H. Adam Stevens, NQ5H wrote:
We need to add
inductance to slow down the wave so it gets back in phase.

Exactly. The coil provides a delay and a phase shift. A series
stub will accomplish the same thing. In the middle of an
electrical 1/4WL antenna, any delay through the coil ensures
unequal net current at the bottom and top of the coil even
if the coil is lossless and non-radiating.
--
73, Cecil http://www.qsl.net/w5dxp


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Jim Kelley wrote:
It seems that the phase shift you
described earlier would have to cause a change in the standing wave
pattern along the radiator.

It does and that is why it is so difficult to write an equation for
it. There are reflections in both directions at the top and bottom of
the coil in addition to the 100% reflection at the tip of the antenna.
My solution is to get Reg to write a new program. :-)

If the loading coil was at the feedpoint,
then the maximum current would appear only at the feedpoint.

I hesitate to introduce secondary effects before most have understood
the primary effects, but that is not a true statement. Some percentage
of what the other side is saying is true. However, the other side
considers those effects to be supreme when they are only secondary -
but they are NOT negligible secondary effects. The maximum current
in the base-loaded system does not appear at the feedpoint. The
maximum current in a base-loaded system appears inside the coil and
that current is of greater magnitude than the feedpoint current. I'm
sorry to muddy the waters even farther with that tidbit.
--
73, Cecil http://www.qsl.net/w5dxp


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"Jim Kelley" wrote in message
...


H. Adam Stevens, NQ5H wrote:
"Jim Kelley" wrote in message
...
deletia....


I have a question. If a loading coil only makes a physically short
antenna look like it's an electrical quarter wavelength reactively, why
does its position along the radiator make such an apparent difference in
performance?

73, Jim AC6XG



My first reaction is to point out that this was (is?) a question on the
Extra exam.

I think you may be right.

Now how can I explain qualitatively why this is?

Start with the answer to the exam question? :-)

Consider an end-fed wire antenna.
An electromagnetic wave goes through the conduction electrons down to
the
end and reflects back.
At 1/4 wavelength, the reflected wave is exactly in phase with the
source so
the load looks minimal and resistive, loss plus radiation. As the
antenna
gets shorter the radiation resistance gets lower and the reflected wave
gets
back to the feed point sooner (becomes capacitive). We need to add
inductance to slow down the wave so it gets back in phase.

Is that the controversial phase shift?

We cannot, alas,
raise the radiation resistance; this is a short antenna. If I place the
inductor at the feed point all the current must flow through it,
maximizing
loss. If I place it at the top little current flows through it,
minimizing
effectiveness. If I distribute it the antenna's resonance is broader,
but at
what cost? Lower Q. The signal strength is less. So I make the coil as
short
as I can, put it in the middle and it's juuust right.

73, H. NQ5H

Sounds like you're describing a sort of 'current drop'. Is I^2R loss
entirely responsible for this drop? It seems that the phase shift you
described earlier would have to cause a change in the standing wave
pattern along the radiator. If the loading coil was at the feedpoint,
then the maximum current would appear only at the feedpoint. Above the
coil, the currents would be out of phase, as you described, because of
the shortened radiator, and the maximum available current would not flow
along any point on the radiator. Moving the coil higher would allow
maximum current to flow along at least the lower portion of the
radiator. Loss is certainly a factor, but I can't see how it is the
entire explanation for the rather pronounced effect. Hence my question.

73, Jim AC6XG

Hi Jim
Clearly an entire explanation would require a rigorous solution to Maxwell's
equations, but you state it better than I did.
The current below the loading coil is as if the antenna were full length,
max power radiated; the voltage above the loading coil is as if the antenna
were full length. And you're right, moving the coil away from the current
max (feed point) reduces I^2R losses in the coil. Are there E^2/R losses in
the coil if we mount it at the top?
Looks like if we make the boundary conditions at the ends of the antenna as
if it were full-length it works best.
Remember the boundary conditions; the current is max at the feedpoint and
zero at the end (can't go anywhere).
The empirical fact is a lumped L in the center of the antenna works best and
one can "sort of" intuitively see why placing the coil at either end has
problems. Hence I use a 4 foot screwdriver and a 4 foot whip. The antenna at
resonance on 40 and 80 is about 20 ohms which I match with a toroidal
autoformer. Then 2 feet of coax to the TS480HX.
73, H.
NQ5H

H. Adam Stevens, NQ5H wrote:
The current below the loading coil is as if the antenna were full length,
max power radiated; the voltage above the loading coil is as if the antenna
were full length.

