It sounds like the predictions are in. Among the several people who
believe that the current out of a small inductor doesn't equal the
current in, only Yuri was able to calculate a predicted value for the
test, of 2.5 - 5% reduction in current at the output compared to the
input, with a phase shift of about 18 degrees.

What I measured was a 3.1% reduction in magnitude from input to output,
with no discernible phase shift. The 3.1% is an average of two readings,
with the input and output probes exchanged. The output was smaller than
the input in both measurements, about 2% and 4%. So I believe there's a
real difference between output and input current, although with the
accuracy of my measurements, I only have reasonable confidence it's
somewhere between 1 and 5%. I can resolve about 2-3 degrees of phase
shift, though, and I couldn't discern any at all. (Yes, the scope
trigger was from one channel, not alternating.) So I have very high
confidence that Yuri's prediction of 18 degrees is incorrect.

I don't subscribe to the notion that the current out of a very small
inductor should be different than the current in due to some magical
property it acquires when connected to an antenna. My working hypothesis
is that the currrent difference I did see was due to stray capacitance,
either from the probes or simply to the Earth and other objects. It
would take an equivalent of 6.8 pF at the output of the coil (that is,
between the coil output and the current probe) to get 3% reduction, and
only about 1/3 that amount to see the minimum value of reduction of 1% I
estimate was actually present.

I repeated the test on the bench, with a 36 ohm resistor in series with
a 220 pF capacitor substituting for the antenna. The result was a 2.3%
output:input reduction, again with no discernible phase shift. This is
within the measurement error of being the same result. This is what
should be expected -- except for unintentional coupling to the antenna's
field, the inductor's environment is the same on the bench as at the
antenna base, in these single frequency, steady state tests. (That also
contradicts what some newsgroup participants have been claiming.)

So, although the small output:input current reduction was within Yuri's
prediction, the phase shift certainly wasn't. If time permits, I'll make
a more idealized antenna and repeat the measurements with a larger
inductor at the base of a more reactive antenna. I'll predict in advance
that if I double the amount of loading L, I'll approximately double the
amount of current magnitude attenuation -- that is, to somewhere around
6%. That's what should be expected if the cause of the attenuation is
stray C or a similar phenomenon.

I've added a picture to the
http://eznec.com/rraa/Inductor_Current_Measurement.html page, showing
the overall setup including the scope. It gives a little better
perspective on the relative sizes of various objects.

Roy Lewallen, W7EL

Today's project was to construct and measure a more idealized antenna.

The antenna is 33 feet high, made of #16 insulated wire. I put out 23
radials on the surface of the wet ground. Radials were of various
lengths, most about 30 feet long. The feedpoint impedance of the
antenna, measured with a GR bridge, was 15.8 - j437 ohms at 3.8 MHz.
Allowing 3% lengthening effect for insulation, EZNEC says a lossless
vertical of that height and diameter should have an input Z of 7.5 -
j478. 8.3 ohms loss resistance is reasonable for that number of radials,
and the somewhat lower than predicted reactance is likely due to the
fact that the radial wires were grouped together as they came up a few
inches to the antenna base, and not immediately coming in contact with
the ground. That would add a bit of inductive reactance.

I wound an inductor on a T-106-6 core as before, but with more turns,
for a measured Z of 1.3 + j387 ohms. After putting it in series with the
antenna at the base, the base impedance measured 17.1 - j54 ohms. This
is only 4 ohms from the expected reactance, and spot on the expected
resistance, so measurements are consistent.

Analyzing verticals with EZNEC, made from #16 wire at 3.8 MHz, shows that:

-- An antenna 63.2' high is resonant.
-- An antenna 35.9' high has a feedpoint reactance of -j437 ohms.
-- An antenna 59.35' high has a feepoint reactance of -j54 ohms.

With a resonant height of 63.2', you could say that 63.2' is "90
electrical degrees" as far as the antenna is concerned. So you might say
that my inductor has "replaced 33.4 electrical degrees" of the antenna.

Using Yuri's cosine rule, we should then expect the inductor output
current to be cos(33.4 deg) times the input current, or 16.5% less.
Also, we should expect to see those 33 degrees of "replaced antenna" as
phase shift from the input to the output of the inductor. That is, the
current change from the input to output of the inductor is the same as
it would be for the portion of the antenna it "replaces". (I think Jim
Kelley subscribes to this theory also, but I'm not sure.)

In contrast, conventional electrical circuit theory predicts no current
difference between the input and output for a physically very small
inductor with no radiation or stray coupling. I saw about 3% in the
previous measurement, which I believe can be attributed to stray
capacitance. So I predicted that we should see about twice that amount
with the higher valued inductor used for this experiment (387 vs 192
ohms reactance). I didn't see any measurable phase shift between input
and output before, so I didn't expect to see it this time.

So for this test, there's quite a difference in predictions for
output:input current --

**Yuri's method predicts a reduction of output current magnitude of
16.5% and a phase shift of 33 degrees.

**I predict around 6% magnitude reduction (due to stray C) and no
measurable phase shift (less than 2 or 3 degrees).

I have very high confidence that my measurements are good enough to
resolve the difference between these two possibilities.

