What does 'near field' 'far field' mean?
W6RCecilA
From: "Antenna Theory" by Balanis where r is the radian distance
and k is 2pi/WL, The near-field is the (kr << 1) region and the
far-field is the (kr >> 1) region.
--
73, Cecil, W6RCA http://www.mindspring.com/~w6rca
Reg Edwards
alternating current, a magnetic field surrounds and spreads out from
the conductor. There is a voltage difference between any two places
along the conductor and so an electric field surrounds and spreads out
from the conductor. In fact you can't have one sort of field without
the other. This pair of co-existent fields is known as the Near Field
because it rapidly decreases in strength as the distance from the the
conductor increases. It doesn't spread out very far.
When the near field is alternating* at a high frequency as in a radio
antenna, some of the energy contained in the Near Field becomes
detached. It's all bound up with the velocity of light, the time it
takes for the Near Field to spread out from the antenna conductor, and
the frequency of the alternations.
The energy which escapes for ever from the Near Field is known as the
Radiation or Far Field. It decreases with distance at a slower rate
than the Near Field and travels outwards into the vastness of space.
But it still consists of a pair of inseperable alternating magnetic
and electric fields, travelling in all directions from the antenna at
the speed of light.
So you can say the Far Field is what remains after the Near Field has
fizzled out. For practical purposes, the radiation and near fields are
equal to each other at a distance of approx 1/6 of a wavelength from a
simple wire radiator. At distances further away than a handful of
wavelengths the near field is so weak only the radiated or Far Field
can be said to exist.
Some folks might say the far field is the radiation field at great
distances like thousands of miles. It's a matter of opinion. I like
chatting from one side of the town to the other on 160m and, as far as
I'm concerned, my pal's antenna is in the far field even though,
technically, he may still be in the near field.
* Alternating means the current in the conductor alternates between
one direction and and the other. So the electric field also alternates
between one polarity and the other, between +ve and -ve.
--
================================
Season's Greetings from Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
================================
Gary Danaher
affected by the frequency of the emission as well as the strength. The
FCC established standards for exposure for near field rf radiation which
are based on these factors.
Gary
AB5RM
Reg Edwards
extends from the antenna may be dependent on frequency but is
independent of wavelength when measured in wavelengths. And,
importantly, field stength depends strongly on direction and
elevation.
Can you remember the days of wave(length)meters rather than frequency
meters ? ;O))
================================
Season's Greetings from Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
================================
Gary Danaher wrote ...
C Andersson
by graphs.
Wavelength on the X-axis and strength on the Y-axis.
Can anyone help me with this? I saw this diagram some time ago but now i
cannot find it...
/Chris
sm6pxj
Richard Harrison
comment anyway.
The near field energy is returned to the sender just as it would be from
storage in a reactance.
But, reactive current adds to an ammeter reading. The r-f ammeter
reading is often used to measure the real power delivered by the
transmitter. We also know that the antenna has often been adjusted to
eliminate any reactance at its feedpoint, So, the near field must
produce a returned current which is out of phase and not at quadrature
with the excitation.
The far field energy is gone for good when it leaves the antenna, just
as if it had been lost in a resistor. We know that the volts and amps
are in phase in this radiation resistance as are the E and H radiation
fields which result from those volts and amps. The directions of the
force lines are at right angles between the E and H fields and also at
right angles to the direction of travel in the plane wave which results
from the radiation at great distance from the source. This wavefront is
plane or flat just as the earth appears plane or flat on its surface due
to the distance of this terrestrial surface from the earth`s center.
But, the instantaneous magnitudes of the E and H fields are in-phase.
Best regards, Richard Harrison, KB5WZI
Stephen
Reg Edwards <G4fgq...@btinternet.com> wrote in message
news:84h3hm$h07$1...@plutonium.btinternet.com...
> When an electric current flows in a conductor, either direct or
> alternating current, a magnetic field surrounds and spreads out from
<snip lotsa good stuff>
Now THAT'S what I call a good explanation that even MY students could
understand!
Thanks Reg
73 Stephen G4SJP
Tom Bruhns
Wireless" that goes into more detail. You can see it at
http://www.noblepub.com/archives2/1999/November1999/nov1999-p58.pdf. In
fact, the transition from near field to far (as commonly accepted)
depends on the antenna; for an antenna which small and non-directional,
it's a short distance. For a high-gain parabolic reflector antenna,
it's a whole lot more than you'd think from kr = 1. The reference above
is a one-page tutorial you can download and keep as a reference. The
article mentions four criteria for deciding if you're in the far field;
all must be satisfied: first, only 1/r terms remain significant.
Second, the angular field distribution becomes independent of distance.
Third, only transverse field components remain, and the ratio of
electric to magnetic approaches the impedance of the medium (377 ohms
for freespace). Fourth, for a receiving antenna probing the field, the
wavefront is nearly planar across the aperture of the antenna.
Cheers,
Tom
Reg Edwards
understand it myself. Ph.D's are a menace to society. Hope you're
not a Ph.D
Reg.
--
================================
Season's Greetings from Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
================================
news:84i5ei$6md$1...@lure.pipex.net...
Stephen
> Stephen, I'm self taught. I had to make it sound simple so I could
> understand it myself. Ph.D's are a menace to society. Hope you're
> not a Ph.D
> Reg.
> --
No Reg, I'm not a PhD! So we're on the same side!
Happy New Year
73 Stephen G4SJP
R.P.Haviland
C Andersson wrote in message ...
>I'm looking for a diagram with the far field and the near field represented
>by graphs.
>
w4mb
Roy Lewallen
>
> Stephen, I'm self taught. I had to make it sound simple so I could
> understand it myself. Ph.D's are a menace to society. Hope you're
> not a Ph.D
> Reg.
Some of the most clear, understandable, and accurate explanations posted
on this newsgroup come from people who have a Ph.D. degree. Some simply
don't publicly advertise it, likely because of sweeping statements like
this.
Roy Lewallen, W7EL
Richard Harrison
"Some simply don`t publicly advertise it (a Ph.D. degree), likely
because of sweeping statements like this."
Reg did make an unwarranted Ph.D. put-down. But, a response from Roy
sounds like professional jealousy, even if he`s right. I hope both
forget they ever said anything.
Reg Edwards
of humour and who have the ability to provide clear, understanadable
and accurate explanations of simple matters in a simple manner, for
lumping you together with other Ph.D's who have managed to pass the
amateur radio examinations and who imagine folded 1/2-wave dipoles
behave as squashed magloops without tuning capacitors.
--
================================
Season's Greetings from Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
Gary Danaher
Gary
AB5RM