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Revised
Dec 22, 2005
corrected text
errors and reworded
some areas to make
clearer
Revised
June 13 2006 to add
link.
Related
pages coaxial
cable and
especially skin
depth
(Please
read the Radiation
and Fields page)
Small
Receiving Loops
Small
loops are often
referred to as
“magnetic
radiators”.
Folklore claims a
small
“shielded”
loop antenna behaves
like an electrical sieve
or filter,
sorting “good
magnetic
signals” from
“bad electrical
noise”.
Nothing
is further from the
truth! At relatively
small distances a
small magnetic loop
is more sensitive to
electric fields than
a small electric
field probe.
Field
Impedance
The
ratio of electric to
magnetic field
sensitivity is
sometimes called or
can be described as
the “field
impedance”.
In the near field
region, a high
field impedance
indicates a dominant
electric field. A low
field impedance
indicate has a
dominant magnetic
field. The field
impedance actually
varies with distance
when close to the
antenna, and it can
also vary with
direction or
angle.
Although
fields have
different ratios
close to the
antenna, at
distances of about
one wavelength
the field impedances
of small antennas are
virtually
indistinguishable
from each other.
Small antennas would include “magnetic” loops, verticals, and dipoles. It
would not include wide-spaced stacked beam antennas, or antenna physically
significantly more than a wavelength in widest dimension. Once we understand
fields near an antenna, we might
picture our antennas
differently. We
often find antennas
don’t respond to
signals and noise
like we thought.
Best of all, we
won’t be as
susceptible to the
“noise”
caused by rumors and
folklore!
Loop
Antenna Fields
It
is the energy storage or reactive induction
field response
within
λ/10 distance from the
antenna that gives
small “magnetic
loop” and
“electric
dipole”
antennas their
names.
Very
close to a small
loop antenna (but
not necessarily near
the open ends of the
small loop where the
tuning capacitor is)
the magnetic field
dominates. Current
is essentially
uniform all around
the loop
circumference, while
voltage has a nearly
straight-line
increase as we move
to the tuning
capacitor area.
Since most of the
loop area has
uniform current and
only a small
concentrated area
has the highest
voltage, the
magnetic field
clearly dominates
over most of the
loop area.
Magnetic
fields are effects
derived from the
actual moving of
charges (current
flow). The magnetic
effect is related to
current, and if it
is proportionally
large compared to
voltage or
capacitive coupling
the field is
described as having
a “low”
impedance. This is
similar to the
description used in
circuits, where a
system with high
current and low
voltage is said to
have “low
impedance”.
Short
Dipole or Vertical
Fields
Near
a small open-ended
dipole or monopole
the electric field
dominates. A short
antenna has very
high voltages
(compared to
current) all along
it’s length. It
normally has highest
current only near
the feedpoint, with
current tapering or
reducing in a
straight line
(linear fashion) to
zero at the
antenna’s open end.
It is the electrical
compliment of a
small closed loop.
The dominant
coupling to objects
immediately next to
the antenna is from
the very high
voltages that appear
all along the
antenna, which we
can also consider as
capacitive coupling.
We say the antenna
has a “high
field
impedance”
right next to the
antenna, and we
might call it a
voltage probe or
e-field probe
antenna.
In
the electrically
small antennas, such
as the loop antenna
and the sort dipole
or vertical
described above, the
dominant field
descriptions only
apply within l/10
distance! You’ll
see why as you read
further.
Radiation
The
induction fields
very close to the
antenna that I
described in the
text above are NOT
what radiates or
receives at a
distance. The
induction effects
are a necessary and
unavoidable result
of:
-
Uneven
charge
distribution (a
difference in
voltage) causing
a physical force
on other
charges. We call
this effect the electric
field.
-
Moving
charges (a
current) causing
a force on other
charges. We call
this effect the magnetic
field.
Signals
from any real
distance are coupled
by a third totally
different charge
interaction at
distance. Acceleration
of charges causes a
very unique force on
other charges in the
Universe. We call
this effect electromagnetic
radiation. It
is a totally
different effect,
and it is independent
of induction fields.
