Mobile antennas, short verticals, loading coil loss,and loading coil current

Related pages:Mobile W7EL’s
How Does

Toroid with hat 
Small 2×2″ coil with 24″ hat up 24″ 
Long 12×3″ coil with 24″ hat up 24″ 
Long 12×3″coil with 6′ whip 

Current below 
100%  100  100  100 
Current above 
100  94.4  73  76% 
Toroid with whip 
long 12×1.5″ coil with 6′ whip 
long 12×1.5″ coil with 24″ hat 
Small 2×2″ coil with 6′ whip 

Current below 
100%  100  100  100 
Current above coil 
100%  79%  75%  96% 
Current in whip 1ft above top of coil 
73% 
The
most revealing thing
was how noticeably
small changes in
stray capacitance
near the middle and
top of the loading
coil affect current
distribution. It was
quite evident
hanging a large
meter on each end of
the coil would
greatly perturb the
system.
Clearly
we do NOT want:
 A
large hat just
above a large
coil  A
long large coil
and a short whip  A
coil near large
sheetmetal
More
data along with
photographs will
appear on a new page
over the next month
or so. Until then, I
can assure everyone
the conventional
theories presented
below are accurate,
and the theory that
“electrical
length” the
coil
“replaces”
is incorrect.
Loading coils indeed
behave like any
other inductor in
the world.
Independent
measurements by a
reliable engineer
have agreed with my
measurements above.
Anyone doubting my
data need only read
the following email
from W7EL.
The Incorrect
Assumption
Another commonly
misconception is,
since voltage
increases at the far
end of the loading
coil, current must
logically decrease.
After all, we have a
fixed amount of
power and voltage
has increased. The
assumption is:
1.) We multiply
voltage times
current to get
power.
2.) If voltage
increases current
must decrease.
Unfortunately,
this is not
correct in reactive
systems! Simple P
(power) = I
(current) times E
(voltage) only works
when the system is
nonreactive. This
condition only
occurs at resonance,
and only below
the
loading coil at the
antenna
feedpoint!
In a reactive
system, like in a
mobile whip above a
loading coil,
voltage and current
are no longer in
phase. As a matter
of fact, voltage and
current can closely
approach being 90
degrees outofphase
when the whip is
electrically very
short. Since the
antenna area above
the loading coil is
highly reactive
(voltage is not in
phase with current),
we can not multiply
voltage times
current without
considering phase
differences.
You may have
heard the term
“reactive
power” or VAR
(voltamperesreactive).
Reactive power is
voltage times
current without
consideration of
phase angle. We can
have kilowatts of
VAR power with only
a low power
transmitter, and
that is what we
actually have in the
reactive part of the
small
antenna.
Coil Q and
Changes in
Efficiency
Current taper or
reduction has been
cited as a reason
coil
“Q”
has little effect on
signal level in
mobile systems.
Speculation is only
the first few turns
of the loading coil
carry significant
current because the
coil
“leaks”
magnetic fields and
radiates, and this
is why the coil Q
has little
effect.
Another idea
proposes the loading
coil “makes
up” a certain
missing part of the
antenna. It goes on
to conclude the
loading coil can be
accounted for in
“electrical
degrees”,
making up the
“missing
difference” in
antenna degrees.
This isn’t true
either. The inductor
doesn’t know where
it is and suddenly
change from
“x” ohms
reactance to
electrical degrees!
It responds to AC
currents and
voltages as any
inductor in any
circuit does. It
doesn’t suddenly
change measurement
units.
As an example of
this, try to define
a 45degree
electrical length
inductor at 1.8 MHz.
That would mean it
is a capacitor at
3.61MHz, where it is
over 90degrees
long! How many
turns at what length
and diameter is a
45degree
inductor??
Where is a formula
that allows
converting a given
size inductor to
electrical degrees?
This shows how
useless and
meaningless that
definition
is!
