radiation and fields
Radiation and Fields
Related
pages Radiation
Resistance , Skin depth
and Shields ,
“Magnetic” Loops
We
tend to think of
electric, magnetic,
and radiation fields
as physical
“things”…much
like a chemical element
would be in
chemistry. This can
cause visualization problems for us!
One
popular (but
incorrect)
assumption is we can
combine, mix, or
blend fields into
another field. After
all…. we have
electric fields…we
have magnetic
fields…surely we
can mix them and
create an
electromagnetic
field. This would
have a distinct
advantage, because
we all know
nearfield losses are
higher than farfield
losses. We know the
farfield field ratio
(or field impedance)
is set by the media
the wave propagates
through. With
reasonably dry air
or vacuum, at normal
radio frequencies,
we know the
impedance is 377
ohms. Why not
just mix the
two fields and
create the third
field without all
that needless
nearfield
loss?
We
logically might
assume, just by
producing fields by their name, we can mix
various fields and
make a new combined field. We
logically might
assume we can split
the combined fields
back apart if we
don’t like what is
happening to one of
them. After all, in our mind’s eye, if
we can mix fields,
the next logical
step would be to
assume we can
also separate, filter, or
sort the fields out
whenever necessary. For example, if
we have a problem
with electrical
interference, we can
just eliminate the
electric field. If
we have a problem
with field
intensity
(defined as
volts-per-meter)
causing problems
with the neighbor’s
VCR, we can reduce
the electric field
and cure the
problem. After all,
it is electrical
interference, isn’t
it?
Unfortunately,
none of the above is
true! The above
conceptual problems
started at the very
beginning, because
we assumed electric and magnetic fields
were physical things,
like building
blocks, that are combined to make an electromagnetic field. They
aren’t. Not only are the various fields rooted in different causes, they behave
much differently with distance. If they were the same, they would behave the
same in all aspects.
“Fields” are really just mathematical descriptions of forces
between charges. There are three simple conditions creating physical forces
or interactions at a distance between charges. These forces are electric, magnetic, and electromagnetic (radiation)
forces. They are all created by distinctly different physical actions in a
system. We can’t “mix” the various fields (forces) caused by specific charge actions
and create a new field! Fields
describe the end-effects
of certain causes,
fields don’t create
these causes!
Electric
Field
“Electric
field” describes a
force created by
uneven charge
distribution. Nature
wants charges to be
evenly distributed,
she can only take so
much piling up of
charges in one
spot and tries to “even them out”. The force
between charges,
caused by nature
trying to balance or
even-out
distribution of
charges, is called
an electric
field.
Uneven
charge distribution
goes hand-in-hand
with a voltage
difference between
two physical points.
We can obviously can
have a difference in
charge distribution
in insulators as
well as conductors.
A comb,
“charged”
by running through
our hair (if we have
any left), can have
an electric field.
The force of this
field can pick up
tiny bits of paper
as nature tries to
equalize the charge
distribution. The
terminals of a
battery have an
electric field
between them, and
when a
conductor is placed in that
field the charges
try to
equalize. Another
example would be an
antenna, where a
voltage difference
(uneven charge
distribution)
between two points
creates an electric
induction or reactive field.
Any
difference in charge
distribution,
whether charges
are
“moving”
or standing, causes
a physical force. We
name this effect an electric
field, and
describe it
mathematically in
volts over a certain
distance (like
millivolts per
meter). This
field (or force)
decreases rapidly
with distance. Decay is at the rate of 1/r3
Magnetic
Field
Magnetic
field describes a
force created by
moving charges. When
charges are moving,
they exert a force
on all other charges
around them. We call
this effect the magnetic
field.
One
example of a
magnetic field is a
conductor carrying
current. Perhaps it
is a wire connected
between two
terminals of a
battery. Another
example would be a
RF-current carrying
conductor in an
antenna.
The
movement of charges
causes a magnetic
field, and somewhere
rooted in the
creation of that
magnetic field is an
uneven distribution
of charges causing
an electric
field! Once
again, this field
(or force) decreases
rapidly with
distance. Decay is at the rate of 1/r2
Electromagnetic
Field
An electromagnetic
field is created
whenever charges are
accelerated.
