An easy experiment with a coil
 

The results of the measurement of current at the bottom and top of the coil
in an off-center-fed dipole configuration should leave no room for doubt.

------1/4WL------FP------1/4WL------

Start out with a 1/2WL dipole. Measure the resonant fundamental frequency.

Then, to one end, add a coil and another 1/4WL of wire

------1/4WL------FP------1/4WL------//////------1/4WL------

Adjust the coil inductance so the antenna is resonant on three times
the fundamental frequency of the original dipole.

Measure the current at the left(bottom) of the coil and measure the
current at the right(top) of the coil.

The current at the left of the coil will be nearly zero. The current
at the right of the coil will be approximately the same as the feedpoint
current, somewhere around 1-2 amps for 100w input. This can probably be
demonstrated using the helix feature of EZNEC.
--
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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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.

Regards,

Frank

Code:

CM free space dipole antenna
CE
GW 1 99 -33 0 0 66 0 0 0.0026706
GS 0 0 .3048
GE 0
EX 0 1 33 0 1 0
LD 4 1 67 67 1200 0
LD 5 1 1 99 5.8001E7
FR 0 31 0 0 20 0.1
RP 0 181 1 1000 -90 90 1 1
EN

"Cecil Moore" wrote in message
...
The results of the measurement of current at the bottom and top of the
coil
in an off-center-fed dipole configuration should leave no room for doubt.

------1/4WL------FP------1/4WL------

Start out with a 1/2WL dipole. Measure the resonant fundamental frequency.

Then, to one end, add a coil and another 1/4WL of wire

------1/4WL------FP------1/4WL------//////------1/4WL------

Adjust the coil inductance so the antenna is resonant on three times
the fundamental frequency of the original dipole.

Measure the current at the left(bottom) of the coil and measure the
current at the right(top) of the coil.

The current at the left of the coil will be nearly zero. The current
at the right of the coil will be approximately the same as the feedpoint
current, somewhere around 1-2 amps for 100w input. This can probably be
demonstrated using the helix feature of EZNEC.
--
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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PS should I have used an actual helix? Very tedious to structure with code,
although have done it in other models. Noticed that the current in and out
of the coil is vastly different when compared with using the lumped element
model.

Frank


"Knarf" wrote in message
news:Z4jfd.743$VA5.640@clgrps13...
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.

Regards,

Frank

Code:

CM free space dipole antenna
CE
GW 1 99 -33 0 0 66 0 0 0.0026706
GS 0 0 .3048
GE 0
EX 0 1 33 0 1 0
LD 4 1 67 67 1200 0
LD 5 1 1 99 5.8001E7
FR 0 31 0 0 20 0.1
RP 0 181 1 1000 -90 90 1 1
EN

"Cecil Moore" wrote in message
...
The results of the measurement of current at the bottom and top of the
coil
in an off-center-fed dipole configuration should leave no room for doubt.

------1/4WL------FP------1/4WL------

Start out with a 1/2WL dipole. Measure the resonant fundamental
frequency.

Then, to one end, add a coil and another 1/4WL of wire

------1/4WL------FP------1/4WL------//////------1/4WL------

Adjust the coil inductance so the antenna is resonant on three times
the fundamental frequency of the original dipole.

Measure the current at the left(bottom) of the coil and measure the
current at the right(top) of the coil.

The current at the left of the coil will be nearly zero. The current
at the right of the coil will be approximately the same as the feedpoint
current, somewhere around 1-2 amps for 100w input. This can probably be
demonstrated using the helix feature of EZNEC.
--
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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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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Knarf wrote:
PS should I have used an actual helix? Very tedious to structure with code,
although have done it in other models. Noticed that the current in and out
of the coil is vastly different when compared with using the lumped element
model.

Yes, use an actual helix. If it will help, download octcoil2.ez from my
web page by clicking he

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

For this particular case:

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

a lumped point inductance gives bogus results.
--
73, Cecil http://www.qsl.net/w5dxp


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Knarf wrote:

PS should I have used an actual helix? Very tedious to structure with code,
although have done it in other models. Noticed that the current in and out
of the coil is vastly different when compared with using the lumped element
model.

Most of the available implementations of NEC-2 include the ability to
generate a helix. Look for information on a 'GH' command.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Knarf wrote:

PS should I have used an actual helix? Very tedious to structure with
code, although have done it in other models. Noticed that the current in
and out of the coil is vastly different when compared with using the
lumped element model.

Most of the available implementations of NEC-2 include the ability to
generate a helix. Look for information on a 'GH' command.

Roy Lewallen, W7EL

Thanks for the info. Should have studied the NEC 2 manual more carefully.
It is on page 20 with no card picture.

