Here are some preliminary details about the inductor current measurement
I made.

My antenna isn't nearly as ideal as the one Yuri described. (But if my
results are different from the ones reported at the web site Yuri
referenced, I'll be eager to hear why.) It's about 33 feet high, and has
only 7 buried radials. The feedpoint impedance indicates a loss of about
25 ohms at 7 MHz, so I'd expect it to be a bit more at 3.8. It's bolted
to a galvanized fence line post which protrudes nearly four feet from
the ground, with spacing between the antenna and the post of about 1/4".
This mounting has only a minor effect on the feedpoint impedance at 7
MHz, which is the antenna's intended frequency of use. It's quite
profound at 3.8 MHz, though. The expected 370 or so ohms of capacitive
reactance is transformed to 185, while the feedpoint R is 35 ohms, not
far from the expected value. So the overall feedpoint Z is 35 - j185
ohms at 3.8 MHz, measured with a GR 1606A impedance meter. (I found that
my MFJ 269 was about right with the X, but measured R as zero --
apparently the combination of low frequency and large X is a problem for
it in resolving the R.) So I built an inductor with measured impedance
of 0.6 + j193 ohms. It's 26 turns on a T-106-6 toroid core. Q is a bit
over 300. This was placed in series at the antenna feedpoint.

For current measurements, I made two identical current probes. Each one
consists of 10 turns wound on an FT-37-73B ferrite core. The two leads
from the winding are twisted and wound in bifilar fashion on another
FT-37-73B core, 10 turns. This is then connected to an oscilloscope
input via a two-foot (approx.) piece of RG-58. A 50 ohm termination is
also at the scope input. This gives the probe a theoretical insertion
impedance of 0.5 ohm. While making the measurements, I moved, grabbed,
and re-oriented the coax cables, with no noticeable effect. This gave me
confidence that the outsides of the coax weren't carrying any
significant current.

One probe went to each channel of the scope. I left the two scope inputs
in the cal position, put both probes on the wire at the input end of the
inductor, and recorded the p-p values with the scope's digital
measurement feature. Then I reversed the order of the probes and
remeasured. I found a slight prejudice toward the probe closest to the
source -- 1.2% in one ordering, and 2.1% in the other. Averaging the two
channels, though, showed them to be the same within less than 1%. (Each
probe was always connected to the same scope channel, so this is a test
of the probe-scope channel combinations.)

Then I moved one probe to the output side of the inductor, and measured
input and output current. And I reversed the probe positions on inductor
input and output. The ratio of output to input current in the two tests
differed by only 1.4%.

When I encounter an astrologist, they invariably ask what "sign" I "am",
then proceed to tell me how my personality meets their expectations. So
what I do instead is to have them tell *me* what "sign" I "am" *first*
-- which they should easily be able to do, based on my personality.
Well, they don't find that to be fair, for some reason (although I
certainly find it amusing). And so, I doubt if the following challenge
will be regarded to be fair, for much the same reason. Those with
alternative rules for solving circuit problems are challenged to predict
what the ratio of output current to input current will be. I'm
particularly targeting Cecil, and others who have bandied about a lot of
pseudo-analysis about electrical length, reflections, and the like. And,
Richard (Harrison), who said something like "an inductor without phase
shift is like". . . I don't recall. . .hot dog without ketchup or
something. Pull out your theories, and calculate it, like any competent
engineer should be able to do. For cryin' out loud, it's a simple series
circuit (except for Cecil, where it's some kind of distributed thing).

First post your answers, then I'll post the result of my measurements.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Pull out your theories, and calculate it, like any competent
engineer should be able to do. For cryin' out loud, it's a simple series
circuit (except for Cecil, where it's some kind of distributed thing).

First post your answers, then I'll post the result of my measurements.

What is the value of the distributed capacitance between each two
turns on the toroid? That distributed capacitance is what makes a
75m mobile loading coil act like a transmission line.

Question: Why didn't you use a 75m bugcatcher coil for the experiment?
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
I'm
particularly targeting Cecil, and others who have bandied about a lot of
pseudo-analysis about electrical length, reflections, and the like.