The inductor adds
a certain amount of
series reactance,
that’s all. A 300uH
inductor is not
20degrees long, nor
is it 80degrees
long, so far as
radiation goes
unless it is really
that long physically. It
is a certain number
of ohms reactance at
a certain frequency,
or a certain number
of units called
Henries. It is not
“electrical
degrees” that
it adds, it is a
nondissipative
reactance (in
combination with a
loss resistance
because of finite
quality) at a
certain frequency!
A loading
inductor can
“insert” a
large amount of
phase shift, but the
phase shift is
between voltage and
current. The only
exception to this
would be if the
inductor had
considerable
distributed shunt
capacitance to the
outside world, and
acted like a string
of series inductors
(with the antenna)
and shunt capacitors
(shunting to the
ground system). In
that case we could
expect coil Q to be
extremely low, since
it would be the
electrical
equivalent of a
lossy transmission
line. That’s either
an awful loading
coil, or it is a
lessefficient
helical loaded
antenna!
The Correct
View
Another group of
people don’t argue
against established
and proven circuit
theory. They
understand charges
flowing into one end
of the loading coil
must have someplace
to branch off (a
virtual third
terminal), or they
must flow out the
other end. Without
that additional
“virtual”
path, charges
flowing into the
coil would always
equal charges
flowing out. This is
true regardless of
radiation, losses,
or induction fields.
This makes
perfect sense when
we think of any dc
circuit, antenna, or
RF system.
Electrical rules are
satisfied, the
system behaves as it
does in the real
world.
There is very
little change in
current, unless the
coil is physically
very long compared
to the rest of the
antenna above the
coil or unless the
coil is laid right
against
“grounded”
conductors and the
whip above the coil
is very short. This
fits perfectly with
helical verticals,
where the coil is
“stretched
out” over the
length of the
antenna.
It also agrees
with base loaded
antennas, which have
nearly as much
current into the
antenna above the
coil as at the
feedpoint. It agrees
with center loaded
antennas, where
current below the
coil is essentially
uniform and the whip
above has triangular
distribution.
Current can
be different in
various areas of an
inductor, but only
if shunting
capacitances
(impedances) to the
outside world are
significant compared
to load capacitance
(impedance). Another
condition where
current can vary
substantially is
with operation near
the condition of
selfresonance in
what is normally
considered or
defined as a
“seriesresonant”
mode. This
would be a very poor
and inefficient
loading inductor,
such as when a
160meter antenna is
used at a secondary
resonant frequency
in upper HF.
Circuit Model of
a Mobile Antenna
This model
represents what might be
a typical mobile
antenna
installation if the coil is compact and has very good flux coupling through the
length of the coil.
 Rgnd ground
resistance of
vehicle
normalized to
feedpoint  V1 coaxial
feed line  C2 base
capacitance  Rrbase Radiation
resistance
of the base area
of the antenna  L1 loading
coil  Rcoil coil
equivalent
series loss
resistance  C3 coil shunt
C to ground  Rr top area radiation
resistance  Cant
equivalent
antenna
capacitance
above coil area
My 160 Mobile
Antenna
I’ve worked all
continents except
Africa while
160meter mobile. I
have CW contacts at
over 10,000 miles,
and SSB as far as
Europe (4000 miles).
My mobile antenna
consists of an
eight foot antenna
with a sixfoot hat
(made from surplus
Ford or Chrysler car
antennas) at the
top. The loading
coil is at 5 feet.
This antenna has
been on the truck
for thousands of
miles, without
mechanical failure.
It is mounted at the
upper left corner of
the truck bed, about
one foot back from
the cab.
The following is
a model of the
current antenna
system on my Ford
F250 HD long bed
super cab truck:
The base
impedance in the
model is:
Frequency = 1.854
MHz.
Source 1 Voltage =
24 V. at 2.35 deg.
Current = 1 A. at
0.0 deg.