This is the only
effect or force that
works to move
charges at a very
large distance, and
it cannot be created
by mixing induction
fields.
(See CFA or EH
antenna hoaxes.)
This
graph shows the
field impedance at
various distances
near a small dipole
or loop. Since the
distance of a
wavelength in the
above graph is 100
meters, we can
considered the
bottom scale as the
percentage of a
wavelength. We can
see at about 11
percent of a
wavelength (which
would be about 60
feet on 160 meters),
there is no
difference in field
impedance between a
small loop and a
small dipole. At
distances beyond 11
percent of a
wavelength or about
60 feet on 160
meters, the magnetic
loop actually has a higher
field impedance than
a dipole. That means
a magnetic loop is
actually electric
field dominant at a
very modest distance
in the near field
area.
If
the noise source is
coupled to the
antenna within a
distance of about
1/2 wl or so, you
might find a
difference in noise
coupling between the
short dipole and
small loop systems.
At larger distances,
only directivity and
polarization would
make a difference.
So
much for the myth
that a receiving
antenna can sort
good signals from
bad signals (noise)
by virtue of being
“magnetic”!
We not only don’t
have the field
response we might
have imagined, we
also almost
certainly have no
idea if close-by
unwanted noise or
signal sources are
radiated from
electric or magnetic
field dominant
sources. Successful
noise reduction by
virtue of by
antenna
“style”
would mostly be a
matter of hitting a
lucky combination
through
experimentation.
Loop
Shielding and
Balance
Loop
shields do not sort
noise out, nor do
they prevent
electric fields from
affecting the
antenna. They do not
change the field
impedance of the
antenna. For a
description of how
shields work, look
at the Concentric
and Coaxial
Transmission lines
page and also skin
depth. You
can also read a few
pages of “Transmission
Lines, Antennas, and
Waveguides”
(no longer protected
by copyright) that
deals with loop
shielding and
balance.
From
those pages you will
see the shield
actually becomes the
antenna in a
“shielded”
loop.
There
are many
construction
articles about small
loop antenna
available. It is VERY
important that
all conductors exit
the loop at the
ground point of the
shield, and that the
loop is grounded
exactly at the
electrical center of
the shield. The
loop must also be
symmetrical, each
side must be excited
equally, and you
must mount the loop
so the feed line and
any metallic
supports leave the
center area of the
loop with maximum
symmetry. If you
DON’T do this, the
loop can actually
use the feed line as
the antenna. This
can greatly increase
sensitivity of the
loop to conducted
noises! Improper
design or
construction can
also distort the
pattern.
Remember
the following
guidelines:
-
The
shield is the
actual
antenna
-
The
shield must be
perfectly
symmetrical
moving away
from the inner
conductor exit
point
-
The
gap in the
shield must be
exactly opposite
the grounded
point
-
The
ground must be
at the inner
wire exit point
-
The
shield will not
make an
unshielded loop
that is properly
balanced any
quieter
-
The
shield only is a
tool to help you
balance the
system. The
shield helps
only when the
shield is
properly
implemented
Examples
of Small Loops and
Analysis of
Loop Construction
From
my 1988 ARRL
Handbook
Good
Points:
A
moderately good feed
system, although no
ability to match
impedances is
included. Notice the
coaxial feed line
feeds each terminal
of the center
conductor in
differential. It
does not feed the
terminals in
parallel or with one
terminal hanging
loose as some loop
designs do.
Bad
Points:
Overall,
this is actually a poor
design and
it includes a very poor
description of
operation.
First,
the shield doesn’t
shield or filter the
time-varying
electric field. It
cannot do so without
also removing the
magnetic field.
Since neither the
electric or magnetic
field passes through
the many skin depths
of shield wall
thickness, any claim
the shield passes
one field and limits
the other is wrong.
The mechanism of
field behavior is
described in detail
in Concentric
and Coaxial
Transmission lines
and skin
depth.
pages.
-
EM
fields excite
the shield’s
outer wall.
