Current through coils
 

On Wed, 8 Mar 2006 23:37:57 -0600, (Richard
Harrison) wrote:

Where is the myth?

Hi Richard,

From "The Man Who Shot Liberty Valance:"
"This is the west, sir. When the legend becomes fact,
print the legend."

73's
Richard Clark, KB7QHC
(coulda' been worse, I just finished watching 2 hours of Monty Python)

Current through coils
 

"Cecil Moore" wrote
Reg Edwards wrote:
Has it ever occurred to you guys that a coil is a coil wherever it
is
used and always behaves in the same way.

The coil always behaves in the same way. Unfortunately, the models
used to explain the operation of the coil don't work in the same
way. Please see:

http://www.ttr.com/corum/index.htm
========================================

Cecil, thanks, I speed-read your extensive pointer.

The whole thing could be summarised in one short sentence -

"Coils are distributed transmission lines."

The same general equations apply to coils of all dimensions, for any
number of turns, at all frequencies, in all applications. There's no
need to unnecessarily complicate things by artificially dividing them
into lumped and other varieties.

I had to skip over the bit about Smith Charts. As you are aware I
don't know how to use one.

It seems Tesla, with HIS coils, knew what it was all about and he
couldn't use a Smith Chart either.
----
Reg.

Current through coils
 

Richard Harrison wrote:
Tom, W8JI wrote:
"Thinking the inductor or loading coil represents 60 degrees of
electrical length is EXACTLY where the big myth is at and it can easily
be proven to be a myth!"

Suppose the vertical is only 2/3 the height needed for self resonance,
or 60-degrees high. The loading coil must replace about 30-degrees of
missing antenna to bring the vertical to resonance.

The myth is in thinking your 60-degree vertical is 90 degree resonant
by virtue of an inductor "replaces about 30-degrees of missing
antenna".

It does not do that.

Every real-world inductor behave to some extent as a transmission line,
but unless the inductor is spatially large in terms of wavelength or
unless the inductor has a termination impedance significantly higher
than the shunting impedance caused by its own self-capacitance to the
outside world the "transmission line" or radiation mode effects are
negligible.

The ultimate in misunderstanding is when people think the loading
inductor replaces a missing electrical degree length, rather than
understanding a reasonably compact inductor primarily inserts a
reactance that compensates for the capacitive reactance of the antenna
system. Does it have some other behavior based on the fact it is less
than perfect? Of course. No one is saying it doesn't have some limited
effects!

If I have a 10 degree tall base loaded antenna it is a ten degree tall
antenna. It is NOT 90 degree resonant antenna with "80 degrees of
missing length" in the inductor, nor with that 80 degree long inductor
behave like 80 degrees of antenna length would.

The very incorrect concept that an inductor and loaded antenna acts as
a "80 degree coil and ten degree antenna" (substitute the numbers you
want) is what kicked this whole thing off several years ago.

73 Tom

Current through coils
 

"Amos Keag" wrote
My 60 meter mobile antenna is 90 degrees long, 1/4 wavelength resonant at
18 +j0 ohms [MFJ analyzer]. etc
_____________

But how much of that resistance term is the true radiation resistance of
your short radiator, and how much is contributed by the coil and "ground
plane" losses?

Resonating an electrically short radiator with a loading coil doesn't change
the radiation resistance of the short vertical antenna (whip) itself -- and
the r-f current able to flow in that whip is the source of practically all
of the useful radiation that system can produce.

The loading coil makes it possible for a practical transmitter to deliver
r-f power into that short antenna system, but doesn't change the fact that
it is only the short radiator itself that provides the useful radiation.
The system may have the net reactance of a resonant, 1/4-wave vertical, but
it will still have the radiation resistance of the original, short radiator,
and that radiation resistance is the primary determinant of system radiation
efficiency when using these coil-loaded antennas.

RF

Current through coils
 

Ian White GM3SEK wrote:
. . .
It is true that we agree on a lot of things, but there's only one reason
for that: because physical reality is the same in Oregon, England and
Scotland as it is in Georgia. There is a very high probability that it's
the same in Texas too.

No, I really think there's some kind of reality vortex in Texas. Adding
to the already overwhelming evidence is the recent news that Tom DeLay
overwhelmingly won the primary election so is well on the way to
re-election. Reality is quite obviously different there.

Roy Lewallen, W7EL

Current through coils
 

Current through coils
 

Richard Harrison wrote:
Suppose the vertical is only 2/3 the height needed for self resonance,
or 60-degrees high. The loading coil must replace about 30-degrees of
missing antenna to bring the vertical to resonance.

Here's the reason why. In a standing wave antenna,
like a mobile antenna, the forward current and the reflected
current are 180 degrees out of phase at the tip of the antenna
and their phasor sum is zero. That is one of the laws of reflection
physics. Their phasors are rotating in opposite directions so they
must each rotate 90 degrees to be in phase and additive at the
feedpoint. That's simple geometry. Your above antenna loading coil
must contribute ~60 deg to the rotation of the forward and reflected
currents. There is nothing else existing in the antenna that can
accomplish that absolutely necessary function.

Where is the myth?