Impedance = 23.98 +
J 0.9853 ohms
Power = 23.98 watts
SWR (50 ohm system)
= 2.087
Actual
measurement at my
Johnstonville, GA
farm in open flat
pastures on August
17 at 8AM. Wet
soil 25.8 ohms
0j base impedance,
pretty close
agreement to EZNEC
model and earlier
data! (I
did have to adjust
the model for very
low ground
conductivity,
otherwise the
resistance was far
too low. It’s my
belief that NEC2
underestimates
ground losses in
small radial or
counterpoise systems
that are close to
earth.) Earlier text
shows a base
resistance of 28
ohms, that
was dry soil with a
slightly different
loading coil and
antenna.
The modeled
current distribution
for 1ampere applied
at the base (in
1foot intervals)
is:
1ft=
1.0031
2 ft= 1.0091
3ft= 1.0178
4ft= 1.0318
<Coil>
5ft= 1.0175
6ft= .97512
7ft= .92984
8ft = .89522
Measuring the
current into and out
of the loading coil
with a small
thermocouple RF
meter, I detect no
difference This is
in close agreement
with the
model.
The efficiency of
this antenna knowing
coil Q, radiation
resistance, and base
resistance
calculates just
under 1 percent. The
model indicates
about 1/3 percent
efficiency. This is
reasonably
close.
Removing the hat
(in the model only)
shows the following
changes:
1ft = 1.0043
2ft = 1.0133
3 ft= 1.0279
4 ft= 1.0566
<coil>
5 ft= .95508
6 ft= .72232
7 ft= .27813
8 ft = open
I haven’t tested
the above, but with
the same loading
coil loss resistance
the model says
efficiency is now
around 3dB worse.
Removing the hat,
with NO change in
coil resistance,
shows nearly loss
nearly doubles. Of
course the coil
resistance would
increase, because
the loading coil
nearly quadruples in
size. Bandwidth is
less and efficiency
is less, even if I
could maintain the
same coil
resistance.
Examples of
Unequal
Current
In the above
models, we see that
current into and out
of the onefoot long
coil is about the
same. There is only
about 2%
change in current
even though the coil
occupies 12% of
antenna length in
the
“hatloaded”
antenna, but in
fairness I couldn’t
resolve that change
with a reasonably
good RF current
meter.
The model
predicts 10%
change in a nonhat
antenna, but
I never measured
that antenna to
confirm it.
Clearly there is
no basis to the
claim current is
high only in the
first few turns of
an inductor, or that
current tapers in
relationship to
“electrical
degrees”. The
most accurate way to
state the effect
would be to say:
“When the
loading coil is
short and the
capacitance of the
antenna beyond the
coil is reasonable
(in this case 3000
ohms Xc or less),
there is an
immeasurable
reduction in current
in the coil. When
the required loading
reactance is very
high (in this case
8000 ohms), the
reduction in current
is about what we
would expect for an
equivalent length of
antenna replacing
the
coil.”
Degrees Vs
Radiation Resistance
This upper four
feet of this antenna
resonates near 24
MHz with the hat. We
can assume it is 90
degrees long at 24
MHz, which would
translate to 6.9
degrees on 1.85 MHz.
Following that same
logic, this would
mean the loading
coil would be about
83 degrees long
electrically. Using
the incorrect logic
proposed by others
where the loading
coil “makes up
the difference in
electrical
degrees”, there
would be almost no
current past the
loading coil.
Obviously this is
not the case, the
loading coil has
very little
“electrical
length”.
As a matter of fact,
in this case because the inductor is terminated in a fairly low capacitance and
the inductor is compact with good flux linkage
through all the turns, electrical
length is much closer to the
physical length of the coil than the “missing antenna length” in degrees!
This goes back to
radiation theory,
and my favorite
saying: “Five
hundred feet of wire
in a one foot long
tube is still one
foot of
antenna”. Some
CB manufactures sell
antennas to
consumers with the
claim they use 5/8
or 3/4 wavelength of
wire in an
eightfoot
fiberglass whip, so
the antenna has more
gain. Obviously this
is not true. Let’s
not let such silly
claims spread into
amateur radio!
