Hi,

Could someone please provide a correct near field equation that a
computer programmer can understand? I'm using the following equation
in a simple simulation but it's not working in all cases.

E = a * sin(theta) * q / ( 4 * pi * e * c^2 * r )

The computer program uses the above equation to create tiny segments
of oscillating charge. The program simulates a wire carrying current
and a ferrite material. The way I am simulating the ferrite material
is by placing small loop currents.

According to the above equation, the near field from an infinitely
long wire should not change with distance. That is, the near field at
1 inch should be the same as 10 inches. This also agrees with the
near field results of an antenna-designing program. Now if that's
true, then how can an infinitely long wire generate a net voltage
across a closed circuit? If the electric field is uniform then the
net induced voltage around a closed circuit that is near the
infinitely long wire should be zero. But experimental results show
that to be completely false. OK, I couldn't find an infinitely long
wire, but a pretty long one. I entered the exact dimensions of my
experiment and the simulation results were completely off from the
experimental results. In fact, there was significant induced voltage
from a very long wire.

Can any one please shed some light on my error? BTW, the simulation
successfully predicted several other experiments when the theta angle
was zero. Obviously the theta angle goes from zero to 180 degrees in
an infinitely long wire. Am I using the wrong equation?

Any help is greatly appreciated,
CJ

On Fri, 12 Nov 2004 19:07:34 +0000 (UTC),
(curiousjohn4) wrote:
In fact, there was significant induced voltage from a very long wire.

Hi OM,

You are treading water here. There is more than radiation at work
very close to an antenna. Consider your own language above: Induction
(or even capacitive coupling).

73's
Richard Clark, KB7QHC

curiousjohn4 wrote:


Could someone please provide a correct near field equation that a
computer programmer can understand? I'm using the following equation
in a simple simulation but it's not working in all cases.

E = a * sin(theta) * q / ( 4 * pi * e * c^2 * r )



I don't know where you got that equation, but I'm not surprised it's not
giving the right answer. In the near field, there are components
inversely proportional to r, r^2, and r^3. The equations are too complex
to easily write in ASCII, and the terms require some definition. But you
can find the relevant equations in Jordan & Balmain, _Electromagnetic
Waves and Radiating Systems_, eq. 10-41 and 10-42; Kraus,
_Electromagnetics_, eq. 13-49 and 13-50; Johnk, _Engineering
Electromagnetic Fields and Waves_, eq. 11-41a and 11-41b; or most other
electromagnetics and some antenna texts. Note that two equations are
given, one for the radial component and another for the circular (theta)
component, which decay at different rates as the distance from the
current element increases.

Roy Lewallen, W7EL

"Frank" wrote in message news:...
Funny, posted this earlier, but did not seem to take.

"Frank" wrote in message news:...
"Richard Clark" wrote in message
...
On Fri, 12 Nov 2004 19:07:34 +0000 (UTC),
(curiousjohn4) wrote:
In fact, there was significant induced voltage from a very long wire.

Hi OM,

You are treading water here. There is more than radiation at work
very close to an antenna. Consider your own language above: Induction
(or even capacitive coupling).

73's
Richard Clark, KB7QHC

I think Richard has hit the nail on the head. I wish calculation of
electromagnetic fields could be reduced to a simple equation. Any
undergraduate text on electromagnetics barely scratches the surface. I
suggest you check the site at www.nec2.org and download the .pdf version
of the "NEC Program Description -- Theory". Gives an overview of "Method
of Moments". As a foundation Kraus' Electromagnetics may help, and is
available used (and cheap) at www.bn.com. Also at the same site a
reprint of "Field Computation by Moment Methods" by Roger F. Harrington
can be obtained for $33.50. Then there is the "Finite Element" method as
used by "Ansoft's" HFSS".


Regards,

Frank

"Richard Clark" wrote in message
...
On Fri, 12 Nov 2004 19:07:34 +0000 (UTC),
(curiousjohn4) wrote:
In fact, there was significant induced voltage from a very long wire.

Hi OM,

You are treading water here. There is more than radiation at work
very close to an antenna. Consider your own language above: Induction
(or even capacitive coupling).