Acceleration occurs
whenever a charge
changes direction or
velocity. When a
charge accelerates,
all the other
charges in the
universe feel a
force trying to make
them move. The only
thing preventing
that force from
going on for infinite distance, weakening only by spreading of wavefront area, is
when another charge
(or combination of
charges) accelerate
to create an
opposing force. We
call the velocity at
which this force or
effect ripples
through the universe
the speed of
light.
One
example of
electromagnetic
fields is in an AC
current carrying
conductor, like a
power line. The
time-varying voltage
causes charges to
move back and forth,
and the change in
velocity and
direction causes the
effect called
electromagnetic
radiation.
It
is easy to see why
our antennas have
all three fields,
and why we can
communicate so well
over large distances
with low power.
While the very
strong electric and
magnetic induction
fields drop off
rapidly with
distance, the
initially much
weaker
electromagnetic
radiation field goes
on until something
cancels it. The radiation field allows us to
communicate, not the
electric or magnetic
induction fields!
Decay of the E-M radiation field is linear with distance,
weakening in density only by increase of surface area of an imaginary sphere
expanded out from the antenna. This means if we looked at the total energy in
the imaginary sphere at any distance out of the antenna, it is always the same.
Radiation, while weak near the antenna, goes on forever through space (unless
cancelled)! Unless something “gets in the way”, EM radiation is inversely
proportional to the square of the distance from our antennas.
| Signal Level = | ____1____ |
| distance ^2 |
(Electric
and magnetic
induction fields
store and return
energy to the
system, the forces
from that effect are
very strong, but
they decay very
rapidly with
distance.)
Near
the Antenna
Near
any antenna fields
are a complex
mixture or ”
soup” of
various effects from
charges. When viewed
from distances very
close to the
antenna, charges are
almost always moving
in multiple
directions and
distributed over
vary distances from
our viewpoint. It
isn’t always easy to
picture or get a
feel for what
actually happens,
especially when the
area of the antenna
is very large
compared to the
distance from which
we observe the
effects of charges.
Near the antenna,
pattern and field
impedance is
generally nothing
like we might
intuitively
imagine!
It
is the response in
this area, generally
within l/10
distance from the
antenna, that small
“magnetic
loop” and
“electric
dipole”
antennas get their
names.
Very
close to the
high-current area of
a small loop antenna
(but not near the
capacitor end, because
that is where the
electric field
dominates), the
magnetic field
dominates. Magnetic
fields are
mathematical
descriptions of
forces derived from
moving charges, or
current flow. This
effect, when large
compared to the
electric
field, is
sometimes described
by saying the “field
impedance”
is “low“.
Conversely,
near a small dipole
or monopole with
high voltage and
little current, the
electric field
dominates. The
largest force is
from the very high
open-end voltages,
and very uneven
charge distribution.
We might say such an
antenna has a “
high field
impedance”
in the area where
the electric field
dominates any forces
cause by moving
charges.
In
all of these cases,
if the antenna is
electrically small,
the dominant fields
apply only within
approximately l/10
distance from
the antenna!
As
we move out further
the weaker radiation
field, because it is
attenuated less with
distance, starts to
have a noticeable
contribution to the
charge forces.
Because the phase of
the fields (fields
are a way of
describing effects)
is different at the
antenna, the sum of
the effects is
different with
distance. At some
distance the low
field impedance of a
small loop becomes
high, and the high
field impedance of a
small dipole becomes
low!
Since
the distance of a
wavelength in the
above graph (thanks
W7EL) is 100 meters,
we can also
considered the
bottom scale as a
percentage of a
wavelength. We can
see at about 11
percent of a
wavelength (which
would be about 50
feet on 160 meters),
there is no field
impedance difference
between a small
“magnetic”
loop and a small
“electric”
dipole. At distances
beyond 50 feet on
160 meters, the loop
actually has a higher
field impedance than
a dipole.