Related topics:
The spice
inductor model shows
one example of how
unequal current is
created. The model
demonstrates a coil
having significant
distributed
capacitance to the
point of current
return in the system
compared to
terminating
impedance of the
coil. In a monopole
this return path
would be to the
groundplane, or
anything closer to
the potential of the
groundplane than the
area above the
loading coil’s
position in the
antenna system.
Another
Practical Antenna
Example
Let’s assume we
have a lossless 15.3
foot long 0.2 inch
diameter conductor
over a perfect
groundplane. Eznec
gives the 1.821 MHz
base impedance as
.3004 2169j. In
other words, the
antenna “looks
like” .3004
ohms of load
resistance in series
with 40.32pF on
1821kHz. The return
path for current is
through the .3004
ohm resistance and
40.32pF capacitance,
back to the ground
of the antenna (it
is a Marconi
antenna).
Such a
termination (load)
would require a
series inductance of
2169j (189.57µH) to
cancel feedpoint
capacitive
reactance. A typical
190µH inductor
would be rather
large, requiring
somewhere around 53
turns when using a
4″ by 4″
form factor. One
would expect a
physically large
inductor to have
noticeable but very
small displacement
currents to the
groundplane, when
the small stray coil
capacitance is
compared to the
40.32pF termination
capacitance. This
raises two very
important design
guidelines:
 When
installing a
loading coil of
substantial
inductance in an
electrically
short antenna,
sheetmetal and
dielectrics
should be kept
away from the
coil and areas
of antenna above
the loading
coil. This would
include
dielectrics on
or near the
inductor, since
the presence of
dielectrics
would increase
undesirable
capacitance.  When
inductive
reactance
requirements are
large, as when
short thin
“stingers”
without hats are
used above a
coil, the coil
form factor
should lean more
towards long and
thin.
Capacitances
near the open
end of the coil
(high voltage
end) should be
minimized. This
would be true
even when the
coil length
increase results
in a small
reduction in
mutual turns
coupling, since
the stray
capacitance may
result in a
larger loss
penalty
than the slight
increase in
accumulated
resistance from
additional wire
length.
Efficiency
Efficiency in any
antenna near earth
is almost always
dominated by ground
related losses,
shortheight Marconi
antennas are no
exception. The
overall effect of
loading inductor Q
and matching system
losses are
“diluted”
or
“swampedout”
by ground losses.
Ground losses cause
most systems to have
greatly reduced
sensitivity to
inductor design.
The only
consistently
predictable factor
in efficiency in
fractional
wavelength Marconi
antennas with
limited size ground
systems is radiation
resistance.
Efficiency increases
almost directly in
proportion to
radiation
resistance.
Radiation
Resistance and Power
Radiated
Radiation
resistance is
probably the most
poorly defined term
used with antennas.
The lack of clear
definition creates
errors and
misjudgments when
predicting antenna
performance. If you
wish more detailed
information, this
page contains
information on
radiation
resistance. For the
purposes of this
discussion and to
avoid pitfalls
associated with
using feedpoint
impedance as
radiation
resistance, I’ll use
the same definitions
Jasik, Balmain, and
others have used.
This definition is
based on the IRE
definition of
radiation resistance
being equal to the
net or effective
current causing
radiation squared
divided by the power
radiated as EM
energy, or
Rr=Pr/I^2.
Using this
definition, a folded
dipole has a
radiation resistance
identical to a
conventional dipole
of the same physical
dimensions ( ~70
ohms).
Radiation is
caused by charge
acceleration,
there is no magic.
