Current through coils
 

"Richard Fry" wrote in message
...
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.

But Richard, that is what is meant by "electrical" length, that it has the
same phase shift as a ~90 degree antenna.
--
73, Cecil, W5DXP

Current through coils
 

On Thu, 09 Mar 2006 08:32:44 -0500, 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.


I beg to differ.

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! 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.

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.

Tom has quite adequately addressed this, however, if you go he

http://www.k6mhe.com/n7ws

and look at either Note 1 or 2 and then look at figures 1 and 2 this
might change your mind about the current distribution.

[snip]

Current through coils
 

Cecil Moo
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. (R. Fry quote)

But Richard, that is what is meant by "electrical" length, that it has
the same phase shift as a ~90 degree antenna.
_______________

So the definition of electrical length you use excludes radiation
resistance? That resistance is the only parameter giving any antenna the
ability produce useful EM radiation in a practical antenna system. And that
resistance is a function of the physical properties and configuration of the
radiator with respect to the operating frequency.

Model a short vertical radiator in NEC, and check its impedance. If short
enough, it could be something like 0.1 -j2500 ohms. Now add an inductive
reactance to the system to reach resonance. NEC then will show 0.1 +/-0
ohms. Note that the radiation resistance term did not change.

That short system is resonant, but it certainly won't have the practical
radiation efficiency of a full, 1/4-wave, linear radiator, even though they
both have the same "electrical length" by your definition.

RF

Current through coils
 

On Wed, 8 Mar 2006 23:37:57 -0600, (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!"

A vertical antenna is often driven against a reflecting ground system.
It is desirable that it be self resonant at nearly 1/4-wavelength
(90-degrees), in many instances, to eliminate reactive impedance to
current into the antenna, avoid loading coil loss, and avoid bandwidth
limitation which comes with high-Q coils. Even with its drawbacks, a
base loading coil is often the practical way to resonate a too-short
antenna.

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.

30-degrees is not an inductance value. An inductor is impure because it
has resistance and capacitance in addition to inductance. Also, the
inductance needed to replace the missing 30-degrees of antenna depends
on where it is sited, high, low, or in-between.

Siting affects performance as it determines current distribution along
the antenna.

Where is the myth?

Maybe part of the myth is that the antenna must be resonant to work.
Nothing could be further from the truth.

How about this thought experiment: Assume a too short for resonance
monopole, that has its feedpoint impedance made non-reactive by the
insertion of a "base loading coil." All kinds of arguments, including
this one, arise about what the role of the coil is, what its current
distribution is, how it affects efficiency and so forth.

To minimize these arguments, let's stop calling the inductor a "base
loading coil" and call it part of an "L-network feedpoint matching
network." Now the radiator isn't resonant and the resulting feedpoint
reactance (and resistance) is matched separately with the external
network.

Has the radiator current distribution changed? No. Does the inductor
in the L-network "make up" some number of electrical degrees in the
radiator? Not from my viewpoint.

Current through coils
 

"Richard Fry" wrote in message
...
So the definition of electrical length you use excludes radiation
resistance?

Yes, of course a 60 degree coil obviously doesn't radiate like a 60 degree
piece of wire. The 60 degrees is merely the phase shift that a traveling
wave
current undergoes while traveling through the coil. I hope this is not
just a semantic problem. When someone says a coil replaces 60 degrees
of an antenna, he certainly doesn't mean for radiation purposes (unless
he is an absolute dummy). He simply means the coil causes a 60 degree
phase shift in the forward current, much like a 60 degree length of wire.
And that's all it means.

Model a short vertical radiator in NEC, and check its impedance. If short
enough, it could be something like 0.1 -j2500 ohms. Now add an inductive
reactance to the system to reach resonance. NEC then will show 0.1 +/-0
ohms. Note that the radiation resistance term did not change.

Of course not! Nobody is arguing otherwise. You are obviously confused
about what I, and others, are saying.

That short system is resonant, but it certainly won't have the practical
radiation efficiency of a full, 1/4-wave, linear radiator, even though
they
both have the same "electrical length" by your definition.

