"Cecil Moore" wrote in message
...
Knarf wrote:
Could not get it to work. Below is my NEC2 code ( heavily segmented as
it is easier to figure out where to place the source and load). What am
I doing wrong? Resonant on 7.25 MHz before extending the antenna.
Approaches resonance on 22 MHz, when extended, with very high series L.
A 40 m dipole is resonant on the 3rd harmonic anyway so all you are doing
is isolating the extra 1/4 wave by placing a very high impedance at the
quarter wave point. I guess I missed something, be interested in any
comments.

The lumped point inductance will definitely not give the correct results.
The coil needs to use the helix feature of EZNEC+ 4.0. I constructed an
8-sided coil out of segments in EZNEC 2.0 and the results can be viewed
at:

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

The dimensions and inductance are not perfect but the idea is perfectly
clear. There's 0.11 amp at the bottom of the coil and 0.57 amps at the
top of the coil. By playing with lengths of wire and frequency, I've seen
the current at the bottom of the coil as low as 0.005 amps while the
current at the top of the coil was about 0.6 amps.
--
73, Cecil http://www.qsl.net/w5dxp


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Sorry that was really dumb, did not think before I posted the response. I
was certainly aware that the currents in, and out, of a lumped element
inductor are the same. I had recently modeled a short monopole with a
physical, octagonal, helix (Now I learn about the GH card!). The difference
between the lumped element and distributed inductor is significant, although
the gains are almost identical from both models.

73,

Frank

On Tue, 26 Oct 2004 11:51:10 GMT, "Knarf"
wrote:

The difference
between the lumped element and distributed inductor is significant, although
the gains are almost identical from both models.

Hi Frank,

You've hit the nail on the head (although I've seen it claimed it
makes a 12dB difference!).

Rarely do we get any practical correlation from this "sky is falling"
oops "current is dropping" argument.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 26 Oct 2004 11:51:10 GMT, "Knarf"
wrote:

The difference
between the lumped element and distributed inductor is significant,
although
the gains are almost identical from both models.

Hi Frank,

You've hit the nail on the head (although I've seen it claimed it
makes a 12dB difference!).

Rarely do we get any practical correlation from this "sky is falling"
oops "current is dropping" argument.

73's
Richard Clark, KB7QHC

Hi Richard,

Cannot see where anybody could get a12 dB difference, since you can only
model a lumped element inductance, you could not build it to test the
performance. I have spent many hours this past week modeling short, loaded,
monopoles, over a perfect ground -- triggered by the previous thread -- just
to see what results I could get. An 86.5" vertical, center loaded, with a
lumped element inductor resonating in the 21 MHz range, exhibits an input
impedance of 20.91 Ohms, and a maximum gain of +4.754 dBi. The same antenna
with a distributed 12 turn helix, of 2.5" diameter, and 6" long, has an
input impedance of 18.98 Ohms, and a gain of +4.783 dBi. The helix alone
has a gain of -25 dBi. Transcribing the NEC output file to an Excel spread
sheet produces some very interesting current plots.

73,

Frank

Knarf wrote:
Cannot see where anybody could get a12 dB difference, ...

Unless I missed something, no one ever asserted a 12 dB difference.
Such is just a logical diversion away from the "current through the
coil" issue.
--
73, Cecil http://www.qsl.net/w5dxp


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Richard Clark mentioned that some people claim a 12 dB difference between
lumped element and distributed loading.

73,

Frank.


"Cecil Moore" wrote in message
...
Knarf wrote:
Cannot see where anybody could get a12 dB difference, ...

Unless I missed something, no one ever asserted a 12 dB difference.
Such is just a logical diversion away from the "current through the
coil" issue.
--
73, Cecil http://www.qsl.net/w5dxp


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Knarf wrote:
Richard Clark mentioned that some people claim a 12 dB difference between
lumped element and distributed loading.

Ask him to please produce the posting. A 100% difference between
lumped element and distributed loading wouldn't produce much of
a difference in the radiation pattern. Sorry Reg, most of the
radiation happens below the coil, whether lumped or distributed.
--
73, Cecil http://www.qsl.net/w5dxp


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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.

Nice attempt at changing the subject - didn't work.

In the process of learning why the superposed current is not constant
through a loading coil in a standing-wave antenna, you will also learn
something about standing-wave antennas in general.
--
73, Cecil http://www.qsl.net/w5dxp


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"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