Regards,

Frank

"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

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


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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

"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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Knarf wrote:
"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



Hi Frank,
you're right, there shouldn't be any argument. There is more
than one way to make a loading coil. If you will visit Tom Rauch's web
page, you will see that he believes he has a method for making a
superior loading coil for small, mobile antennas. Some of the characters
on this newsgroup have misrepresented what he wrote to assert that he
believes that all loading coils behave as an ideal inductance where
the current at both ends of the inductor are the same. Read what Tom
actually wrote and then go back and look at what these people said he
wrote and you'll see the two aren't the same.
73,
Tom Donaly, KA6RUH

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

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

Hi Frank,
you're right, there shouldn't be any argument. There is more
than one way to make a loading coil. If you will visit Tom Rauch's web
page, you will see that he believes he has a method for making a superior
loading coil for small, mobile antennas. Some of the characters
on this newsgroup have misrepresented what he wrote to assert that he
believes that all loading coils behave as an ideal inductance where
the current at both ends of the inductor are the same. Read what Tom
actually wrote and then go back and look at what these people said he
wrote and you'll see the two aren't the same.
73,
Tom Donaly, KA6RUH

Thanks Tom, will read the links so will better understand what the arguments
are about.

73,

Frank

Tom Donaly wrote:
you're right, there shouldn't be any argument. There is more
than one way to make a loading coil. If you will visit Tom Rauch's web
page, you will see that he believes he has a method for making a
superior loading coil for small, mobile antennas. Some of the characters
on this newsgroup have misrepresented what he wrote to assert that he
believes that all loading coils behave as an ideal inductance where
the current at both ends of the inductor are the same. Read what Tom
actually wrote and then go back and look at what these people said he
wrote and you'll see the two aren't the same.

From: http://www.eham.net/articles/5998
In Search of 'The Perfect Mobile Antenna':
Reply by W8JI on August 10, 2003

"If you look at HOW an inductor works, the current flowing in one terminal
ALWAYS equals the current flowing out the other terminal. THE VOLTAGE can be
(and is) different on each end of the inductor, NOT the current."

Would you please share with us what that posting really means?
--
73, Cecil http://www.qsl.net/w5dxp


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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

Roy Lewallen wrote:

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.

The technical information published on this particular subject is
non-existent. Therefore, there is nothing published that contradicts
what I am saying. Why do you think that gives you an advantage?
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:
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.

I didn't mean to imply that I don't know the equations - I do. I just
don't know the value of all the constants in the equations.

Given that a horizontal dipole 24 ft. above ground and constructed from
#16 wire will have a natural Z0 of 600 ohms:

The forward current will be an If-max value multiplied by an exponential
relating to frequency multiplied by an exponential relating to the loss
of energy due to conductor resistance and radiation.

The reflected current will be an Ir-max value multiplied by the same
exponential relating to frequency multiplied by the same exponential
relating to the loss of energy due to conductor resistance and radiation.

We know that (Vf+Vr)/(If+Ir) equals 50 ohms for a dipole whose feedpoint
impedance is 50 ohms. With a 1/2WL dipole the current equation is clear.

Here is the equation you asked for, unfortunately in ASCII:

Itot = If-max*e^-yz*e^-2az + Ir-max*e^+yz*e^-2az

same as it is for a transmission line. The I^2*R losses plus the
radiation "losses" are combined into the attenuation factor 'a'.

So I can indeed write you an equation for a wire dipole. The coil
in the mobile antenna causes another level of complication, and
that is the equation with which I am struggling at the moment.
In addition, the vertical nature of a mobile antenna means that
the Z0 is changing with length. That is a minor problem compared
to including the reflections from both ends of a loading coil in
both directions. But I have no doubt that I can solve that problem.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
. . .
Here is the equation you asked for, unfortunately in ASCII:

Itot = If-max*e^-yz*e^-2az + Ir-max*e^+yz*e^-2az

same as it is for a transmission line. The I^2*R losses plus the
radiation "losses" are combined into the attenuation factor 'a'.
. . .

No, that isn't the equation I asked for. Nowhere in your equation are
L1, L2, L3, L, I, or F. Whatever your equation is supposedly solving
for, it isn't what I asked.

I feel strongly that if you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations. I haven't seen any evidence of this.

Roy Lewallen, W7EL

If you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations.

Roy Lewallen, W7EL

-----------------------------------------------------

By far, the most sensible statement yet made in these interminable 'coil'
threads.

No need to quote Kelvin.

Maths comes first - THEN the arguments if there are any.
----
Reg.

Roy Lewallen wrote:
I feel strongly that if you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations. I haven't seen any evidence of this.

Well, You're right. I should be able to express it as an equation. Truth
is, personality wise, I tend to deal in concepts, not equations. That's
why the field of digital electronics was so appealing to me. "If it's
not a zero or a one, it's broke!" I seem to have been born with a Boolean
Algebra processor built in. (It's similar to the fact that I can read
Spanish but I can't speak it.)

