"Cecil Moore" wrote in message
...
Roy Lewallen wrote:
Most of the available implementations of NEC-2 include the ability to
generate a helix. Look for information on a 'GH' command.

Some time ago, I generated an 8-sided coil for EZNEC 2.0. It was a
lot easier than I thought at first. Here's one turn of dia=13" at
about one turn per inch. Note 'y' always equals the x value from two
lines up.

x y z
.5, .2, 4.00
.2, .5, 4.01
-.2, .5, 4.02
-.5, .2, 4.03
-.5, -.2, 4.04
-.2, -.5, 4.05
.2, -.5, 4.06
.5, -.2, 4.07

--
73, Cecil http://www.qsl.net/w5dxp

Your helix is far more elegant than my octagonal structure, where each turn
is in the same plane, and a 90 deg segment then connects to the next turn.

I have just experimented with the GH card -- combined with the GM card. It
appears to be an excellent method of constructing a helix. I have not
seriously attempted to determine the optimal segmentation, but as long as
the segments are less than or equal to the turn spacing the results seem
acceptable -- as with parallel transmission line models. I have also
attempted to maintain the same segmentation on wires external to the helix,
and to use the same wire size. While these structures are interesting, from
the point of view of analyzing current distribution on an antenna, there
seems to be very little difference in the actual performance of an antenna
modeled with lumped element components. What I have learned (as mentioned
in a previous posting) is that it is possible to predict, with a fair degree
of accuracy, the actual inductance of a helix.

73,

Frank

"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

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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Knarf wrote:
While these structures are interesting, from
the point of view of analyzing current distribution on an antenna, there
seems to be very little difference in the actual performance of an antenna
modeled with lumped element components.

Don't let the logical diversions throw you. The argument is, and always
has been, about the current in a loading coil, not about the radiation
pattern. The radiation pattern is absolutely irrelevant to the argument.
--
73, Cecil http://www.qsl.net/w5dxp


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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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"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

Knarf wrote:
Sorry, this may sound dumb, I think I must have missed the point.

Doesn't sound dumb - you just missed the original argument. It
occurred over on eHam.net, was titled "In Search of 'The Perfect
Mobile Antenna'" and overflowed to this newsgroup. W8JI said there
is essentially no current change from end to end in a mobile loading
coil and used the lumped ideal dimensionless inductor from EZNEC to
"prove" his statement. K3BU took issue and the argument still rages.

The article on eHam.net is at:

http://www.eham.net/articles/5998

W8JI's take on the subject is on his web page at:

http://www.w8ji.com/mobile_and_loaded_antenna.htm

K3BU's take on the subject is on his web page at:

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

My take on the subject is on my web page at:

http://www.qsl.net/w5dxp/current.htm

Why are
people arguing about current in a loading coil? NEC, and experiment, seem
to provide the answer.

Lumped point inductances used by some models are dimensionless
and therefore have no current drop. Of course, lumped point
inductances don't exist in reality. The Helix feature of EZNEC+
illustrates the change in current through the coil. Based on
the quote from Balanis below, the total current through the
coil is If+Ib where 'If' is the forward current and 'Ib' is
the backward (reflected) current. That is, of course, phasor
addition. The net total current, If+Ib, for a 1/2WL dipole is
close to a cosine function with the center feedpoint at zero
degrees. So Itot = If+Ib = ~Ifeed*cos(degrees), where (degrees)
is the number of degrees away from the feedpoint of the dipole.
Halfway out one leg of a dipole the current is approximately
Ifeed*cos(45) = 0.707*Ifeed (Ifeed is feedpoint current)

The current at the tip of a dipole is Ifeed*cos(zero) = 0
Itot = If+Ib = 0 at the tip of the dipole because the two
currents, If and Ib, are equal in magnitude and opposite in
phase. (Note: the convention is that the current reflected at
an open-circuit changes phase by 180 degrees. Even though
the current is a phasor, only the Real part exists in reality.
Current in a wire has a magnitude and direction. There are
only two possible directions.)

The total current on the dipole is a standing wave with the
total current decreasing to zero at the tip of the dipole.
If and Ib are flowing in opposite directions and therefore
their phases are rotating in opposite directions. It is that
phase rotation difference that causes If+Ib to be different
at each end of a typical loaded mobile antenna. It's a pretty
simple concept and is explained on my web page above.

Incidentally, the feedpoint impedance of a 50 ohm dipole depends
upon the magnitude of the reflected voltage and reflected current
on the antenna. If a 1/2WL dipole were turned into a traveling-
wave antenna by terminating both ends, the feedpoint impedance
would be hundreds of ohms.
--
73, Cecil http://www.qsl.net/w5dxp
"The current and voltage distributions on open-ended wire antennas are
similar to the standing wave patterns on open-ended transmission lines ...
Standing wave antennas, such as the dipole, can be analyzed as traveling
wave antennas with waves propagating in opposite directions (forward and
backward) and represented by traveling wave currents If and Ib ..."
_Antenna_Theory_, Balanis, Second Edition, Chapter 10, page 488 & 489


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