sure you know all this, but others may not.