That's semi-close but not entirely true. As Tom and Roy say, the coil
indeed does distort the current away from the cosine pattern common in
1/4WL wire antennas. It just doesn't distort the current as much as they
say. But the current at the top of the coil is a greater magnitude than
it would be if your above statement were true. In one case, it is 66%
higher than in a wire antenna.

The current at the top of the coil is greater than it would be for the
same stinger mounted on a physical 1/4WL antenna but it is not equal to
the current at the bottom of the coil. The coil probably causes a larger
phase angle between the voltage and current than exists in a wire antenna.
If theta is small, V*I*cos(theta) can be fixed while V and I become larger
than they are in a wire antenna. There's a lot happening around that coil.

Remember the boundary conditions; the current is max at the feedpoint and
zero at the end (can't go anywhere).

Sorry, the current is not max at the feedpoint. There is a current maximum
point located inside the coil that is a greater magnitude than the feedpoint
current. If we say there is 90 degrees from current max inside the coil to
the tip of the antenna, a center-loaded mobile antenna is longer than 90
degrees. One in particular, calculates out to be 110 degrees long.

The coil causes an impedance discontinuity at each end in the standing-
wave antenna. It is somewhat like the following where the 10k ohm feedline
represents the coil with a Z0=SQRT(L/C):

---600 ohm feedline---+---10k ohm feedline---+---600 ohm feedline---open

One can see that there would be reflections in both directions at the
'+' points. That's why there will probably never be an equation to
represent a mobile antenna.

Here's the key. (Vfor+Vref) must be in phase with (Ifor+Iref) at the
feedpoint for the feedpoint impedance to be purely resistive. But
Vfor doesn't necessarily have to be in phase with Ifor for that to
happen. Neither does Vref necessarily have to be in phase with Iref
for that to happen. The coil has a different phase effect on voltage
than it does on current and the natural Z0 of a vertical antenna is
not fixed since every point on the antenna is a different distance
from ground. It's probably impossible to quantify using equations.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Seems everyone must be suffering from post tramatic syndrome on this one.

Here's the bottom line.
Does a center-loaded mobile antenna have a cosine current distribution? No.

Does just the coil have a cosine current distribution? Close to one but
it may not start at zero degrees like a thin-wire dipole does.

Does displacement current have an effect? Yes, but the effect is
close to the same for the forward current and reflected current.

Do I^2*R losses have an effect? Yes, but the effect is close to
the same for the forward current and reflected current.

Does radiation "loss" have an effect? Yes, but the effect is close
to the same for the forward current and reflected current.

Whatever effects exist that affect the current, the forward current
and reflected current are affected close to equally.

The net current anywhere on the antenna is still the phasor sum of
whatever forward current and reflected current exists at that point.

Any phase shift through the coil is multiplied by two by the two
currents traveling in opposite directions with opposite rotations.

Can one subtract the number of degrees occupied by the conductors
from 90 degrees to get the number of degrees occupied by the coil?
No, it's not that simple.

Is the current at the top of the coil higher than predicted by the
above simple calculation? Yes.

Does the coil occupy zero degrees? No.

If the coil doesn't occupy zero degrees, the net current at the bottom
of a coil and at the top of the coil cannot be the same value when the
coil is located in the middle of an electrical 1/4WL antenna.

Are there special cases where the magnitude of the current at the top
and bottom of a coil can be equal? Of course, but the typical loaded
mobile antenna is not one of those special cases!
--
73, Cecil, W5DXP


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Cec sez,

My solution is to get Reg to write a new program. :-)

=======================================

My patience is wearing thin.

Nevertheless, once more into the breach.

There's no need to write a new program. There's not even any need for an old
one.

Knowledge of the current distribution along a loading coil has no practical
use except to assist with drawing pictures of it in books and magazines.

FOR PRESENT PURPOSES THE LENGTH AND IMPEDANCE OF THE ANTENNA BELOW THE COIL
IS NOT RELEVANT. IT CAN BE CONSIDERED TO BE PART OF THE GENERATOR.

LET THE ANGULAR LENGTH OF THE COIL = THETA DEGREES. OTHERWISE KNOWN AS THE
PHASE SHIFT. THIS IS A FIXED, IMMUTABLE QUANTITY SET BY THE PROPAGATION
VELOCITY = 1 / SQRT( L * C ).