-
Skin
effect isolates
the outer shield
wall from the
inner shield
wall.
-
The
current on the
outside of the
shield produces
a voltage (EMF)
across the open
gap in the
shield, and that
voltage in turn
excites a
current flow on
the inner wall
of the shield.
-
The
current flowing
on the inner
wall of the
shield creates a
current in the
inner conductor
through
induction field
coupling, and
that current
(and voltage) is
coupled to the
receiver
feed line.
-
The
inside of the
loop is a simple
transformer, the
gap is the
feedpoint, and
the outside of
the shield is
the antenna.
The
flaw with the ARRL
Handbook’s DF loop
pictured above is
the shield is not
symmetrically placed
with reference to
ground, and so it is
susceptible to
common mode or
parallel currents.
The shield
arrangement, in this
case, actually unbalances
the loop.
This
loop could be
corrected by moving
the shield gap to a
point exactly
opposite the
feedpoint
“ground”.
Each half of the
loop would be
symmetrical on the
outside.
The
next design cures
the shield symmetry
problem, but creates
an additional
unnecessary problem
inside the cable in
the process.
Typical
Magnetic Loop (found
on Internet and
other places)
This
loop probably works
better than the ARRL
Handbook loop above,
although still not
optimally designed.
Good
Points:
The
shield is
symmetrical in
reference to the
ground point. Since
the shield is the
actual coupling
mechanism or
antenna, it must be
symmetrically
distributed to
prevent acting like
an extension of the
feed line. Shield
design is excellent.
Bad
Points:
If
we look at how
energy transfers
from the inner
conductor to the
feed line closely, we
see an inherent
design problem.
Let’s look at the
ideal coupling
method:
G
always has to be
referenced to ground
or an electrical
neutral or zero
voltage point.
1
and 2 are fed
differentially. It
is the voltage
difference between 1
and 2 that causes
current to flow
around the internal
conductor. Ideally
terminals 1 and 2
should have about
the same voltage
level with respect
to G but opposite
polarity voltages
(balanced) in
reference to G. This
would ensure each
half of the loop
exterior wall
is excited with
reasonable symmetry.
Circuit
Representations of
Shielded Loops
Shown
above, ideal
differential
excitation of the
inner conductor.
Shown
above left,
unbalanced stray
capacitance
excitation of the
inner conductor.
Note the lack of a
clear return path
for the inner wire
current to the
feed line. The
differential current
path exciting the
inner conductor is
actually through the
stray capacitance of
the right cable
only, although both
sides can affect
SWR. This has the
potential to
unbalance the loop
shield currents and
cause energy
transfer problems.
The feed system SWR
is critically
dependent on loop
coaxial length and
loop coaxial
impedance
(distributed
capacitance).
Above
right, electrical
equivalent. This
system depends on
cable capacitance to
form a voltage
divider that excites
the inner conductor
and matches the loop
impedance to the
feed line impedance.
C right has a very
high impedance
compared to C left,
since C right is
shunted by the very
low feed line
impedance.
Shown
above, unbalanced
differential mode
excitation of the
center conductor
without SWR
adjustment. This is
still distributed
capacitance critical
with the left side
having limited
primary control of
SWR, but it has
better balance than
the system with a
floating center
conductor and more
symmetrical shield
currents.
Above,
feedpoint with
adjustable matching.
Impedances shunting
C left and right are
both low, but
impedance balance is
slightly flawed.
Above
circuit would
balance both loop
sides, assuming
matching
capacitances C1 and
C2 are nearly equal.
Above,
series-tuned
isolated/ balanced
feed using matching
transformer. I used
this system in large
multi-element arrays
of phased
“unshielded”
loops in the early
1970’s.
Above,
isolated unshielded
with matching.
I
hope this article is
useful in helping
you select a GOOD
design, or to
experiment and find
a better system.
You
might also research
older antenna
textbooks,
like
“Transmission
Lines Antennas and
Wave Guides” by
King, Mimno, and
Wing
 since
June 2004
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