"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