The only thing
affecting radiation
resistance in a
short vertical
antenna near ground
is current
distribution over
the linear area
occupied by the
radiation portion of
the antenna. The
general rules are:
Radiation
resistance of a
Marconi vertical in
the maximum possible
radiation resistance
case for a given
height (this is the
case where current
is uniform
throughout the
structure) is equal
to 1580*(H/L)^2
where H equals
height and L equals
wavelength and both
are expressed in the
same units. Using
degrees, we see a
10degree tall
antenna has a
maximum possible
radiation resistance
of 1580*(10/360)^2
or 1580*.000772 =
1.22 ohms. This
would apply even if
the antenna is a
vertical, DDRR,
Fractal, or folded
unipole with
considerable top
loading.
If current is
triangular,
radiation resistance
would decrease by a
factor of four to
0.305 ohms.
Power radiated is
given by I^2*Rr
With 100watts
applied to a
10degree tall
antenna, net current
in a lossless
antenna with uniform
current distribution
would be 9.05
amperes. With
triangular
distribution, such
as appears in a
small diameter short
base loaded whip,
current would be
approximately 18.1
amperes. We are in
serious problems if
the inductor reduces
current along its
length, since the
only possible way to
radiate 100 watts
would be to have
somewhere around 9
amperes of effective
current integrated
over the 10degree
vertical area of
space for the
radiator!
Ground Losses
All current
flowing (or
displaced)
vertically into the
antenna must equal
current flowing out
of the ground or
counterpoise system.
Even though ground
losses are
distributed losses,
we must normalize
all losses to the
feedpoint in order
to compare systems.
There are cases
where this will not
always occur,
causing us to
falsely assume we
have lower losses
than really
exist.
In this tutorial
and comparison, I
have normalized
ground losses to the
same point where
radiation resistance
is
considered.
System Losses
(Measured
data below of actual
antenna given below
was from 1995 data
taken at a different
location near
Atlanta with a
slightly different
loading coil and
antenna. There is a
slight disagreement
with current data. I
left this all in so
you can see the
departure from
measurements and
models using 8 year
old data.)
Base Loaded
(Triangular Antenna
Current
Distribution) with
no ground loss
Assuming we have
a baseloaded
antenna, and the
operating frequency
has a wavelength of
550 feet (around the
160meter band), a
15.3 foot vertical
would fit the above
10degree value.
Interestingly enough
when we compare
Eznec to formulas
available in older
(1950 vintage)
engineering
textbooks, we find
radiation resistance
predicted by Eznec
is .3003 ohms while
the triangular
current estimate for
the same height
radiator is .305
ohms! This is an
amazing degree of
agreement,
illustrating what we
could do before
modeling programs
became available.
(With perfect top
loading, both Eznec
and longhand
calculations show
approximately 1.2
ohms of radiation
resistance.)
Assuming our 15.3
foot tall
(10degree)
baseloaded antenna
uses a coil Q of
200, the coil has
10.845 ohms of ESR.
Total resistance
with a perfect
ground would be
10.85+.3= 11.15
ohms. Current into
this system with 100
watts applied would
be around 3 amperes,
resulting in ~2.7
watts radiated and
~97.3 watts lost as
heat in the
inductor.
Doubling coil Q
(400) would provide
5.73 ohms of base
resistance with 4.18
amperes. Power
radiated would be
5.2 watts, power
lost as heat would
be 94.8 watts.
Efficiency does not
quite double,
changing from 2.7 to
5.2%. This results
in a 2.8dB change in
signal
level.
Top Loaded (with
no ground loss)
If we added a
fourwire hat with
15foot wires,
current would no
longer be
triangular. While we
wouldn’t quite reach
the optimum uniform
distribution,
current at the top
would be about 78%
of current at the
antenna base.
Feedpoint impedance
would become 0.97
551j, and the
antenna would look
like 0.97 ohms in
series with 159pF.
Using a coil Q of
200, we would now
have 2.76 ohms of
inductor loss.
Current becomes 5.18
amperes. Radiated
power is 26 watts,
while power lost as
heat becomes 74
watts. Even in the
perfect ground case,
the change in
efficiency caused by
top loading is
large. Top loading
(with only the hat)
results in 9.8 dB
change in signal
level when compared
to the base loaded
case when coil Q
remains 200.
Efficiency is 26%.
The coil remains
at ground
level for easy
matching and
frequency change.
In this case
current at each
terminal of the
loading coil would
be essentially the
same regardless of
poor coil mounting
techniques. In order
to have significant
current taper in the
coil or in the
bottom of the mast,
shunt capacitance
would have to be a
significant compared
to 160pF. The
antenna’s high input
capacitance relaxes
inductor and antenna
mounting electrical
requirements.
Base Loaded
(high ground loss)
My F250HD Super
Cab pickup truck,
when parked over
open medium quality
pasture land, has a
ground resistance of
about 20 ohms
(normalized to the
feedpoint) on 160
meters. Applying
this ground loss to
the base loaded
antenna, the system
has a feedpoint
resistance of
20+.3=20.3 ohms.
(This is reasonably
close to actual
feedpoint
resistances measured
with a similar
operating antenna.)
Adding coil losses,
the system has
20.3+10.85=31.15
ohms. (NOTE:
Current coil is ~8
ohms ESR, 10.85 ohms
is from ~8 year old
data) Current is
sqrt (100/31.15) or
1.79 amperes.
This results in
.96 watts radiated,
and 99.04 watts lost
as heat. Efficiency
is now around .96%.
Substitution of a
coil with a Q of 400
results in 25.7 ohms
feed resistance, or
1.97 amperes antenna
current at 100
watts. In this case
efficiency is now
1.16% for 1.16 watts
radiated. The change
caused by doubling
coil Q with high
system ground losses
is about 0.8dB,
compared to almost
3dB in the perfect
ground case! With a
poor ground (in this
case typical of a
very large vehicle),
a large change in
coil Q produces
little change in
system efficiency.
Another Top
Loaded (high ground
loss) System Example
(made prior to the
EZNEC model above)
Using a large hat
isn’t practical in a
moving mobile,
although it could
apply to fixed
stations suffering
with poor ground
systems. When the
hat is smaller, such
as a mobile
requires, the
loading inductor can
be moved higher in
the system. Such a
move would produce
uniform current
below the loading
coil, with a current
shape above the coil
dictated by the
construction of the
upper portion of the
antenna. My own
mobile uses a
sixfoot diameter
hat manufactured
from stainless steel
automobile antennas
arranged in a spoke.
I have no problems
with wind or
occasional
obstructions. While
unsightly, a modest
hat is workable.
In order to keep
the systems
comparable I’ll use
the same radiation
resistance provided
by a large hat, but
intentionally add
high ground loss as
a lumped resistance.
This model ignores
field losses near
the antenna.
In this case we
have 0.97 551j as
the inductor
termination
presented by the
antenna. With ground
losses normalized at
20 ohms and an
inductor Q of 200,
we have 20+2.76+.97
= 23.73 ohms of
feedpoint
resistance. Current
is 2.05 amperes, and
power radiated is
4.1 watts. Power
lost is 95.9
watts.
Efficiency is
4.1%, a 6.3dB
increase over a
baseloaded
triangular current
system with the same
lossy ground. This
system is 8dB
down from the same
“toploaded”
distribution using a
perfect ground.
When the system
has significant
fixed losses,
increasing radiation
resistance four
times by top loading
provides a similar
dividend in system
efficiency. At the
same time a
substantial increase
in coil Q provides
only minimal change
in field
strength.
Current Through
Coil
Related pages:
There has been
some speculation
that current is high
only in the first
few turns of a
loading inductor.
Radiation comes
solely from charge
acceleration or
current over spatial
(in line)
distance.
If any loading
inductor shows
substantial decrease
in current over the
length of the
inductor, it is an
absolute certainty
that the inductor is
poorly designed and
that the system
above the loading
inductor is not
contributing to
system efficiency.
The reason for this
is very simple and
straight forward.
