"Cecil Moore" wrote in message
...
Richard Clark wrote:
Rarely do we get any practical correlation from this "sky is falling"
oops "current is dropping" argument.

Asserting that the argument is about any practical correlation is
a diversion of the issue. THE ARGUMENT IS ABOUT THE CURRENT IN A
LOADING COIL, not about the radiation pattern. The radiation pattern
is completely irrelevant to the argument. One side says the current
is absolutely constant except for radiation. The other side says it
is not constant (except for special cases). An electrical 1/4WL loaded
mobile antenna is not one of the special cases.

Sorry, this may sound dumb, I think I must have missed the point. Why are
people arguing about current in a loading coil? NEC, and experiment, seem
to provide the answer.

73,

Frank

Sorry, this may sound dumb, I think I must have missed the point. Why are
people arguing about current in a loading coil? NEC, and experiment, seem
to provide the answer.

73,

Frank


If you didn't read the stuff on my web page, have a look, the story is there.

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

73 Yuri, K3BU.us

Sorry, this may sound dumb, I think I must have missed the point. Why are
people arguing about current in a loading coil? NEC, and experiment, seem
to provide the answer.

73,

Frank


If you didn't read the stuff on my web page, have a look, the story is
there.

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

73 Yuri, K3BU.us

Thanks for the link Yuri. Read the web page, and now understand what is
going on. I have an Excel spreadsheet, complete with graph, prepared from a
NEC2 model of an inductively loaded monopole. The graph clearly shows the
current distribution across the coil. If you are interested I can e-mail it
to you, or can post it on the NG. It is only about 50kB, but not sure if it
is acceptable to post attachments on a NG.

73,

Frank

Knarf wrote:
Thanks for the link Yuri. Read the web page, and now understand what is
going on. I have an Excel spreadsheet, complete with graph, prepared from a
NEC2 model of an inductively loaded monopole. The graph clearly shows the
current distribution across the coil. If you are interested I can e-mail it
to you, or can post it on the NG. It is only about 50kB, but not sure if it
is acceptable to post attachments on a NG.

The netnews rules prohibit posting binary files. If you don't have
a web page, you could post it to alt.binary and point to it from
here.

Modeling a helical loading coil in EZNEC and putting loads at the
various segments also clearly illustrates the current taper. All
real-world air-core loading coils are distributed networks. In
a distributed network with reflections, the standing-wave currents
are tapered within a sinusoidal envelope.

Here's an unanswered question: If the loading coil occupies zero
degrees, how can the remaining eight feet of the antenna occupy
the entire 90 electrical degrees? Wouldn't the coil have to
change the frequency for that to happen?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil,

In a simple monopole with one inductor, let L1 be the distance from the
base of an antenna to the bottom of the loading coil in meters, L2 the
length of the loading coil, L3 the distance from the top of the loading
coil to the top of the antenna. I is the base current, L the inductance
value and F the frequency. You can assume the antenna is very thin.

Since your theory is so elegant and well developed, and you've had such
an excellent education at Texas A&M, it shouldn't be difficult at all
for you to write a couple of simple equations which give the currents at
the two ends of the coil. In the time-honored methods of science, your
equations can then be tested against modeled and measured results to
prove the validity of your theory.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Cecil,

In a simple monopole with one inductor, let L1 be the distance from the
base of an antenna to the bottom of the loading coil in meters, L2 the
length of the loading coil, L3 the distance from the top of the loading
coil to the top of the antenna. I is the base current, L the inductance
value and F the frequency. You can assume the antenna is very thin.

Since your theory is so elegant and well developed, and you've had such
an excellent education at Texas A&M, it shouldn't be difficult at all
for you to write a couple of simple equations which give the currents at
the two ends of the coil. In the time-honored methods of science, your
equations can then be tested against modeled and measured results to
prove the validity of your theory.

Sorry, Roy, my theory is not elegant and/or well developed. Equations may
be possible in the future, but not right now. At the present time, the
theory is qualitative, not quantitative. We are out on the edge of what
has been published so far and are in the process of discovery. It is hard
for me to believe that this material hasn't been covered some time, somewhere,
in a Master's thesis or a PhD dissertation or somewhere in the IEEE proceedings.
I regret that I don't have access to such.

