RadioBanter - Electromagnetic wave question

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Electromagnetic wave question

In the electric power industry there is increasing public concern regarding
fields around power lines. The general public often refers to these fields
as electromagnetic radiation, which it is not. We generally are concerned
with magnetic fields and less often with the electric charge fields. The two
are treated as separate issues.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important electromagnetic
wave?

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Electromagnetic wave question

On 9 jun, 11:01, "Peter" wrote:
In the electric power industry there is increasing public concern regarding
fields around power lines. The general public often refers to these fields
as electromagnetic radiation, which it is not. We generally are concerned
with magnetic fields and less often with the electric charge fields. The two
are treated as separate issues.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important electromagnetic
wave?

Peter *VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Hello Peter,

The point where the fields merge to the EM wave depends on various
things.

There are three field zones used within the antenna community:

1. The far field zone (Fraunhofer region), where there is a true
spherical wave behavior, and the radiation pattern of the antenna is
independent of the distance to the antenna. The antenna can be treated
as a true point source emitting EM waves. The far field zone is mostly
taken r 2*b^2/lambda, where b is the largest size of the antenna.
This formula is very conservative. Depending on the current or
aperture distribution, the far field can start from r 0.5*b^2/
lambda. E- and H- field is proportional with 1/r (r=distance).

2. The transition field zone (Fresnel region), where the fields have a
reasonable wave character, but are not spherical. The radiation
pattern of the antenna depends on the measuring distance. This one is
tricky. An antenna with a null in the far field pattern may show
significant radiation (in same direction) in the transition field. So
within the transition field, you cannot for sure assess field strength
levels based on the far field radiation pattern. Also the main lobe
in the far field pattern may not be the main lobe in the radiation
pattern measured at distance far below the far field distance. You can
compare that with coherent light passing through a round aperture, you
get an onion shaped ring pattern on the wall (airy disk). You can even
get a black hole in the center at certain distance. So here field is
not proportional with 1/r.

3. The reactive field zone. This is the region where you can calculate
the fields using electrostatics and induction. In many cases, fields
are 90 degrees out of (time) phase. r0.16*lambda is used frequently.
Of course, there is no hard distance where the reactive field zone
stops and the transition field starts. The decay of the H and E field
are different depending on the antenna and the orientation of the
antenna.

For small loop antennas, r diameter AND r0.16*lambda, H field decay
is proportional with 1/r^3.

Another example is a HW dipole. It does not radiate on the axis of the
dipole. But when you are at (for example) 0.5*lambda from the ends
(but on the axis), you will sure measure an E-field (because of the
reactive fields that decay with r^3 to r^2).

Another example is the field between a wide parallel strip
transmission line that is well terminated (strips face each other)….
Both E and H are virtually homogenous between the strips, are in time
phase and are spatially 90 degrees out of phase. So there is a plane
EM wave traveling between the strips (no reactive issues).

The situation becomes more complicated when interaction with other
materials is present also (for example reflection on metal sheet). You
can have points where you have only E, but no H, and vice versa. This
is comparable with reflection in a transmission line.

Sorry for the long text, but I hope it is useful.

Best regards,

Wim
PA3DJS
www.tetech.nl
please remove the obvious three letters in the PM

Electromagnetic wave question

Kraus has a graphic about this in ANTENNAS, 3rd edition...

http://i62.photobucket.com/albums/h8...hicFields-.jpg

RF

Electromagnetic wave question

Peter wrote:
In the electric power industry there is increasing public concern regarding
fields around power lines. The general public often refers to these fields
as electromagnetic radiation, which it is not. We generally are concerned
with magnetic fields and less often with the electric charge fields. The two
are treated as separate issues.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important electromagnetic
wave?

by "electromagnetic wave" you really mean freely propagating wave... and
by definition it's at the point where the ratio of E/H = 377 ohms, aka
"the far field".

Closer in, the ratio between E and H may not be 377 ohms, and energy is
moving back and forth between the fields and the antenna (or between the
E and H fields, depending on your conceptual model).. a region also
called the "reactive near field" or "near field"..

Lots of folks also talk about a "transition zone" where there's both
significant propagating wave and stored energy.

None of these boundaries are hard and fast.. there's some conventions
used in connection with things like antenna ranges. The conventions are
derived from an underlying assumption that the real behavior isn't
significantly different (in a measurement sense) from what you measure.

e.g. the "far field" assumption for antenna range distance
(2*D^2/lambda) is where the deviation of the spherical wavefront from
the assumed perfectly flat is small enough that the error in the
boresight gain measurement is "small" compared to other effects. (It's
comparable to the Rayleigh criterion for optical reflectors)

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Electromagnetic wave question

"Peter" wrote
. au...
In the electric power industry there is increasing public concern
regarding fields around power lines. The general public often refers to
these fields as electromagnetic radiation, which it is not.

Yes. Radiation is proportional to f^4. So at 50Hz practically no radiation.

We generally are concerned with magnetic fields and less often with the
electric charge fields. The two are treated as separate issues.

The two were seperate in ancient ages and now in the textbooks. It is the
necessary simplification.
But the adults have the two ways:
1. The way (EM theory) by Biot-Savart and Maxwell where the current create
the magnetic whirl,
2. The way by Ampere and many others where the moving charge create also
electric field. Here the magnetism is an illusion.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important electromagnetic
wave?