Any twoterminal
component (even
considering wire as
a
“component”
applies) MUST have
equal charges
flowing into and out
of each terminal.
Voltages to other
reference points can
be different, but
for every charge
moving into one
terminal a like
number of charges
MUST move out of the
other terminal.
Radiation, induction
fields, and loss
resistances have no
influence on this
rule.
In order to have
any change in
current, there must
be an additional
path or paths for
charges. This path
can be through
leakage resistances,
or through
fictitious currents
called displacement
currents. Whatever
the path, the total
charge movements
must be
reconcilable. We
simply can not have
current
“disappear”.
The normal path
upsetting
“unbalancing”
current into and out
of each terminal in
an inductor is
provided by
displacement
currents through
electric fields. As
with any system, the
amount of current
flow is proportional
to potential
difference and
impedance of the
path. In order to
shunt a substantial
current out of an
inductor, the
potential difference
between the ends of
the path has to be
high compared to the
impedance of the
path. The impedance
of the stray path
must also be
reasonably low
compared to the
normal desired path.
Current diversion
is problematic in
very large inductors
operated at (or very
near) internal self
resonance, when the
self resonance is
what we typically
refer to as a
“seriesresonant”
condition. This
condition is common
in plate chokes used
in vacuum tube power
amplifiers, where
the system operates
over many octaves of
frequency range.
“Series
resonances”
inside components
occur when
distributed
inductance forms a
pair (or multiples
of pairs) of
“L”
networks. The large
series inductance
from each end of a
winding reacts with
the small stray
capacitance at the
center, and forms a
very high impedance
transformation L
network. The
electrical potential
at the center of the
system becomes
extremely high, and
even the smallest
amount of
capacitance to
surrounding objects
will carry a
substantial
displacement
current. The large
displacement
currents cause the
terminal impedances
to drop, and allow
considerable current
to concentrate in
small areas of the
component. At the
same time,
considerable voltage
can be present. The
normal result is
arcing or
destruction of the
component, or
failure of the
system depending on
the choke to
operate.
Series resonance
always occurs at a
frequency higher
than the self parallel
resonant frequency
of the component. A
loading coil
operating under such
conditions would be
required to have
serious design
errors to fall into
this category, since
the end termination
capacitances should
always be
substantially higher
than stray
capacitance
throughout the
component. Failure
to follow this rule
would result in
needless loss and
reduced SWR
bandwidth in an
antenna.
The speculation
or supposition that
the first few turns
of a loading coil
carry most of the
current is clearly
untrue. In order to
shunt current off,
high series
impedances would
have to exist along
with high stray
shunting capacitance
to areas removed
from the radiator.
Additionally, the
remaining coil area
connected to the top
area of the antenna
above the loading
coil would have to
present a high
impedance to the
area where current
reduction occurs.
This would never be
the case, unless the
top area of the
antenna and loading
coil are not
resonant near the
operating frequency.
A reasonable test
for proper inductor
and system design
would be to remove
the antenna above
the loading coil,
measuring system
resonance. If
resonance does not
change
substantially, the
area above the coil
is not correctly
terminating the
system. Firstorder
self resonance of
the inductor
(parallel
resonance), when
removed from the
system, should also
be far above the
operating frequency
of the system. If
selfresonance comes
within three or four
times the operating
frequency range, the
loading coil almost
certainly will have
needless performance
shortfalls.
Conclusion
A normally
functioning inductor
has essentially
equal currents
throughout the
inductor, loading
coils are no
exception. Any
current difference
requires “missing current”
flow through
undesired stray
capacitances (displacement current) or leakage currents.
In a reasonably
welldesigned
system, current into
and out of the
loading inductor
should be
substantially equal.
Large differences in
current would
indicate excessive
and problematic
undesired stray
capacitance in the
loading coil or antenna system
design.
Reduced
sensitivity to coil
Q is primarily a
function of
additional losses in
the system, not
reduction of current
through the coil.
Also see Loading Inductors
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visits
since February 11,
2004.