I have been satisfied all my life to let someone else provide the equations
and so far, I have been able to stand on the shoulders of giants. But in this
case, if anyone has ever provided the equations, I am not aware of it. If
one is so inclined, one might get to be famous by generating those equations.

The S-parameter equations should work just fine at each individual impedance
discontinuity. The trick is in knowing how much to reduce the incident and
reflected voltages because of radiation. It's one approach to think about.

One possible solution would be to model the antenna as a transmission line,
as Balanis and Kraus suggest. If we made a 1/4WL open-circuit stub out of
resistance wire with Z0=600 ohms such that it's feedpoint impedance is 50
ohms, it should be a good approximation to an antenna wire.

In any case, the loading coil has a steady-state forward current (If) and
reflected current (Ib) each of which undergo a phase shift through the coil.
Any phase shift in phasors traveling in opposite directions is cause for
their sums to be different at each end of the coil. I'm surprised that such
a concept is controversial. Exactly the same concept applies to 'X' degrees
of a transmission line with reflections.
--
73, Cecil http://www.qsl.net/w5dxp


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Reg Edwards wrote:

If you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations.

By far, the most sensible statement yet made in these interminable 'coil'
threads.

Well, I have already posted the equations. During steady-state, there will
be some magnitude of forward current (If) through the coil and some magnitude
of reflected current (Ib) flowing backwards through the coil caused by
reflections from the tip of the standing-wave antenna. The net current at
any position up and down the antenna is Ir+Ib. That's a pretty simple
equation. Since there is a phase shift in Ir and Ib through the large
bugcatcher coil, the sum of Ir and Ib will not be the same at each end.

In an electrical 1/4WL antenna, like a typical 75m mobile antenna with
no top hat, the If+Ib sum at the bottom will always be a larger magnitude
than the If+Ib sum at the top. Modeling the coil as an eight-sided helix
in EZNEC resulted in:

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

The net current at the bottom of the coil is obviously of a greater
magnitude than the net current at the top of the coil. That's because
If and Ib at the bottom of the coil are nearly in phase. Ir and Ib
are about 58 degrees different in phase at the top of the coil. That
tells me that the coil causes about a 29 degree phase shift when either
Ir or Ib flows through it. It's not that simple but let's stick with
simple for right now.
--
73, Cecil http://www.qsl.net/w5dxp


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Perhaps it's your background as a manager that's in evidence here. I was
one of the people that "concept" people like you "let" provide the
equations. I have an apropos Doonesbury cartoon on my office wall:

Pointy-haired boss, pointing to simple graph labeled "Sales": "Sales are
dropping like a rock."

Pointy-haired boss, pointing to a graph labeled "Future", with single
upward line: "Our plan is to invent some sort of doohickey that everyone
wants to buy."

Pointy-haired boss, to Dilbert: "The visionary leadership work is done.
How long will your part take?"

At review time, the boss judged me on whether the "doohickey" worked
according to specifications that he and some marketing people came up
with in "concept" meetings. All I had to do was to understand the
science, come up with the equations, develop new technology as required,
create the device, and make it work -- all on a schedule and within a
budget which were also dictated by the "visionaries". Just grunt work,
not worthy of the visionary people who were doing more important things.

I don't believe for a minute that the boss really understood how the
device worked. And for the same reason, I don't believe that you really
understand the "concepts" you're promoting. Real science and engineering
are done by understanding the basic concepts, developing mathematical
models of them based on those understandings, then using those models to
test the theory. Without the understanding and models, the theories
can't be tested. Then all you have are smoke, mirrors, and hand waving.
Instead of solid, testable evidence, you just about have to resort to
diversion, evasion, misinterpretation, and the other tools of the
politician and upper level manager. As important and richly rewarded as
those skills are, it isn't science and it isn't engineering, and it
doesn't constitute evidence of any knowledge or understanding.

You seem to have convinced a few readers of the group that you know what
you're talking about. Since you're apparently not able to express your
ideas in concrete form, perhaps one of them will volunteer to do the
mundane work of developing a coherent theory to explain it in scientific
and mathematical terms. My only question to them is:

How long will your part take?

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

I feel strongly that if you really understand what you're talking
about, you should be able to express it mathematically as an equation
or equations. I haven't seen any evidence of this.


Well, You're right. I should be able to express it as an equation. Truth
is, personality wise, I tend to deal in concepts, not equations. That's
why the field of digital electronics was so appealing to me. "If it's
not a zero or a one, it's broke!" I seem to have been born with a Boolean
Algebra processor built in. (It's similar to the fact that I can read
Spanish but I can't speak it.)

I have been satisfied all my life to let someone else provide the equations
and so far, I have been able to stand on the shoulders of giants. But in
this
case, if anyone has ever provided the equations, I am not aware of it. If
one is so inclined, one might get to be famous by generating those
equations.
. . .

