| Hi!
If you arrived from an eHam article link,
please read this carefully. If you read carefully, you will find this
page actually does not, and never did, say what the eHam article tells
you this page says. I accented important areas to make it easier to spot
misrepresentations of this page’s contents.
It’s my belief that some people just like to argue for the sport of
arguing, even where there is nothing useful to be learned or gained. To
enjoy the sport of arguing, they sometimes find it necessary to alter
what other people say. They also reach conclusions not supported by
evidence or fact, like the following quote from the eHam article:
Conclusion Number Two:
Any 75m air-core loading coil must necessarily
be in the ballpark of 30-45 electrical degrees
long which falls under the classification of a
distributed network, NOT a lumped circuit. |
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If you look carefully at the eHam article, you will not find anything
supporting that conclusion. As a matter of fact I can offer solid
evidence that proves that statement is wrong, and I am willing to wager
money on it. It is more than a little silly to say “[a]ny
75m air-core loading coil must necessarily be in the ballpark of 30-45
electrical degrees long”. Is every antenna the same? Is every
loading inductor the same?
Anyone can write an article and claim:
W5DXP loaned his 75m
Bugcatcher Loading Coil to Louisiana Tech
University where EE graduate students ran some
simple measurements. The current magnitude and
phase at each end of the coil was measured with
Tektronix CT2 current probes at 4 MHz under
various resistive loads. Here is the test setup
(based on an EZNEC graphic). |
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Generally when someone says “here is the test setup”, they describe
in detail and/or show detailed pictures of the test setup. They also
generally offer a screen shot or print of data. Without knowing many
details, such as test fixture design and measurement equipment, we
really don’t know what was done. An EZNEC graphic is not a test setup!
Also, there is little value to any of this to anyone building an
antenna, except guidelines or suggestions I highlighted in color below.
Thanks for visiting my website, no matter how you got here. Please feel
free to email me by using my callsign at w8ji.com with any suggestions
for improvement. I may not always answer, but I always read, and try to
implement any suggestions, or correct any errors.
|
Time
Delay through
Inductor
Many people
visualize current, in
a small loading
inductor, as starting
at one end and
traveling through
the conductor
turn-by-turn. It is
this visualization
that causes us to
conjure up all sorts
of untrue ideas of
what a loading
inductor does. One example is where people think an 80 meter vertical needs 67
feet of conductor length to make a 1/4 wave vertical, so they wind 67 feet of
wire (one quarter wavelength) around an insulated pole just a few yards long. One
way to prove or
disprove the
perception current
travels through the
conductor turn-by-turn is by
examining time taken
for current at one
end of the inductor
to
“appear”
at the other
end.
The sample
inductor in this
test is a typical
80-meter loading
coil. It is 100
turns, ten turns-per-inch, and 2 inches
inside diameter. The
wire is tinned
#18 buss wire. Inductor Q measures
290 at 4 MHz on an HP4101A, and on an Agilent vector network analyzer. This
is a reasonably high
Q 80-meter loading
coil.
We know many
things about this
inductor right away, based only on physical dimensions.
We know conductor
length making up the
coil is about 53
feet. We know light
travels at
982,100,000 feet per
second in freespace.
We know physical
length is 10 inches,
plus about one-foot of
total connection
length in my open-air test
fixture. We also
know the very
fastest speed
electromagnetic
energy can travel is
the speed of light
in freespace, other
things like nearby dielectrics only slow it
down.
If current winds
through the
conductor length,
time delay should be
about .98
nanoseconds per foot
of conductor length.
Time delay would be
54*.98 = 53
nanoseconds.
How long does it
take current reach
the other end of
this inductor?
Here’s a plot of
time delay at
various with
frequencies:
On 80-meters, and
actually over a
fairly wide
frequency range,
time delay is about
3nS. 3nS is
equivalent to 3.06
feet of distance. We
know one foot is
occupied by the test
fixture connections,
so the ten-inch long
inductor appears to
be about two feet
long, so far as
current propagation
delay.
How does the
current travel
through the inductor
so fast? After all, the wire is about 53 feet long.
At first this
seems impossible,
but the answer
is actually quite
obvious.
Time-varying current
gives rise to
time-varying
magnetic flux. This
magnetic flux, since
conductor spacing is
close and
distance very small,
links each
turn very tightly to
the adjacent turn. The
rapidly changing
magnetic flux causes
charges to move in
the adjacent conductor. The changing
magnetic field
couples through all
the close-spaced
turns with very
little time delay.
It is this magnetic
flux coupling that
provides the primary
mechanism for energy
transfer through this
inductor, and the
path is much shorter
than the circuitous
and much longer path
along the conductor.
The above measured data, shown in the screen capture above, shows time delay
of
current is very
close to zero.
Current at BOTH
ends of inductor, when there is no
shunting capacitance present
to increase phase
shift, lags voltage
at the source-end by
a value that depends
on system termination impedance and the
inductor’s
reactance! Without shunting capacitance throughout the length of the inductor,
current would be equal at both ends and phase delay would be zero. In the case
of an inductor, voltage
is out-of-phase with
current. Voltage leads
current at the
generator. Current
lags the generator
voltage an equal
amount at either end
of the inductor,
even though delay
time is finite, unless we have shunting capacitances and imperfect mutual
coupling from end-to-end of the inductor.
If the inductor
in this test were an
ideal inductor, time
delay would be just
under 2nS in this
test system. Since
it is
less-than-perfect
and does not have
perfect flux
coupling, and because it has stray capacitance to the fixture groundplane, time delay
is longer (about
double) than we might
expect.
Depending on shunt capacitance, termination, and mutual coupling from
turn-to-turn, overall system time delay can vary. A compact lumped loading coil does
not represent a certain fixed number of missing antenna degrees, it simply cancels
reactance. The very best loading coils, mounted in the best positions, have
the least current taper and current phase delay from end-to-end.
The important point is current does not slowly wind its way through the
loading coil turn-by-turn, and a loading coil does not replace a certain number
of “missing” antenna electrical degrees. Mutual magnetic coupling from turn-to-turn tries to
make charges in the inductor move at nearly the same instant of time. Flux leakage and shunt
capacitance do the opposite, they allow, and even encourage, delay. The better the loading coil and system design, the more uniform
current distribution is, and the less time delay occurs.
This is why, with a base loading coil in a well-designed system,
current is equal on each end of the loading coil. Phase delay of current
is also essentially zero. Only voltage decreases and changes phase along the
length of the loading coil. The essentially equal currents at each end of the
coil are also true with large capacitance hats, or with long whips above the loading coil.
In such cases, termination is primarily by shunt reactances at the open end of the inductor.
A large current taper though a loading coil indicates the system is poorly
designed, and that displacement currents are robbing the upper areas of the
antenna from current. This reduces effective antenna height and radiation
resistance.
Conclusion (new)
The bottom line, with very high reactance coils, is we should keep large
loading coils away from anything that increases capacitance shunting the coil.
This means the coil should be away from sheet metal, and capacitance hats kept
up away from the coil.
Mobile and other Inductors
Page
 since
3/11/2006
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