I use lumped circuit analysis when dealing with lumped circuits, and
distributed circuit analysis when dealing with distributed circuits.
EZNEC's loads are lumped elements, so when you're talking about EZNEC
loads, you're talking about lumped elements.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Of course it doesn't account for phase shifts of current, since there
aren't any. It does account for voltage phase shift. It uses the same
equations I learned in freshman circuits class. Perhaps they taught
those same equations in Texas, too, but I can't be sure.


Roy,
We are talking about distributed networks. Of course, there is a phase
shift in the current as well as the voltage. You and W8JI seem to be
using lumped circuit analysis when you should be using distributed
network analysis. The center loading coil for a 75m mobile antenna
is an appreciable percentage of an electrical wavelength so you cannot
use your lumped circuit analysis without introducing errors.

Yes, I disagree with that.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Of course it doesn't account for phase shifts of current, since there
aren't any.


You seem to be disagreeing with John Devoldere's "Bible" - "ON4UN's Low
Band DXing", 3rd Edition, on page 9-34 at:

http://www.k3bu.us/loadingcoils.htm

A little thought should prove there is a current phase shift (delay)
through
the coil. Let's look at an 8 foot long center-loaded mobile antenna for
75m.
The 4 feet below the coil gives a phase shift of about 5 degrees. Assume
zero
phase shift through the coil. The 4 feet above the coil gives a phase shift
of another 5 degrees for a total of 10 degrees at the end reflection point.
It's an open circuit, so a 180 degree phase shift takes place. That puts
the
reflected current at 190 degrees. Add the 10 degrees coming back and we see
the reflected current arrives mostly out of phase with the forward current
at the feedpoint. Since the feedpoint impedance is known to be around 15
ohms,
these superposed currents cannot possibly be out of phase and must
necessarily
be in phase.

The phase shift (delay) of the current simply cannot be the same with and
without the coil.

The "simplified assumptions" made by EZNEC (and NEC in general) are the
same ones you'll find in any circuit analysis or electromagnetics text.
EZNEC includes a model of a lumped inductor (or "load"), which is
accurately represented. It also includes an accurate model of a straight
conductor which has physical length. If you could build an antenna from
straight conductors and lumped inductors, the result would be very close
to EZNEC's predictions.

EZNEC does not have a model of a coil which has physical length. Neither
the straight wire model nor the lumped inductor model can or should be
expected to behave exactly like a coil which has physical length. As
I've mentioned before, a useful approximation can be made by inserting
one or more lumped inductor models into a model wire. I don't have any
measurements to assess the accuracy of that approximation, however.

Roy Lewallen, W7EL

Cecil Moore wrote:

The antenna current reported by EZNEC is inaccurate because of
simplified assumptions. EZNEC assumes that the current doesn't
change through the single point inductive load. Therefore, EZNEC
cannot be used to prove that the current doesn't change.

Tell us, Cecil, at steady state at one frequency, can a lumped inductor
(presumably like the experimenter's toroid) tell whether it's at the
base of an antenna or simply in series between a generator and load
impedance?

Yes_____

No______

If you answered "yes", please explain how and why, and how we'd
calculate the current through and voltage across the inductor. If we
moved it an inch up the transmission line from the antenna base, can it
still tell?

If you answered "no", please write us the equations showing just how
much the current should be expected to be different from one end of the
inductor to the other. And where those coulombs are going, that go into
one end and don't come out the other. Going to the fourth dimension as
virtual photons, perhaps?

Roy Lewallen, W7EL

Cecil Moore wrote:
Yuri Blanarovich wrote:

in other words, the highest current point on the structure is at the
inductor.

That's what W8JI calculated in EZnec, does it make sense? Like 2+2 is
4.5? Why
would inductor "suck" the current up? We should then use "those"
inductors to
suck the current all the way to the top of the whip - perfect antenna?
Cecil, can you 'splain that?