Would anyone care to comment before I post the measurement results? And,
Yuri, please correct me if I've misinterpreted your theory.

Roy Lewallen, W7EL

Would anyone care to comment before I post the measurement results? And,
Yuri, please correct me if I've misinterpreted your theory.

Roy Lewallen, W7EL



It is not my theory. My argument with W8JI and his followers: is the current in
typical loading coil in quarter wave radiator same at both ends or does it drop
with distance from the feedpoint. I have made temperature observations, W9UCW
measured the difference, W5DXP provided some explanation. Based on Cecils
analysis of data you provided, and on my understanding of the phenomena I
guestimated drop in current in your setup. No theory, no mathematical procedure
(yet) just attempt (using degrees replaced by coil in a radiator) at
explanation of what is happening. I will measure things myself, try to verify
previous measurements and then come up with conclusions and "theory". So far
Cecils (and ON4UN book) theory seems to be closest to the truth.

As far as your measurements, it appears that you are trying to use the worse
case extreme situation (feed point, toroid) to prove your case. Why don't you
use thermo ammeters or current probe without leads and normal coil and do it on
typical mobile whip antenna.
Here is the info on homebrew current probe:
http://www.isd.net/~lyle/currprob/currprob.htm

I am going to build one too, it is handy to check the current while sliding
along the radiator, which easier than inserting ammeter.

I posted my 7 points, so far not one argument against, had few agreements.

What's this guessing game anyway? Why don't you try to prove that W9UCW
measurements are off the rocker?

Yuri, K3BU

Ok,

For anyone who cares, the magnitude of the current out of the inductor
in the later test measured 5.4% less than the current in. No phase shift
was discernible. An analytical person could build on this information to
investigate the properties of longer inductors placed elsewhere in the
antenna.

Thank you for the comments, Cecil, Yuri, Richards, Art, and others. I've
learned a good lesson from this -- that this isn't an appropriate forum
or appropriate audience for the sort of quantitative analysis and
reasoning I'm familiar and comfortable with. And that the considerable
time and effort required to make careful measurements is really of very
little benefit -- certainly not anywhere near enough to justify it.

With a great sigh of relief from everyone, I'm sure, I'll now turn this
thread back over to Yuri, Cecil, et al.

My apologies to everyone for taking up so much bandwidth.

73,
Roy Lewallen, W7EL

Yuri Blanarovich wrote:
Would anyone care to comment before I post the measurement results? And,
Yuri, please correct me if I've misinterpreted your theory.

Roy Lewallen, W7EL




It is not my theory. My argument with W8JI and his followers: is the current in
typical loading coil in quarter wave radiator same at both ends or does it drop
with distance from the feedpoint. I have made temperature observations, W9UCW
measured the difference, W5DXP provided some explanation. Based on Cecils
analysis of data you provided, and on my understanding of the phenomena I
guestimated drop in current in your setup. No theory, no mathematical procedure
(yet) just attempt (using degrees replaced by coil in a radiator) at
explanation of what is happening. I will measure things myself, try to verify
previous measurements and then come up with conclusions and "theory". So far
Cecils (and ON4UN book) theory seems to be closest to the truth.

As far as your measurements, it appears that you are trying to use the worse
case extreme situation (feed point, toroid) to prove your case. Why don't you
use thermo ammeters or current probe without leads and normal coil and do it on
typical mobile whip antenna.
Here is the info on homebrew current probe:
http://www.isd.net/~lyle/currprob/currprob.htm

I am going to build one too, it is handy to check the current while sliding
along the radiator, which easier than inserting ammeter.

I posted my 7 points, so far not one argument against, had few agreements.

What's this guessing game anyway? Why don't you try to prove that W9UCW
measurements are off the rocker?

Yuri, K3BU

Roy Lewallen wrote:
For anyone who cares, the magnitude of the current out of the inductor
in the later test measured 5.4% less than the current in. No phase shift
was discernible.

A better way to measure phase shift is to measure the delay between
the zero-crossings of the two currents.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
For anyone who cares, the magnitude of the current out of the inductor
in the later test measured 5.4% less than the current in.

That would be one amp in and 0.9460 amps out. The angle whose cosine
is 1 is zero deg. The angle whose cosine is 0.9460 is 18.9 degrees.
So Yuri's estimate of an 18 degree effect was pretty accurate.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote in message ...
Ok,

For anyone who cares, the magnitude of the current out of the inductor
in the later test measured 5.4% less than the current in. No phase shift
was discernible. An analytical person could build on this information to
investigate the properties of longer inductors placed elsewhere in the
antenna.

Thank you for the comments, Cecil, Yuri, Richards, Art, and others. I've
learned a good lesson from this -- that this isn't an appropriate forum
or appropriate audience for the sort of quantitative analysis and
reasoning I'm familiar and comfortable with. And that the considerable
time and effort required to make careful measurements is really of very
little benefit -- certainly not anywhere near enough to justify it.

Interesting though. I think I may try to rig up some couplers so I can
do this myself. I have the dual channel scope, but I need to build the
couplers.

With a great sigh of relief from everyone, I'm sure, I'll now turn this
thread back over to Yuri, Cecil, et al.

My apologies to everyone for taking up so much bandwidth.

None needed. If the group can have multiple postings on amateur
racists, and other assorted problem children, then I see no problem
with this thread, no matter how long it gets. So far, your tests,
while not being a bugcatcher type coil seem to match my expectations
fairly closely. I never expected to see no reduction at all. In my
view, even a large 75m bugcatcher coil is still a lumped coil, and
will pretty much act as one. Why do I think this? Because the overall
form is still very small per wavelength. IE: 90 degrees is appx 65 ft.
So far no one has argued that the current taper UNDER the coil is
suspect when modeled. Most all seem to agree that the current
distribution is dramatically improved when the coil is raised up the
mast. If you model a 10 ft whip, using a center load coil, the model
will show max current at the coil. Here is an example using eznec....

EZNEC Demo ver. 3.0

Vertical over real ground 11/12/03 11:30:20 AM

--------------- CURRENT DATA ---------------

Frequency = 3.85 MHz.

Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Ground 1 0.00
2 1.0013 -0.01
3 1.0036 -0.02
4 1.0072 -0.03
5 1.0122 -0.04
6 1.0192 -0.04
7 1.029 -0.05
8 1.0432 -0.06
9 1.0691 -0.06
10 1.1036 -0.07 ......coil is at segment 10
11 .98384 -0.07
12 .87242 -0.07
13 .77233 -0.07
14 .67604 -0.07
15 .58163 -0.07
16 .48789 -0.08
17 .3938 -0.08
18 .2982 -0.08
19 .19932 -0.08
20 Open .08787 -0.08

OK. Lets say the coil in the real world is one foot long. That is appx
1/10 of the total antenna length. Will there be any argument that max
current will occur at the coil? I hope not...
OK. Lets say that Yuri, et el, are correct and there is a noticable
taper of current across the coil from bottom to top. I still think
they are being fooled by the capacitance above the coil, which is
where they are testing, but thats another issue.
Say you have a 1 ft section of the antenna, "coil" and it is found
that there is a noticable current taper across it. What would this
amount to in the real world? To me, nothing much at all. I don't think
it would have any effect on the way I build mobile antennas. It won't
have any effect on where I mount my coil, because I am already using
the best locations possible. These "best" coil locations are old news
and easily calculated using a program such as Reg's "vertload" or even
info in the ARRL antenna handbook.
Would this current taper in a 1/10 section of the antenna drastically
skew any modeling done of this antenna? It's possible, but again, I
really doubt it.
BTW, I think I said earlier that the modeling of these mobile whips
didn't do a good job of showing increases in performance due to
changes in coil position.
But that seems to not be the case. I may have been thinking of
something else. I do show increases in gain when the coil is raised
from a base load, to a center load. As far as the reflected currents,
and phase, etc, I just don't see that causing a major difference in
the current across the coil. Some difference I'm sure, but I don't
think it would be enough to cause a difference in either the
calculation of best coil location, or in the modeling of the antenna.
I'm still of the opinion that if you measure the current at the top of
the coil, where it is attached to the capacitance section, this will
slightly stunt the upper coil measurement. The eznec plot *seems* to
agree. I'm still of the opinion that the current is *fairly* constant
across the coil, but I'm not losing any sleep over it. I'll still be
building my antennas the same way I have been. Nothing will change,
even if it's determined they are correct about this current taper
across the coil. MK

"Mark Keith" wrote in message
om...
So far, your tests,
while not being a bugcatcher type coil seem to match my expectations
fairly closely.

I'd like to hear an explanation for ANY current difference across a coil
that is supposedly behaving as a lumped inductor. But the test really
should be for the same type of antenna used in Yuri's discussion; A
physically short antenna, with an electrically long coil, positioned away
from the feedpoint. One misconception here has been about the physical
length of the coil with respect to wavelength. That's not the most relevant
issue, in my opinion. The wire comprising the coil also has a physical
length. The relationship between physical length and electrical length is
velocity factor. The same thing is true for a coil. The velocity factor
for a wire does not go to infinity simply by virtue of the fact that it has
been wound into a coil. This is basically what is being implied when
someone argues that loading coils do not effectively supliment the
electrical length of an antenna.

73, Jim AC6XG

Jim Kelley wrote:

I'd like to hear an explanation for ANY current difference across a coil
that is supposedly behaving as a lumped inductor. But the test really
should be for the same type of antenna used in Yuri's discussion;

Jim, did you fail to notice that arc-cos(0.95) = 18.2 degrees?
--
73, Cecil http://www.qsl.net/w5dxp



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Mark Keith wrote:
So far, your tests,
while not being a bugcatcher type coil seem to match my expectations
fairly closely.

They seem to have matched Yuri's predictions almost exactly. He predicted
a 5% reduction in current. That was very close. He predicted an 18 degree
effect. Turns out a 5% reduction in current in that area of the cosine
curve is almost exactly 18 degrees. Cos-1(.95) = 18 degrees
--
73, Cecil http://www.qsl.net/w5dxp



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