Cecil Moore wrote:

Reg Edwards wrote:

If you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations.


By far, the most sensible statement yet made in these interminable 'coil'
threads.


Well, I have already posted the equations. . .

Well, no you haven't. Why do you keep saying that? I asked a simple
question, and instead of answering it, you've answered some other
question that you prefer, then claim it's the answer to the question I
asked.

I took only one political science course in college, and that was the
technique used by the majority of students. It's also the universal
technique of political candidates. I'm asking techical and engineering
questions, and you're giving political answers. No wonder we don't
communicate.

This isn't a game I like to play and, since leaving the cube farm, it's
one that I no longer have to. So I'll bow out again and leave it to the
folks who do enjoy this form of discourse.

Roy Lewallen, W7EL

Roy Lewallen wrote:
I feel strongly that if you really understand what you're talking about,
you should be able to express it mathematically as an equation or
equations. I haven't seen any evidence of this.

Taking a look at just the loading coil, the math is very simple.
Since a loaded mobile antenna is a standing wave antenna, there
exists a forward current through the coil. Balanis labels that
current as 'If'. There also exists a reflected current flowing
backwards through the coil. Balanis labels that current as 'Ib'.
The current at any point in the coil is the superposed sum of
If + Ib. The net current within the coil is approximately a section
of a cosine function.

On my web page is a mobile antenna modeled with EZNEC using a
segmented octal shaped coil.

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

EZNEC reports that the current at the bottom of the coil is
0.9956 amps and the current at the top of the coil is 0.5326.
Assuming the current is approximately a cosine function through
the coil, the approximate number of degrees occupied by the coil is:

arccos(0.5326/0.9956) = ~58 degrees

The other eight feet of antenna is equal to about 42 degrees
at 13.5 MHz.

58 + 42 = 100 degrees is within 10% of being 90 degrees and
seems reasonable for a ballpark estimate. This seems to be
a clue that a loaded mobile antenna is electrically slightly
longer than 90 degrees.

The forward current and reflected current magnitudes through the
coil are probably within 90% of each other for a mobile antenna.
Kraus considers them to be equal for a dipole discussion
in his book. Considering them to be equal introduces a small error
but is good enough for a reasonable estimate.

The point is that the difference in the net current at each
end of a coil gives us a rough estimate of the number of electrical
degrees that the coil occupies in the antenna. The whip above the
coil can be considered as a load on the source energy at the top
of the coil. It is unlikely that the coil is capable of changing
the characteristics of the load which would be necessary if Tom
were right.

If the coil occupies zero degrees, as implied by Tom, W8JI, then
the other 8 feet of the antenna would have to occupy 90 degrees
to make the feedpoint impedance resistive. I would say that shoving
90 degrees into 8 feet of whip at 13.5 MHz is impossible. That 8
feet of antenna would have to have a velocity factor of 0.46 which
is unlikely. At 4 MHz, that 8 feet of antenna would have to have a
velocity factor of 0.137 to occupy 90 degrees. That's seems obviously
impossible. Tom's requirement that a wire change velocity factor with
frequency is really hard to accept.
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:
You seem to have convinced a few readers of the group that you know what
you're talking about. Since you're apparently not able to express your
ideas in concrete form, perhaps one of them will volunteer to do the
mundane work of developing a coherent theory to explain it in scientific
and mathematical terms.

You must have missed some of my postings. I have already expressed my
ideas in concrete form. Maybe you don't like the simplicity of

Itot = I+ + I- (I+ is forward current and I- is reflected current)

Standing wave antennas possess standing waves. Standing waves occur
when forward waves and reflected waves are superposed. The phase
rotation of these two component currents are in opposite directions.
The result is a sinusoidal function for both net voltage and net current.

Any real-world air-core coil has a phase delay that affects the forward
current and the reflected current. Since they are phase rotating in
opposite directions, the overall effect is doubled. It's all explained
on my web page. Have you taken time to read it?
--
73, Cecil http://www.qsl.net/w5dxp


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Roy Lewallen wrote:

Cecil Moore wrote:
Well, I have already posted the equations. . .

Well, no you haven't. Why do you keep saying that?

Because it's true. Here are the equations again.

A loading coil exists in a standing-wave antenna.
The forward current through the loading coil is I+.
The reflected current through the loading coil is I-.
The net current at any point in the coil is I+ + I- (phasor addition)
The magnitude of the net current depends upon the phase of I+ and I-.

Itot = I+ + I-

There's the equation that I have already posted many, many times.
Sorry you missed it.

A rough estimate of the net current at the top of a loading coil
can be had by estimating the number of degrees occupied by the
coil and assuming a net current cosine function through the coil.
--
73, Cecil http://www.qsl.net/w5dxp


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