73's
Richard Clark, KB7QHC

I think Richard has hit the nail on the head. I wish calculation of
electromagnetic fields could be reduced to a simple equation. Any
undergraduate text on electromagnetics barely scratches the surface. I
suggest you check the site at www.nec2.org and download the .pdf version of
the "NEC Program Description -- Theory". Gives an overview of "Method of
Moments". As a foundation Kraus' Electromagnetics may help, and is
available used (and cheap) at www.bn.com. Also at the same site a reprint
of "Field Computation by Moment Methods" by Roger F. Harrington can be
obtained for $33.50. Then there is the "Finite Element" method as used by
"Ansoft's" HFSS".


Regards,

Frank

Curious John wrote:
"Can someone please provide a correct near field equation that I can
understand?"

The relation between the current in a wire and the magnetic field
intensity (H) around it at any distance (r) is:

H = 2I / r
I is the current in a long straight wire.

The magnetomotive force around a complete contour around a wire is: 4Pi
(I)

If the current is changing, the magnetic field is likewise changing. An
electric field is induced in the space around the wire. The voltage
induced in a wire properly immersed in the field is:

-d Phi /dt

The induced voltage is proportional to the rate of change of the
magnetic flux and is negative, that is it opposes the changing flux.

When an electric field is imposed on a dielectric, it places an
electrical strain in the dielectric. This causes a dielectric
displacement current (D):

D = dielectric constant x volts/m of the electric field. So D is
equivalent to an electric flux density. Displacement current is greater
in materials with larger values of dielectric constant. The value for
air or space is one.

Best regards, Richard Harrison, KB5WZI

I think Richard has hit the nail on the head. I wish calculation of
electromagnetic fields could be reduced to a simple equation. Any
undergraduate text on electromagnetics barely scratches the surface. I
suggest you check the site at www.nec2.org and download the .pdf version
of the "NEC Program Description -- Theory". Gives an overview of "Method
of Moments". As a foundation Kraus' Electromagnetics may help, and is
available used (and cheap) at www.bn.com. Also at the same site a
reprint of "Field Computation by Moment Methods" by Roger F. Harrington
can be obtained for $33.50. Then there is the "Finite Element" method as
used by "Ansoft's" HFSS".


Regards,

Frank





Allow me post a couple of formulae ?? formulas ??
How about equations?

Balanis 2 Ed. p 32-33
Reactive near field - .62 * A
where A = Square Root of (D^3)/lambda
D= length of a Hertzian dipole (both elements)
lambda is the wavelength of a transmitting frequency
radiating near field - 2*(D^2)/lambda
Outside of that is the far field.
This is area that you seem concerned about.
Krauss uses Reactive near field as inside lamda/(2 * pie)

Do you work for a group of physicists? I have encountered the
field equations from a physics text by Young and Friedman.


Dave

"onyx" wrote in message
. ..
I think Richard..........."Ansoft's" HFSS".
Allow me post a couple of formulae ?? formulas ??
How about equations?

Balanis 2 Ed. p 32-33
Reactive near field - .62 * A
where A = Square Root of (D^3)/lambda
D= length of a Hertzian dipole (both elements)
lambda is the wavelength of a transmitting frequency
radiating near field - 2*(D^2)/lambda
Outside of that is the far field.
This is area that you seem concerned about.
Krauss uses Reactive near field as inside lamda/(2 * pie)

Do you work for a group of physicists? I have encountered the
field equations from a physics text by Young and Friedman.


Dave

Hi Dave:

By inspection, both of your equations show that the units of the solution
will be in meters. E field strength units are in V/m, and in the near
field, will be complex numbers. From memory, I believe your expression for
the near field/far field transition is correct. I do not have my own copy
of Balanis, but will check it when I get a chance.

Even for a non-realizable "Hertzian dipole", where the current is assumed to
be constant along its entire length, the math is fairly involved. Not sure
if a physics text is the best source for antenna studies. By what I have
read; Young and Friedman's texts have not had very good reviews -- by
students anyway!

No, I do not work for physicists. Most of the time I have worked in EE
labs. Electromagnetics is more of a hobby for me. So far I have been using
undergraduate texts, such as Paul and Nasar's "Introduction to
Electromagnetic fields". The problem with this book is that there are no
answers in the back.

Frank