Again, the current can either stay the same, increase, or decrease through
an inductor depending upon where it is located. Has that statement sunk
in on anyone? If you install a coil 1/8WL up on a 1/2WL vertical, the
current through the coil will *INCREASE*. If you install it in the center,
the current magnitude will be the same in and out of the coil and opposite
in phase. If you install it 1/8WL from the top, the current will decrease
through the coil like it does on a 1/4WL mobile antenna.

On Sun, 02 Nov 2003 15:14:28 -0700, I made a booboo:

[snip]
|It doesn't "suck it up." Haven't you ever hear of circulating
|current?
|

I meant to type, "heard of circulating current".

Roy, W7EL addressed several provocative questions to Cecil. Anyone can
comment, so I will.

Roy wrote:
"And where those coulombs are going that go into one end of the inductor
and don`t come out the other."

Coulombs travel back and forth in an inductor and may go actually
nowhere. Their movement in an unshielded inductance may result in
radiation and certainly produces some heat.

The purpose of a loading coil in a short loaded vertical antenna is
often to add to the existing degrees of antenna length to reach a
resonant length of 90-degrees, as shown in Fig 9-22 of ON4UN`s "Low-Band
DXing", and included on Yuri`s web pages.

Fig 9-22 is illustrative. First, a full-size 90-degree vertical is
shown. Current is maximum at the base and zero at the top. This is also
true for what Kraus calls a "normal-mode helical antenna". A normal-mode
helical antenna has its principal radiation at right-angles to the axis
of the helix.

The normal-mode helix is fed from a generator with two terminals. One
terminal feeds the base end of the helix directly. The other generator
terminal feeds the ground end of a capacitance between the ground,
various turns, and the tip end of the helix.

The impedance is only a few ohms at the ground end of the helix and
perhaps several thousand ohms at the tip end of the helix. This means a
lot more amps at the ground end of the helix than at the tip end, though
the power flow through the generator`s terminals is the same in either
terminal.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen wrote:
Yes, I disagree with that.

Then you disagree with Balanis.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
If you could build an antenna from
straight conductors and lumped inductors, the result would be very close
to EZNEC's predictions.

Hard to prove since lumped inductors are impossible in reality. Why
does EZNEC show so much difference between lumped inductors and stub
inductors? The difference in coils Vs stubs in reality is virtually
nill.
--
73, Cecil http://www.qsl.net/w5dxp



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Can you post a link to this so I know what it is that you think I
said, or what I said that I don't remember saying?

Wes Stewart N7WS


You are right. It wasn't you, it was and credit belongs to Richard Clark,
KB7QHC.
I apologize for the error, which was caused by my confusion between two of you
7's.
Now that you made posting on eHam.net I went back to the archives and found the
posting and its rightful author.
Sorry if it caused any problems and made you to jump to conclusion that I am
making things up. I will be more careful with references.

Yuri, K3BU.us

Roy Lewallen wrote:
Tell us, Cecil, at steady state at one frequency, can a lumped inductor
(presumably like the experimenter's toroid) tell whether it's at the
base of an antenna or simply in series between a generator and load
impedance?

This question proves you don't understand the problem. The inductor
cannot tell if it is installed in an antenna or transmission line.

So I will turn the question around: Does a standing wave antenna
have standing waves? Reference _Antenna_Theory- by Balanis, page 17,
section 1.4 Current Distrubution on a Thin Wire Antenna. Is Balanis
correct when he says: "If the diameter of each wire is very small,
the ideal standing wave pattern of the current along the arms of
the (1/2WL) dipole is sinusoidal with a null at the end."

This is after he takes an unterminated transmission line, discusses
standing waves, and then slowly opens up the ends of the transmission
line to create a 1/2WL dipole.

I took Balanis' antenna course at ASU in 1995. I asked a lot of
questions about inductively loaded antennas. The current and
standing wave pattern on each side of a loading coil is NOT the same.
--
73, Cecil http://www.qsl.net/w5dxp



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