Losses
in the area around
the antenna
Field impedance
close to the
antenna is modified by rapidly decaying induction (also called reactive) fields.
The electric induction and magnetic induction fields (fields are forces) add to
the weaker electromagnetic radiation forces. This changes impedance from the 377 ohms
of a pure electromagnetic wave.
Near the antenna we have a largely unpredictable soup or mix of the three
primary causes of force between charges.
Losses are directly
related to the field
density, and when we
are close to any
antenna the fields
are very intense.
Losses are not a
field ratio problem,
they are field
intensity related.
In very small
antennas, virtually
ALL of the losses
are related to
reactance canceling
and resistive losses
in the antenna and
any lossy media
around the
antenna!
We
also must be mindful
of the painful truth
that we can not take
either electric or
magnetic fields to
zero or all
radiation stops. By
definition,
radiation is an
electromagnetic
wave. We can’t
modify the field
impedance of an
antenna without
changing the voltage
and current
distribution of the
antenna.
Nearfield
The
nearfield
area is an area
where the ultimate
pattern is not fully
formed, and where
induction fields
(from charge
distribution and
charge movement)
have a noticeable
effect on the forces
we measure or
observe.
It
is possible, with
large arrays of
small elements, to
be out of the
induction field
region but still be
in the area called
the
“nearfield”
area or zone. Let’s
consider individual
groups of elements
as
“cells”,
and the array a
combination of small
directional cells
occupying a very
large physical
area. Each
cell has formed a
radiation field.
Depending on the
size and type of
radiator in each
cell, induction
fields that charge
distribution plays a
role in may be
attenuated so much
as to be
negligible….yet
the radiation
pattern of the
entire array may not
be totally formed.
The radiation
pattern might not be
fully formed even
though the induction
effects are no
longer observable.
We are in the
nearfield, but not
in an area where the
energy storage
fields have a
noticeable effect.
This
is the case with my
phased Beverages and
phased verticals.
The individual
antennas making up
the array are so
distant that the
effects of charge
distribution
(electric induction
field, sometimes
called the
electrostatic field)
or steady movement
(considered at one
infinitely brief
instant of time, or
magnetic induction
field) have no
effect. For example,
at about 1
wavelength distance
the electric and
magnetic induction
fields are
negligible from
either my circle of
eight verticals or
780-foot Beverages,
yet the pattern of
the overall array
established by the
phasing of multiple
cells is not fully
formed. The pattern
would only be fully
formed several
wavelengths from
each array, where
the distance between
cells or elements is
only a small
fraction of the
distance we are
looking back from.
The
total pattern of two
780-foot long
Beverages spaced 350
feet apart is not
fully formed even at
distances of several
thousand feet, yet
nearfield induction
effects are totally
gone at much shorter
distances. The field
impedance is
established, yet the
antenna pattern is
not.
The
nearfield generally
refers to or
includes the area
where
“static”
or induction fields
still have a
noticeable
influence.
Fresnel
Region
The
Fresnel (fre-nel, no
“S” sound)
region is the area
where the radiation field pattern or shape is
still being formed.
It may or may not
include induction
field areas.
Physically
large arrays almost
always have a
physically large
Fresnel zone. Even
simple
omni-verticals have
a Fresnel zone
extending out a few
wavelengths. The
field impedance may
or may not have
already been
established in the
Fresnel zone.
You
may have heard about
Fresnel zones during
discussions of
vertical antenna
loss at low wave
angles, or Fresnel
lenses for
lighthouses or other
beacon lights.
Farfield
The
farfield or Fraunhofer region or zone is the area
where changes in
distance from the antenna
no longer produce a noticeable change
in pattern shape or field
impedance. Losses
are lower in the farfield area
because field
density per unit volume of space is lower.
My phased 160 meter Beverages are a good example of a farfield or Fraunhofer region that starts well beyond normal, far beyond induction and nearfields. Because the antennas are 800 feet long and spaced 300-400 feet apart broadside, the pattern is not completely formed to the final shape for a mile or more from the antennas! The field impedance has long stabilized, it is stable hundreds of feet from the antennas in any direction, but the pattern is not fully formed for many thousands of feet from the antennas.
