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