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Old August 22nd 08, 10:49 PM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

MAYBE NOT EXACTLY THE RIGHT GROUP, but..

If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?

For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?

Any references or information on how to calculate this would be greatly
appreciated.
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Old August 22nd 08, 11:23 PM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

Jon Mcleod wrote:
MAYBE NOT EXACTLY THE RIGHT GROUP, but..

If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?

For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?

Any references or information on how to calculate this would be greatly
appreciated.


Is the sine wave applied and the load, if any, connected such that the
currents in the two wires are equal and opposite? If so, the problem
becomes exactly the same as a single wire suspended above and parallel
to an infinite perfect ground plane. In the plane exactly midway between
the wires -- the position represented by the perfect ground plane in the
simplified model -- the field is zero, since the fields from the two
wires are equal and opposite in that plane. If the currents aren't equal
and opposite, the problem becomes considerably more complex.

By "between the wires", do you mean in the plane of the wires, or some
larger region?

Are you looking for a closed-form solution, or would a numerical
analysis result be adequate?

If this is a homework assignment, the professor assigning it should be
able to steer you to some suitable references.

Roy Lewallen, W7EL
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Old August 23rd 08, 01:51 AM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

On Aug 22, 3:23 pm, Roy Lewallen wrote:
Jon Mcleod wrote:
MAYBE NOT EXACTLY THE RIGHT GROUP, but..


If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?


For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?


Any references or information on how to calculate this would be greatly
appreciated.


Is the sine wave applied and the load, if any, connected such that the
currents in the two wires are equal and opposite? If so, the problem
becomes exactly the same as a single wire suspended above and parallel
to an infinite perfect ground plane. In the plane exactly midway between
the wires -- the position represented by the perfect ground plane in the
simplified model -- the field is zero, since the fields from the two
wires are equal and opposite in that plane. If the currents aren't equal
and opposite, the problem becomes considerably more complex.

By "between the wires", do you mean in the plane of the wires, or some
larger region?

Are you looking for a closed-form solution, or would a numerical
analysis result be adequate?

If this is a homework assignment, the professor assigning it should be
able to steer you to some suitable references.

Roy Lewallen, W7EL


Hmmm...I don't think that's quite right, Roy. I expect the E field
between the wires, in the plane of the wires, to be parallel to the
plane, perpendicular to the wires, if there is a potential between the
wires and they are straight and parallel. For a wire above ground,
the E field must be perpendicular to the ground at the ground (assumed
perfect), but it's not zero in general.

Cheers,
Tom


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Old August 23rd 08, 05:23 AM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

Any references or information on how to calculate this would be greatly
appreciated.



It's called Classical Electrodynamics.

http://www.amazon.com/gp/reader/0471...pt#reader-link


"Jon Mcleod" wrote in message
m...
MAYBE NOT EXACTLY THE RIGHT GROUP, but..

If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?

For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?

Any references or information on how to calculate this would be greatly
appreciated.


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Old August 24th 08, 10:38 PM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires


"Jon Mcleod" wrote in message
m...
MAYBE NOT EXACTLY THE RIGHT GROUP, but..

If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?

For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?

Any references or information on how to calculate this would be greatly
appreciated.


if you are looking for a closed form solution you will need to investigate
some classical electrodynamics methods for wires on a dielectric half plane.
There are probably some not so elegant methods to calculate it using the
relative dielectric constants of the air on one side and the wall material
on the other. The field will of course be different in the wall material
than in the air. as a first approximation you can take an average of them
and use that to estimate the field as if the conductors were embedded in an
infinite block of dielectric material, that is probably a simpler problem.

there are programs that will do finite element analysis and come up with
numerical solutions, that is probably simpler in most cases than the raw
field equations.

For a relatively low frequency like 120khz and those low voltages you can
bound the problem by assuming 2 wires in air... 100v/5cm=20v/cm half way
between the wires... with a material on one side it will usually have a
lower field than in the air by approximately the ratio of the relative
dielectric constants. For wood the constant depends a lot on moisture
content, from a low of maybe 5 up to 100 for saturated wood. The higher the
dielectric constant the lower the field in the material, as a first
approximation with a constant of 5 you could guess that 80% of the field was
in the air and 20% in the wall... and of course the farther from the wires
the lower the field... so from a max of maybe 20v/cm between the wires the
field will decrease as you move away from the wall in air, and decrease
about 5x faster as you move into the wall. the hard part is close to the
wires, there it depends on the diameter of the wire, the smaller the
diameter the higher the field, to get those numbers you need one of the
other methods above.




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Old August 26th 08, 09:10 AM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

K7ITM wrote:
On Aug 22, 3:23 pm, Roy Lewallen wrote:
Is the sine wave applied and the load, if any, connected such that the
currents in the two wires are equal and opposite? If so, the problem
becomes exactly the same as a single wire suspended above and parallel
to an infinite perfect ground plane. In the plane exactly midway between
the wires -- the position represented by the perfect ground plane in the
simplified model -- the field is zero, since the fields from the two
wires are equal and opposite in that plane. If the currents aren't equal
and opposite, the problem becomes considerably more complex.
. . .
Roy Lewallen, W7EL


Hmmm...I don't think that's quite right, Roy. I expect the E field
between the wires, in the plane of the wires, to be parallel to the
plane, perpendicular to the wires, if there is a potential between the
wires and they are straight and parallel. For a wire above ground,
the E field must be perpendicular to the ground at the ground (assumed
perfect), but it's not zero in general.

Cheers,
Tom


Tom is completely right -- as always. The problem is, as I said, the
same as a single wire suspended above a perfect conducting plane. The E
field is perpendicular to the conducting plane (that is, in the plane
containing the wires as Tom said) and not zero as I incorrectly said.
Thanks for the correction, Tom, and my apology to all for the error.

Roy Lewallen, W7EL
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Old August 26th 08, 07:22 PM posted to rec.radio.amateur.antenna
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Default E-Field between 2 parallel wires

On Aug 22, 2:49 pm, Jon Mcleod wrote:
MAYBE NOT EXACTLY THE RIGHT GROUP, but..

If I tape 2 insulated, parallel wires to the wall, x cm apart, and then
drive a sinewave into them (Vo p-p), how can I calculate the field
strength between the 2 wires?

For instance, 120KHz, 100V, 5cm apart, what is the field in V/cm between
the two wires?

Any references or information on how to calculate this would be greatly
appreciated.


I'm tempted to make some statements that I can't quite prove without
doing a bit of math, but I'll refrain...

But I will recommend the program ATLC ("Arbitrary Transmission Line
Calculator"), which does calculate the electric and magnetic fields
numerically.

Except for changes in the current distribution because of changes in
skin depth versus frequency and changes the dielectric versus
frequency, I don't think there's any effect of frequency on the fields
distribution for a TEM-type line, where the current in the two
conductors is equal amplitude but opposite direction and the line is
in a uniform dielectric. Taping it to a wall will make is "quasi-TEM"
but you can expect the electric field direction to be very close to
being in a plane perpendicular to the axis of the wires.

Cheers,
Tom
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