The coil has an 'L' and a 'C' and thus can be regarded as a short piece
of transmission line. For a mental picture, consider two pieces of helix
material, side by side, being used as a balanced transmission line. They
would certainly possess a high velocity factor as does a bugcatcher coil.
Here is the equivalent of 1/2 of a typical loaded dipole using horizontal
#16 wire at a height of 24 feet where Z0=138*sqrt(4h/d).

Feedpoint---Z0=600 ohms---x---coil---y---Z0=600 ohms---

The Z0 of the coil is presently unknown but I am working on getting a
ballpark value for it. In any case since Z0=sqrt(L/C), the Z0 of the
loading coil will be very high. That means, in addition to the
reflections at the tip of the antenna, there will also be reflections
at 'x' and 'y', both ways. That situation is pretty complicated but
the result is apparently to put the forward voltage out of phase with
the forward current at the feedpoint. It also apparently puts the reflected
voltage out of phase with the reflected current at the feedpoint. The only
requirement is that Vf+Vr be in phase with If+Ir at the feedpoint. I hope
you can appreciate the complexity of that situation, stop asking for a
"simple equation", and assist us in the apparently complicated solution.

When someone doesn't understand the topic, one asks for a "simple
equation" and when none is forthcoming, one rationalizes that the
new information is not worth knowing. How about working with me
instead of against me on this complicated problem for which neither
one of us has the complete answer (yet)?

P.S. If you had demanded a "simple equation" from Maxwell, you would
have been disappointed also. :-)
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
For a mental picture, consider two pieces of helix
material, side by side, being used as a balanced transmission line. They
would certainly possess a high velocity factor as does a bugcatcher coil.
^^^^
Sorry, this should have been a *LOW* velocity factor.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Sorry, Roy, my theory is not elegant and/or well developed. Equations may
be possible in the future, but not right now. At the present time, the
theory is qualitative, not quantitative. . .

Somehow I expected this.

Roy Lewallen, W7EL

Cecil,

I decided to take a look at the question you asked below, and I came up
with a really simple modeling experiment.

Set up a simple quarter-wave vertical in EZNEC, resonant at 7 MHz. Run
the source and current functions and save the data. Now change the
frequency to 3.5 MHz and repeat the source and current functions. Do not
scale the antenna or change anything else. I believe most people would
now view this antenna as one-eighth wave at the new frequency, or
perhaps representative of a whip above a loading coil (at 3.5 MHz).

This experiment demonstrates what happens to the "remaining eight feet"
when confronted with the conflict between the "need" for 90 degrees and
the availability of only 45 degrees.

My computer did not blow up, and I suspect yours will survive as well.

Any number of permutations can be tried. Change the length instead of
the frequency, scale up, scale down, and so on. The current always
starts at 1.0, and it always goes to 0.0 at the tip. The reactance and
driving voltage can be awesome, but the current remained unfazed (or is
that unphased?).

This is not a revelation. Antenna books point out that the current in a
short antenna decreases in a straight line, not a sine curve, from the
feed point to the tip. (E.g. Kraus, 2nd Ed. page 216)

Since your traveling wave model seems to be based on a 90 degree
requirement, you may want to consider incorporating this additional
information before submitting your new model for publication.

73,
Gene
W4SZ

(The "eight feet" is taken from your message. In this experiment the
whip length is quite a bit larger, of course. Rescale the entire
experiment if you like.)


Cecil Moore wrote:


Here's an unanswered question: If the loading coil occupies zero
degrees, how can the remaining eight feet of the antenna occupy
the entire 90 electrical degrees? Wouldn't the coil have to
change the frequency for that to happen?

Gene Fuller wrote:

Antenna books point out that the current in a
short antenna decreases in a straight line, not a sine curve, from the
feed point to the tip. (E.g. Kraus, 2nd Ed. page 216)

Isn't that simply because the slope of a sine wave near the zero
crossing closely approximates that of a straight line?

73, Jim AC6XG