THE MAGNITUDE OF THE CURRENT DISTRIBUTION ALONG THE COIL IS ALWAYS THE SAME
COSINE CURVE BUT WHICH IS TRUNCATED (SLICED OFF) AT VARYING ANGULAR DEGREES
AT EITHER OR BOTH ENDS.

THE ANGLE AT WHICH THE TOP END IS TRUNCATED DEPENDS ON THE INPUT IMPEDANCE
OF THE ANTENNA ABOVE IT. IT IS ALSO RELATED TO Zo OF THE COIL WHICH IS THE
USUAL SQRT( L / C ).

THE ANGLE AT WHICH THE BOTTOM END IS TRUNCATED IS ALWAYS THE TOP ANGLE MINUS
THETA. IT CANNOT BE ANYTHING ELSE.

THUS, WHEN THE TOP END IS OPEN CIRCUIT AND THETA = 90 DEGREES WE HAVE A
COMPLETE 1/4-CYCLE OF A COSINE CURVE - A 1/4-WAVE RESONANT HELICAL
ANTENNA.

The foregoing applies to both short, fat coils and long, thin coils,
close-wound or stretched-out.

Coil resistance is the uniformly-distributed radiation resistance plus
conductor resistance.

For useful calculations such as Q, bandwidth, efficiency, etc., you can
forget all about bewildering reflections, standing waves, forward and
reflected power and use the well-known classical transmission line formulae,
the everyday tools of all good engineers.
The final wanted characteristic, the radiation pattern, can be found with
number-crunching EZNEC-type computer programs which work in an entirely
different manner.
----
Reg, G4FGQ

Reg Says:

Knowledge of the current distribution along a loading coil has no practical
use except to assist with drawing pictures of it in books and magazines.

WROOOONG!

Have you ever tried antenna shootouts? Cecil can enlighten you about the
difference in efficiency and signal levels radiated by various configurations
(bottom, center, top loading).
The efficiency is proportional to the AREA under the current curve on the
loaded radiator. That is dependent on the position of the loading element
within the radiator. That also depends on the current distribution (drop :-)
across the coil. Use that in the loaded parasitic element beams and the effect
is magnified.
So obvious, but hard to swallow for Rauchians?

Viva Bush!!! Sayonara sKerry botoxed flipflopping lying girlie man.

Yuri, K3BU.us

Reg
Your patience is that of a saint.
H.
"Reg Edwards" wrote in message
...
Cec sez,

My solution is to get Reg to write a new program. :-)

=======================================

My patience is wearing thin.

Nevertheless, once more into the breach.

There's no need to write a new program. There's not even any need for an
old
one.

Knowledge of the current distribution along a loading coil has no
practical
use except to assist with drawing pictures of it in books and magazines.

FOR PRESENT PURPOSES THE LENGTH AND IMPEDANCE OF THE ANTENNA BELOW THE
COIL
IS NOT RELEVANT. IT CAN BE CONSIDERED TO BE PART OF THE GENERATOR.

LET THE ANGULAR LENGTH OF THE COIL = THETA DEGREES. OTHERWISE KNOWN AS
THE
PHASE SHIFT. THIS IS A FIXED, IMMUTABLE QUANTITY SET BY THE PROPAGATION
VELOCITY = 1 / SQRT( L * C ).

THE MAGNITUDE OF THE CURRENT DISTRIBUTION ALONG THE COIL IS ALWAYS THE
SAME
COSINE CURVE BUT WHICH IS TRUNCATED (SLICED OFF) AT VARYING ANGULAR
DEGREES
AT EITHER OR BOTH ENDS.

THE ANGLE AT WHICH THE TOP END IS TRUNCATED DEPENDS ON THE INPUT IMPEDANCE
OF THE ANTENNA ABOVE IT. IT IS ALSO RELATED TO Zo OF THE COIL WHICH IS
THE
USUAL SQRT( L / C ).

THE ANGLE AT WHICH THE BOTTOM END IS TRUNCATED IS ALWAYS THE TOP ANGLE
MINUS
THETA. IT CANNOT BE ANYTHING ELSE.

THUS, WHEN THE TOP END IS OPEN CIRCUIT AND THETA = 90 DEGREES WE HAVE A
COMPLETE 1/4-CYCLE OF A COSINE CURVE - A 1/4-WAVE RESONANT HELICAL
ANTENNA.