The lumped circuit model is generating a logical blunder. That model
presupposes no phase shift through an inductance. Therefore, there
is no phase shift through an inductance. (Circular Logic) Fortunately,
the coil ignores our mis-applied man-made model and performs the phase
shift anyway. The actual myth is that there is no phase shift through
a 75m bugcatcher coil. That myth is rampant on ham radio web pages.

The lumped circuit model assumes the proof before a solution to
the problem is even attempted. It is a common mistake, known as
'petitio principii', in the solution of logic problems (also known
as 'begging the question').

Some folks have simply forgotten that the lumped circuit model
presupposes that no reflections exist. It cannot be used in a
standing wave antenna environment. Now that these folk have
been reminded of the technical facts, it will be interesting to
observe the results. Will the erroneous web page information be
corrected?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

Reg Edwards wrote:
I had to skip over the bit about Smith Charts. As you are aware I
don't know how to use one.

The basics are really easy. Do you want to learn?
--
73, Cecil http://www.qsl.net/w5dxp

Current through coils
 

David Shrader wrote:
wrote:

SNIPPED



If I have a 10 degree tall base loaded antenna it is a ten degree tall
antenna. It is NOT 90 degree resonant antenna with "80 degrees of
missing length" in the inductor, nor with that 80 degree long inductor
behave like 80 degrees of antenna length would.

If I have a 15 degree long physical antenna, center loaded at 10
degrees, with 5 degrees above the coil I do have a 15 degree physical
antenna. That does not mean the antenna is NOT 90 degrees elctrically
long!

If the antenna is 15 degrees long, it is 15 degrees long. I'm not even
sure "90 degree resonant" is a viable phrase, or what it means.

Can I have a series tuned circuit that is "90 degree resonant"? Is the
radiation resistance the same as a 90 degree tall Marconi? Is the
current distribution the same?

If we go to twice the antenna's frequency is it 180 degree resonant?
Does it have a current maximum in the center and HV at each end?

This is as bad as the MFJ 20 foot tall vertical half-wave that covers
80 through 2 meters.


Resonance requires that the reactive components cancel both in
amplitude and phase! Each reactive component introduces phase shift into
the system. The antenna, without a loading coil, is composed of three
terms: resistance [radiation and loss], self capacitance, and self
inductance. In a shortened antenna the self capacitance dominates and
the resultant phase shift is NOT zero. It is required to add inductance
to achieve resonance [phase shift = 0]. If an antenna is electrically 15
degrees long and the self inducance does not reduce the reactive phase
shift to zero PHASE SHIFT MUST BE ADDED TO THE ANTENNA for resonance.
This phase shift is accomplished by the loading coil.

So what? No one disagrees with that. The series loading inductor
corrects power factor, or brings voltage back in pahse with current.
Nothing I've written or read disagrees with that.

Now, when that antenna is fed with 1 ampere [Imax] at the base of the
antenna and the feed current follows a cosine distribution to the base
of the coil [I = Imax*cos(theta)][theta=10], you claim that the current
exiting the coil is also Imax*cos(theta), or 98.5% of max value.

However, if we start with zero current at the tip, a valid initial
condition, and let current increase by a sine function then I =
Imax*sin(theta1][theta1 = 5 degrees] The result is simply 9% of max
value. There seems to be a disconnect here. 98.5% = 9% ?????????

That's because you have failed to realize the antenna has a triangular
distrubution ABOVE the coil. I suggest you look at this link:

http://www.w8ji.com/Topload_Rad_res.bmp


If I understand you correctly, then the measured value at the base of
the 15 degree antenna is NOT 1 ampere but only 0.07 amperes.

Your misunderstanding appears to be rooted in thinking the antenna
above the loading reactor has a sine distribution. It does not. It is
basically triangular.

Unless distribution is correctly visualized, conclusions will be
flawed.

It take a certain number of ampere-feet to radiate a given amount of
power as EM radiation. When the spatial distance of the radiating
element is reduced, the current has to increase. This is just another
way of saying radiation resistance decreases, and current increases.


Therefore, I offer that the loading coil provides the
required additional inductance for resonance.

I'll agree with that. It corrects power factor. It forms a series
resonant circuit with the capacitance presented by the antenna above
the inductor.

The only thing you are missing is the current curves are nowhere like
you assume in the whip above the coil. It's very well documented in
hundreds of peer reviewed engineering texts that current takes on a
triangular distribution in a very short radiator, and that the
intetgrated current over the length of that radiator for a given
radiated power always equals the same ampere-feet.

73 Tom

Current through coils
 

"Cecil Moore" wrote
It is physically 10 degrees tall. The physical height is what determines
the radiation pattern. It is electrically 90 degrees long. The electrical
length is what determines the feedpoint impedance. Where does that 90
degrees of electrical length come from if not partially from the coil?
________________

The intrinsic impedance of the radiator consists of its radiation resistance
and its reactance. The proper loading coil can provide a net reactance at
the feedpoint of the antenna system of ~ zero ohms, but that hasn't changed
the radiation resistance of that short radiator.

So the coil-loaded, short radiator is not really "electrically" ~90 degrees
long -- it just has the reactance of an antenna that is ~90 degrees long.
The coil-loaded, short radiator itself still doesn't have the radiation
resistance of linear, 1/4-wave antenna. That remains a function of the
electrical length of the whip, itself, apart from the effects of the coil.

RF