Balanis would be surprised to know that you consider the material that
he teaches in his classes at ASU to be pseudo-analysis. Some of the
stuff I have posted is in Balanis' book, _Antenna_Theory_ which you
haven't read. In particular, he says: "Standing wave antennas, such
as the dipole, can be analyzed as traveling wave antennas with waves
propagating in opposite directions (forwards and backwards) as represented
by traveling wave currents If and Ib in Figure 10.1(a)."

I'm just wondering how you can be so sure that what I have offered is pseudo-
analysis and which of the following statements you disagree with. Please
be specific.

1. The feedpoint impedance of a typical traveling wave antenna is in the
hundreds of ohms since there are no reflected waves.

2. The feedpoint impedance of a standing wave antenna is the result of
superposition of forward and reflected waves (which cause the observable
standing waves).

3. At the feedpoint of a 1/2WL resonant dipole, the forward current, reflected
current, and forward voltage are all in phase. The reflected voltage is 180
degrees out of phase. This results in a purely resistive low-voltage/high-current
ratio for the feedpoint impedance.

4. The above relationship is true for any dipole, 1/2WL or physically shorter,
that has a purely resistive feedpoint impedance. (No resistive loading)

5. The phases of the signals at the feedpoint are known. The phases of the signals
at the open tips of the dipole are known. Any loading used in order to increase
the electrical length to 1/2WL must maintain those known phase conditions in
order to achieve a purely resistive feedpoint impedance.

On page 18, in Figure 1.15, Balanis shows how a 1/2WL dipole is achieved by
flaring out the last ~1/4WL of an unterminated transmission line. He says:
"As the section of the transmission line begins to flare, it can be assumed
that the current distribution is essentially unaltered in form in each of
the wires."
--
73, Cecil http://www.qsl.net/w5dxp



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My challenge to you was rhetorical. Based on past experience, I had no
real expectation that you'd be able to actually calculate a current.

Our educations differ a great deal. Mine enabled me to give a numerical
prediction, which as anyone who has read my earlier postings, is 1.
Yours has evidently not prepared you to meet this onerous challenge.

Does anyone else feel up to the task of calculating the currents in a
simple circuit? It used to be that you'd have to be able to do this to
get a first phone license, or probably an amateur extra. Now, it appears
that even American engineering education isn't always up to the task.

Roy Lewallen, W7EL

Cecil Moore wrote:
. . .
Balanis would be surprised to know that you consider the material that
he teaches in his classes at ASU to be pseudo-analysis. Some of the
stuff I have posted is in Balanis' book, _Antenna_Theory_ which you
haven't read. In particular, he says: "Standing wave antennas, such
as the dipole, can be analyzed as traveling wave antennas with waves
propagating in opposite directions (forwards and backwards) as represented
by traveling wave currents If and Ib in Figure 10.1(a)."
. . .

Roy Lewallen wrote:
My challenge to you was rhetorical. Based on past experience, I had no
real expectation that you'd be able to actually calculate a current.

Here's how to mask the effects that we have been discussing:

1. Choose a small inductance that replaces a very small number of
degrees of the antenna.

2. Use a ferrite coil designed to minimize distributed effects.

3. Mount the coil at a place in the antenna where the slope of the
current is virtually zero.

That's what you have done.

Here's how to showcase the effects that we have been discussing:

1. Chose a large inductance that replaces an appreciable number of
degrees of the antenna.

2. Use a typical air-core loading coil like a bugcatcher that has
appreciable distributed effects.

3. Mount the coil at the center of the antenna where the slope of
the current curve is near maximum.

When you perform your experiment with an 8 foot center-loaded
bugcatcher on 75m, then you will be taken seriously.
--
73, Cecil http://www.qsl.net/w5dxp



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W5DXP wrote to W7EL:

Question: Why didn't you use a 75m bugcatcher coil for the experiment?


And why did you put toroid at the base? We are talking about loading coils that
are installed at half to 2/3 up the radiator. Try 12 foot radiator with loading
inductor at ~65% up from the feedpoint. The results should be "magnified". Our
theory is that the current drop across the inductor should be roughly
proportional to
the current in the radiator (in degrees) that it replaces (Cosine law).