You have the answers for the way 1.
For the way 2 we have only the one electric wave. But would be easy to you
to check which way is right.

In the way 1 EM waves are radiated by the AC (current create magnetic whirl
and this create electric whirl and so on). Radiate this part of antenna
where the current is max.

In the way 2 the electric waves are radiated by the two ends of Hertz
dipole. So the dipole radiate the two coupled electric waves. The receiving
antena should detect the doubled frequency (on monopoles no such effect).
You must distinguish it from the harmonics.
S*

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Electromagnetic wave question

"Wimpie" wrote in message
...
On 9 jun, 11:01, "Peter" wrote:
In the electric power industry there is increasing public concern
regarding
fields around power lines. The general public often refers to these fields
as electromagnetic radiation, which it is not. We generally are concerned
with magnetic fields and less often with the electric charge fields. The
two
are treated as separate issues.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important electromagnetic
wave?

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Hello Peter,

The point where the fields merge to the EM wave depends on various
things.

There are three field zones used within the antenna community:

1. The far field zone (Fraunhofer region), where there is a true
spherical wave behavior, and the radiation pattern of the antenna is
independent of the distance to the antenna. The antenna can be treated
as a true point source emitting EM waves. The far field zone is mostly
taken r 2*b^2/lambda, where b is the largest size of the antenna.
This formula is very conservative. Depending on the current or
aperture distribution, the far field can start from r 0.5*b^2/
lambda. E- and H- field is proportional with 1/r (r=distance).

2. The transition field zone (Fresnel region), where the fields have a
reasonable wave character, but are not spherical. The radiation
pattern of the antenna depends on the measuring distance. This one is
tricky. An antenna with a null in the far field pattern may show
significant radiation (in same direction) in the transition field. So
within the transition field, you cannot for sure assess field strength
levels based on the far field radiation pattern. Also the main lobe
in the far field pattern may not be the main lobe in the radiation
pattern measured at distance far below the far field distance. You can
compare that with coherent light passing through a round aperture, you
get an onion shaped ring pattern on the wall (airy disk). You can even
get a black hole in the center at certain distance. So here field is
not proportional with 1/r.

3. The reactive field zone. This is the region where you can calculate
the fields using electrostatics and induction. In many cases, fields
are 90 degrees out of (time) phase. r0.16*lambda is used frequently.
Of course, there is no hard distance where the reactive field zone
stops and the transition field starts. The decay of the H and E field
are different depending on the antenna and the orientation of the
antenna.

For small loop antennas, r diameter AND r0.16*lambda, H field decay
is proportional with 1/r^3.

Another example is a HW dipole. It does not radiate on the axis of the
dipole. But when you are at (for example) 0.5*lambda from the ends
(but on the axis), you will sure measure an E-field (because of the
reactive fields that decay with r^3 to r^2).

Another example is the field between a wide parallel strip
transmission line that is well terminated (strips face each other)….
Both E and H are virtually homogenous between the strips, are in time
phase and are spatially 90 degrees out of phase. So there is a plane
EM wave traveling between the strips (no reactive issues).

The situation becomes more complicated when interaction with other
materials is present also (for example reflection on metal sheet). You
can have points where you have only E, but no H, and vice versa. This
is comparable with reflection in a transmission line.

Sorry for the long text, but I hope it is useful.

Best regards,

Wim
PA3DJS
www.tetech.nl
please remove the obvious three letters in the PM

Thanks for the reply Wim

That is a great explanation. I assume from the explanation that the EM wave
will due to interaction with path feature i.e. building, ground etc. have
more of less of the various zone components. I'm thinking for example as the
EM wave passes a hill it may have some of the Fresnel region characteristics
that would manifest as say the so called knife edge effect.

Cheers

--
Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Electromagnetic wave question

"Szczepan Bia³ek" wrote in message
...

"Peter" wrote
. au...
In the electric power industry there is increasing public concern
regarding fields around power lines. The general public often refers to
these fields as electromagnetic radiation, which it is not.

Yes. Radiation is proportional to f^4. So at 50Hz practically no
radiation.

We generally are concerned with magnetic fields and less often with the
electric charge fields. The two are treated as separate issues.

The two were seperate in ancient ages and now in the textbooks. It is the
necessary simplification.
But the adults have the two ways:
1. The way (EM theory) by Biot-Savart and Maxwell where the current create
the magnetic whirl,
2. The way by Ampere and many others where the moving charge create also
electric field. Here the magnetism is an illusion.

I believe the same is true of antennas, that is the electric and magnetic
fields are separate when you are close in to the antenna in terms of
wave-length.

Question:

If this assumption is correct at what point or distance do the two
relatively independent fields become the one all important
electromagnetic wave?

You have the answers for the way 1.
For the way 2 we have only the one electric wave. But would be easy to you
to check which way is right.

In the way 1 EM waves are radiated by the AC (current create magnetic
whirl and this create electric whirl and so on). Radiate this part of
antenna where the current is max.

In the way 2 the electric waves are radiated by the two ends of Hertz
dipole. So the dipole radiate the two coupled electric waves. The
receiving antena should detect the doubled frequency (on monopoles no such
effect). You must distinguish it from the harmonics.
S*

Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

Thanks to all for the response and relatively detailed explanations that has
given me something to ponder.

Regards Peter YSF
--
Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm