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
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