The foregoing applies to both short, fat coils and long, thin coils,
close-wound or stretched-out.

Coil resistance is the uniformly-distributed radiation resistance plus
conductor resistance.

For useful calculations such as Q, bandwidth, efficiency, etc., you can
forget all about bewildering reflections, standing waves, forward and
reflected power and use the well-known classical transmission line
formulae,
the everyday tools of all good engineers.
The final wanted characteristic, the radiation pattern, can be found with
number-crunching EZNEC-type computer programs which work in an entirely
different manner.
----
Reg, G4FGQ

Yuri Blanarovich wrote:
Reg Says:

Knowledge of the current distribution along a loading coil has no practical
use except to assist with drawing pictures of it in books and magazines.


WROOOONG!

Have you ever tried antenna shootouts? Cecil can enlighten you about the
difference in efficiency and signal levels radiated by various configurations
(bottom, center, top loading).
The efficiency is proportional to the AREA under the current curve on the
loaded radiator. That is dependent on the position of the loading element
within the radiator. That also depends on the current distribution (drop :-)
across the coil. Use that in the loaded parasitic element beams and the effect
is magnified.
So obvious, but hard to swallow for Rauchians?

Viva Bush!!! Sayonara sKerry botoxed flipflopping lying girlie man.

Yuri, K3BU.us

If you're really interested in the "AREA under the current curve,"
you'll have to figure out how to make an efficient, continuously loaded,
short antenna. You'll find, though, that the difference between a
continuously loaded antenna and an antenna with the loading coil,
say, halfway up from the feedpoint won't amount to a hill of beans.
There's still no such thing as a "current drop."
73,
Tom Donaly, KA6RUH

Reg Edwards wrote:

Cec sez,


My solution is to get Reg to write a new program. :-)


=======================================

My patience is wearing thin.

Nevertheless, once more into the breach.

There's no need to write a new program. There's not even any need for an old
one.

Knowledge of the current distribution along a loading coil has no practical
use except to assist with drawing pictures of it in books and magazines.

FOR PRESENT PURPOSES THE LENGTH AND IMPEDANCE OF THE ANTENNA BELOW THE COIL
IS NOT RELEVANT. IT CAN BE CONSIDERED TO BE PART OF THE GENERATOR.

LET THE ANGULAR LENGTH OF THE COIL = THETA DEGREES. OTHERWISE KNOWN AS THE
PHASE SHIFT. THIS IS A FIXED, IMMUTABLE QUANTITY SET BY THE PROPAGATION
VELOCITY = 1 / SQRT( L * C ).

THE MAGNITUDE OF THE CURRENT DISTRIBUTION ALONG THE COIL IS ALWAYS THE SAME
COSINE CURVE BUT WHICH IS TRUNCATED (SLICED OFF) AT VARYING ANGULAR DEGREES
AT EITHER OR BOTH ENDS.

THE ANGLE AT WHICH THE TOP END IS TRUNCATED DEPENDS ON THE INPUT IMPEDANCE
OF THE ANTENNA ABOVE IT. IT IS ALSO RELATED TO Zo OF THE COIL WHICH IS THE
USUAL SQRT( L / C ).

THE ANGLE AT WHICH THE BOTTOM END IS TRUNCATED IS ALWAYS THE TOP ANGLE MINUS
THETA. IT CANNOT BE ANYTHING ELSE.

THUS, WHEN THE TOP END IS OPEN CIRCUIT AND THETA = 90 DEGREES WE HAVE A
COMPLETE 1/4-CYCLE OF A COSINE CURVE - A 1/4-WAVE RESONANT HELICAL
ANTENNA.

The foregoing applies to both short, fat coils and long, thin coils,
close-wound or stretched-out.

Coil resistance is the uniformly-distributed radiation resistance plus
conductor resistance.

For useful calculations such as Q, bandwidth, efficiency, etc., you can
forget all about bewildering reflections, standing waves, forward and
reflected power and use the well-known classical transmission line formulae,
the everyday tools of all good engineers.
The final wanted characteristic, the radiation pattern, can be found with
number-crunching EZNEC-type computer programs which work in an entirely
different manner.
----
Reg, G4FGQ




For a guy who is always excoriating people for believing old wives
tales, you've just told a whopper, Reg. It's true, you might get an
answer using the above theory that is "good enough for who it's for,"
but as an expression of what's actually happening in the antenna, it's
hopelessly simplistic.
73,
KA6RUH