Judging by description, I would guess that there wasn't much difference. Put
that coil up but don't use scope probes, they will detune the antenna, no
wires, use thermocouple meters. Probes at the base they probably do not distort
the measurements much.

Nice disertation on engineers and modeling. The only small problem is what and
how you model. If your modeling uses 0 size inductance and real measurement
shows something else, maybe there is a reason to question modeling how well it
reflects reality. I went to university first, I designed parts that human life
depended on, and I would think twice about relying on some numbers without
testing and verifying it. Space shuttle tiles modeled OK?

Yuri

Yuri Blanarovich wrote:
Judging by description, I would guess that there wasn't much difference.

The feedpoint of the radiator alone is 35-j185. The impedance of the loading
toroid is 0.6+j193. Assuming perfect predictability, that gives the antenna
system a feedpoint impedance of 35.6+j8, i.e. it is *longer* than resonant.
That moves the current maximum point inside the toroid making the current
in and out even closer to equal. If a coil is installed at a current maximum
point or a current minimum point, the current in and out will be the same.
If a coil is installed at a place where the slope of the current envelope
is positive, the current will actually increase through the coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Can I conclude from this that if I were to make a coil with more or less
inductance, then I would see a current difference between the ends of
the coil?

So tell you what. If you'll pull out your equations and calculate the
expected current difference, I'll replace the coil with one of 100 ohms
reactance and remeasure. How much current difference (magnitude andd
phase, of course) between the ends of a 100 ohm inductor at the base of
that same antenna?

Roy Lewallen, W7EL

Cecil Moore wrote:
Yuri Blanarovich wrote:

Judging by description, I would guess that there wasn't much difference.


The feedpoint of the radiator alone is 35-j185. The impedance of the
loading
toroid is 0.6+j193. Assuming perfect predictability, that gives the antenna
system a feedpoint impedance of 35.6+j8, i.e. it is *longer* than resonant.
That moves the current maximum point inside the toroid making the current
in and out even closer to equal. If a coil is installed at a current
maximum
point or a current minimum point, the current in and out will be the same.
If a coil is installed at a place where the slope of the current envelope
is positive, the current will actually increase through the coil.

Yuri wrote,
Our
theory is that the current drop across the inductor should be roughly
proportional to
the current in the radiator (in degrees) that it replaces (Cosine law).

That's a pretty good theory, Yuri. I'd like to know where you got
this "Cosine law" you keep talking about. I can't seem to find
mention of any such _law_ anywhere but on this newsgroup. Does
that mean I should throw away my method of moments software
because I don't need it any more? And what is a current
drop? I've heard of voltage drops and cough drops but never
current drops. Finally, how do you measure the "current in
the radiator (in degrees)?" Why not use amperes like everyone
else?
I won't believe your theory, Yuri, until you and Cecil take the
time to present it in terms of field theory. Since you guys have taken
EM classes in college you should have no trouble doing this, right?
73,
Tom Donaly, KA6RUH

Tdonaly wrote:
And what is a current
drop? I've heard of voltage drops and cough drops but never
current drops.

It's is the decrease in current due to the attenuation
(alpha) factor in equation 1.22 (2) in Ramo, Whinnery, & Van Duzer.
It's all covered in any distributed networks course. According to
Balanis, antennas have an attenuation factor due to radiation and
is similar to (slightly more complicated than) this familiar
transmission line equation for lossy lines.

I = Im(e^-az)d^j(wt-bz)

where a is alpha, w is omega (2*pi*f), and b is beta (phase factor).

I won't believe your theory, Yuri, until you and Cecil take the
time to present it in terms of field theory. Since you guys have taken
EM classes in college you should have no trouble doing this, right?

Please reference Chapter 1 of _Fields_and_Waves_... by Ramo,
Whinnery, and Van Duzer. Start with equations 1.18 (4)&(5)
and 1.22 (1) & (2). Also _Antenna_Theory_ by Balanis, equations
4-81 and 10-1 and one other that I cannot locate right now. :-)
The one I cannot locate is the simplified one for a 1/2WL dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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