What Determines Electromagnetic Radiation
Intensity?
If you’ve been around antennas or Ham radio very long, you may
have heard things like “if you are going to fold an antenna back on itself,
don’t do it in the high current area” or “keep the high current area of a mobile
antenna as high as possible”. There is good reason for that advice. EM radiation
is caused by charge acceleration, and we know current is caused by charge
movement. The amount of radiation is directly determined by the linear spatial
distance charges are accelerated over. Radiation ultimately comes down to one
thing, current over linear spatial distance. The ultimate answer is we need a
certain number ampere-feet to radiate a given power.
If we make an antenna shorter, antenna current has to increase
to radiate the same power. If we fold an antenna back on itself, or spiral a
full-size antenna up in a tight helice, current has to increase to radiate the
same power. If we compress the current into a small linear physical area,
current has to increase to radiate the same power as uniform current over the
same area. This is because we have fewer spatial feet, so we need more amperes
to radiate the same power level.
Cancelling of radiation in one or more directions also increases
current for a given radiated power. If we fold an antenna back upon itself or
bend it into a small loop, so radiation from one area fights or opposes
radiation from other areas of the antenna, current increases until the antenna
radiates the same power. This is the reason current is so high in a small
“magnetic” loop. More turns does not solve the problem, because the spatial area
does not increase. More turns in the same diameter loop just divides the same
total loop current between all of the turns, resulting in the total of current
in each individual turn equaling the current required in one turn.
More of this is explained in
radiation resistance.
Summary
There
really isn’t a clear
distance where
certain effects just
abruptly stop.
Transitions between near field and far field are
smooth and gradual,
because transitions
are the result of
gradual changes over increasing or decreasing distance from the
antenna….nothing else. The intensity of effects on other charges are distance related,
and the effects caused by the
three distinct
conditions of charges
moving, charge
distribution, and
charge acceleration.
Even
if we could somehow
make the
field-impedance, or ratio of electric to magnetic forces, a
certain ratio equal to
farfield electromagnetic ratio, it
would not eliminate
or reduce nearfield losses in the slightest
amount!
Losses
are related only to
the density of
fields in any given
media or
environment, and
nearfield or Fresnel
zone losses decrease
with distance
because of the lower
field intensity in
any given volume of lossy
media as we move away
from the antenna. At
a greater distance,
there is a wider
cross-sectional area
of lossy media
carrying
energy. The
induction fields are
attenuated greatly
with distance. The
field intensity and
field density are
both greatly reduced
with increased
distance. That is
what reduces losses,
not the magical 377-ohm
field impedance.
Larger
antennas, in
general, have larger
boundary areas for
electric and
magnetic fields.
With wider boundary
areas, the system
has
less-concentrated
fields and charge
effects. Very small
antennas of any type
obviously have very
concentrated fields,
and the high field
density or
concentration of
current or voltage
in conductors are at
the root of
increased loss. The
concentration of
energy in a small
area is the real
reason losses are
generally much
higher in smaller
antennas, and this
has little or
nothing to do with
field
impedance!
We
can not mix fields
and create a
different field and
make a small antenna
become an
“artificial
large antenna”,
nor can we
“filter”
fields and remove
the electric field
to reduce electric
noise.
Simple Textbook Explanation
Terman describes radiation below. Note the ONLY criteria for
determining field strength Є are distance, angle,
length of conductor, wavelength or frequency, and the current over the length.
Antenna modeling programs use this formula, based on Maxwell’s equations, to
calculate radiation from small current carrying segments of antennas.
EM radiation comes from only from
current over linear effective spatial distance occupied by conductors. If the
conductor is bent, or another conductor radiates fields, the resulting field
with be the vector sum (includes phase) of the multiple fields. A small loop has
very high current for a given radiated power because every area of the loop
cancels radiation from every other area. This forces current to very high values
for a given radiated power. This is the same reason, when we phase two verticals
to cancel radiation in a certain direction, current in the verticals increases
for a given power.
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2004
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