Cecil Moore wrote in message

2. The second antenna is electrically 3/4 wavelength long. This one
illustrates how the current at the top of the loading coil can be
greater than the current at the bottom of the loading coil.

http://www.qsl.net/w5dxp/octcoil2.gif
http://www.qsl.net/w5dxp/octcoil2.ez

Why the 3/4 wave example? This is NOT what I had in mind. What I would
like to see, is you take a 8 ft mobile whip for 80m. Start with a
center load. Note the current taper. Then place the coil below the
midpoint level. Note the current taper. Then place the coil above the
midpoint level. Note the current taper.
You should be able to find a configuration that provides max current
at the top of the coil, if what you claim is true. I want to see max
current at the top of the coil in the SHORT loaded 8ft mobile whip for
80m. Not a funky overly tall 3/4 wave antenna. I'd try it myself, but
I only have the demo of eznec and don't have the segment capability.
Then if you can see this happen in the model, I'd like to see it
confirmed in the real world. If this occurs, then I might start to
think what you claim holds some water. I'll reserve further comment
until you can try this simple test. MK

Mark Keith wrote:
Why the 3/4 wave example?

In order to illustrate higher current at the top of the coil than
at the bottom, of course.

You should be able to find a configuration that provides max current
at the top of the coil, if what you claim is true.

I just did in the 3/4WL example. One cannot get higher current at
the top of the coil in a 1/4WL electrically long antenna. For a
1/4WL antenna, the maximum current is at the feedpoint and tapers
down to the tip.

I want to see max current at the top of the coil in the SHORT
loaded 8ft mobile whip for 80m.

Well, good luck on that one. I think it's impossible. Blood out
of a turnip comes to mind. If you think I ever said or implied
that I could do that, you are as mistaken as you can possibly be.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Mark Keith wrote:
Why the 3/4 wave example?

In order to illustrate higher current at the top of the coil than
at the bottom, of course.

You should be able to find a configuration that provides max current
at the top of the coil, if what you claim is true.

I just did in the 3/4WL example. One cannot get higher current at
the top of the coil in a 1/4WL electrically long antenna. For a
1/4WL antenna, the maximum current is at the feedpoint and tapers
down to the tip.

uh huh.....

I want to see max current at the top of the coil in the SHORT
loaded 8ft mobile whip for 80m.

Well, good luck on that one. I think it's impossible. Blood out
of a turnip comes to mind. If you think I ever said or implied
that I could do that, you are as mistaken as you can possibly be.

What has all this been about? Bugcatchers and other short loaded mobile
antennas. All shorter than 1/4 wave.
Good grief....I rest my case.
My theory? I still think the current across the coil is *fairly*
constant.
I'm *still* of the opinion that the *apparent* radical taper across the
coil is more due to the presence of the capacitance above the coil. I
think Roy described this in more accurate terms, but I have to reread
the thread.
I'm *still* of the opinion that if you could measure the current INSIDE
the windings of the coil, a couple or three turns in from each end, you
would not see near the difference you all do. I do think it's quite
normal to have a slight taper, as you would with any other radiator that
is shorter than 1/4 wave. So what? But these are just my gut instincts
using my built in BS filter. I'm absolutely certain than any error in
modeling a short mobile antenna using lumped coils is not worth worrying
about. This was the main gist of the argument by Yuri. It was the bottom
line. He claimed we were modeling in fairly gross error due to this new
found revelation of coil current taper. He promised a new revelation in
coil positioning that would turn the mobile antenna world on it's head.
Or at least the hype seemed to imply this. But I'm afraid many beat him
to it years ago. As far as phasing coils, yep, you might have an issue
there. But it appears you may have been enlightened to a workaround even
in that case. Isn't life grand? MK
--
http://web.wt.net/~nm5k

On Fri, 06 Feb 2004 18:33:24 -0600, Mark Keith wrote:

I'm absolutely certain than any error in
modeling a short mobile antenna using lumped coils is not worth worrying
about.

That has been shown several times - unless your skin crawls with
differences of 0.5dB

This was the main gist of the argument by Yuri. It was the bottom
line. He claimed we were modeling in fairly gross error due to this new
found revelation of coil current taper.

And that pilot's error was addressed before the soap opera began.

He promised a new revelation in
coil positioning that would turn the mobile antenna world on it's head.

Umm yes. The excuse is snow, but given the tapering heat wave
reports, the coils should clear that away in half an hour. Patents
pending!!! - any further discussion constitutes a violation of
Intellectual Property rights subject to suit (40R w/2 pair of pants).

73's
Richard Clark, KB7QHC

Mark Keith wrote:
I'm *still* of the opinion that if you could measure the current INSIDE
the windings of the coil, a couple or three turns in from each end, you
would not see near the difference you all do.

Wes's and my modeling show a smooth current taper through the coil.
Here's the taper predicted by EZNEC through the coil for octcoil1.ez

http://www.qsl.net/w5dxp/taper1.gif

I do think it's quite normal to have a slight taper, ....

Then you disagree with the guys who say it has no taper. Guess what,
Mark? That puts you on my and Yuri's side of the argument.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil wrote,

Mark Keith wrote:
I'm *still* of the opinion that if you could measure the current INSIDE
the windings of the coil, a couple or three turns in from each end, you
would not see near the difference you all do.

Wes's and my modeling show a smooth current taper through the coil.
Here's the taper predicted by EZNEC through the coil for octcoil1.ez

http://www.qsl.net/w5dxp/taper1.gif

I do think it's quite normal to have a slight taper, ....

Then you disagree with the guys who say it has no taper. Guess what,
Mark? That puts you on my and Yuri's side of the argument.
--
73, Cecil http://www.qsl.net/w5dxp


This is an order of magnitude error. In the thirteenth century, the
scholastics would tell their pupils that, since the earth is a ball,
if you build two buildings side by side and use a plumb line to build
each one, the buildings will be farther apart at the top than at the bottom.
Also, if you have a swimming pool with a perfectly flat bottom, the
center will be deeper than the sides because it's closer to the center
of the earth.
No normal people paid any attention to these fellows. The scholastics never
specified
what things meant in terms of real numbers. Cecil says there's a taper but he
doesn't
attempt to say how much. Yuri thinks it's a lot. Cecil seems to agree
with him, but neither fellow has been willing to back his theory with a
$$$$ NEW $$$ [{### IMPROVED ###}] mobile antenna. I think any
rational observer would have to conclude that if Yuri and Cecil are right,
they're only right in the 13th cent. scholastic sense, and that their whole
argument doesn't amount to a hill of beans.
73,
Tom Donaly, KA6RUH

Cecil Moore wrote in message

I do think it's quite normal to have a slight taper, ....

Then you disagree with the guys who say it has no taper. Guess what,
Mark? That puts you on my and Yuri's side of the argument.

Maybe so, but I place much less importance on this than he does. To
me, it means very little, if anything. It surely will not effect how I
will design mobile antennas. After all, as Wes pointed out on his web
page, the change in radiation resistance is what really improves the
efficiency of a short loaded antenna. Not the current distribution in
itself. The improved current distibution is just a method used to
change the radiation resistance. Taper or no taper, in the usually
short 1 ft or so space a coil would occupy, "1/8 the length of a 8 ft
whip" the difference either way is not worth worrying about. And the
worrysome taper is only in the upper section of the coil, so really
it's less than 1/8 of the total antenna length. Most of the plots I
see are more bow shaped than a gradual taper due to the peak in
current. I'm glad Wes commented on the current peak...This was
something I had seen previously in modeling the antennas/coils a few
weeks ago, and had wondered about..
If we could do away with the high ground losses, we wouldn't need to
elevate the coils, or use hats. That's why often a base load 10-11m
vertical on a large car roof is about as good as a center load. Less
ground loss due to the better psuedo ground plane under the antenna.
MK

Mark Keith wrote:
"Start with a center load. Note the current taper. Then place the coil
below the midpoint level. Note the current taper."

Mark may want an indication of the better site on a too-short whip for a
loading coil. John Devoldere, ON4UN shows what he`s found in his book
"Low-Band DXing". On page 9-14 John says:
"It is clear now that the real issue with short verticals are EFFICIENCY
and BANDWIDTH. -----Therefore maximum attention must be paid to these
terms by:
Keeping the radiation resistance as high as possible
Keeping the losses of the loading devices as low as possible"

John gives radiation resistance formulas for base loading, top loading,
center loading, combined top and base loading, and linear loading.

John says on page 9-19:
"The rule for keeping the radiation resistance high is simple:
Use as long a vertical as possible (up to 90-degrees)
Use top loading

Low-Band DXing is worth checking out.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
John says on page 9-19:
"The rule for keeping the radiation resistance high is simple:
Use as long a vertical as possible (up to 90-degrees)
Use top loading

I arrived at the same conclusion many years ago by assuming the
current taper in a 75m loading coil would be maximum when the coil
was as far away from the feedpoint as possible. Using that assumption
and a few free parts from my junk box, my top loaded mobile equaled
the best of the best (and most expensive) at one of the CA 75m
shootouts. And that was with a pretty small capacitive top hat.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote in message

2. The second antenna is electrically 3/4 wavelength long. This one
illustrates how the current at the top of the loading coil can be
greater than the current at the bottom of the loading coil.

http://www.qsl.net/w5dxp/octcoil2.gif
http://www.qsl.net/w5dxp/octcoil2.ez

Why the 3/4 wave example? This is NOT what I had in mind. What I would
like to see, is you take a 8 ft mobile whip for 80m. Start with a
center load. Note the current taper. Then place the coil below the
midpoint level. Note the current taper. Then place the coil above the
midpoint level. Note the current taper.
You should be able to find a configuration that provides max current
at the top of the coil, if what you claim is true. I want to see max
current at the top of the coil in the SHORT loaded 8ft mobile whip for
80m. Not a funky overly tall 3/4 wave antenna. I'd try it myself, but
I only have the demo of eznec and don't have the segment capability.
Then if you can see this happen in the model, I'd like to see it
confirmed in the real world. If this occurs, then I might start to
think what you claim holds some water. I'll reserve further comment
until you can try this simple test. MK