Is this a straw man? Nobody has said anything remotely resembling any
argument otherwise. Electrical length doesn't have much to do with
radiation. The radiation resistance and therefore efficiency, is closely
associated with the physical length. Electrical length mainly has to do with
phasor rotation. If a traveling wave current phasor rotates 90 degrees while
flowing through a coil, the coil's electrical length is 90 degrees. That's a
pretty
simple concept. The coil can even be considered to be lossless and non-
radiating in some relatively efficient antenna systems without introducing
much of an error.
--
73, Cecil, W5DXP

Current through coils
 

"Wes Stewart" wrote
if you go he

http://www.k6mhe.com/n7ws

and look at either Note 1 or 2 and then look at figures 1 and 2 this
might change your mind about the current distribution.

Nobody is disputing the current rise through a coil. In fact, I have
been pointing it out. The coil does distort the current away from
the nice cosine envelope of a 1/2WL thin wire dipole.

Your graphs show standing wave current which doesn't flow. (Its
phase angle doesn't rotate.) Therefore, the magnitude of the standing
wave current can be any value depending upon where it is located in
the system. Wes, please take a look at http://www.qsl.net/qrzgif35.gif
to find out why standing wave current can have any value and is thus
unimportant. EZNEC plots the current in much the same way that
you have. So are the EZNEC results wrong and yours right? The
fact is that a standing wave current plot is close to meaningless.
Why are we continuing to discuss standing wave current?

What we need to plot is the forward traveling wave current and
the reflected traveling wave current which are the two components
of your standing wave current graphs. Do you have any simulation
software that will plot the forward current and reflected current?
Nobody is going to understand what is really happening until we
get a plot of those two component waves or at least an estimated
graph of the underlying superposed currents.

In fact, how about your best estimate of a graph of forward and
reflected currents through the coil including phase shifts? Only
then are you likely to understand what we are talking about..
--
73, Cecil, 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.


I beg to differ.

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! 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.

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% ?????????

If you claim Imax @ 98.5% exits the coil and has a value of 96.6% [I2]
at the tip then boundary conditions require total reflection. That
requires a 180 degree phase reversal at the I2 amplitude to satisfy the
boundary condition. Now the reflected current into the top of the coil
is -I3 = 99.6% of -I2. The reflected current exiting the bottom of the
coil is, by your reasoning -I3. The reflected current at the base is
96.6% of -I2, or 93% of Imax.

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.

Obviously, the coil is acting as something more than a simple L. It is
adding and inductive phase shift. The vertical has capacitance to the
local ground. The vertical also has a self inductance!! That self
inducatance is insufficient to complete the 90 phase shift required for
resonance. Therefore, I offer that the loading coil provides the
required additional inductance for resonance.

You can replace the antenna with a box containing a series resistor and
capacitor, and except for the field there's no steady state way to tell
it from an antenna. A physically small inductor such as a toroid will
function exactly the same in both cases. (I limited it to being
physically small, since a larger inductor will interact with the
antenna's field.) So your explanation should work just as well when the
inductor is in series with a simple RC as when it's in series with an
antenna. I can easily write the equations describing the voltage and
current at every part of the resulting RLC circuit, using circuit
analysis techniques which have been around for over a century. You're
saying either that they're wrong, or that you can tell by looking at the
terminals of a black box whether it contains an antenna or a simple RC.
Can you describe the method you'd use, restricting yourself to steady
state measurements, how you'd tell the difference?

Roy Lewallen, W7EL

Current through coils
 

Wes Stewart wrote:

Maybe part of the myth is that the antenna must be resonant to work.
Nothing could be further from the truth.

Has the radiator current distribution changed? No. Does the inductor
in the L-network "make up" some number of electrical degrees in the
radiator? Not from my viewpoint.

I think there are two problems:

1.) Cecil wants everyone to start using reflection wave models to
analyze every antenna system in the world.

2.) Many people think a very short monopole antenna that is resonant is
still 90 electrical degrees long, and that the inductor makes up the
missing number of degrees, and the current taper across that inductor
is some form of sine shaped curve.

Cecil is free to use whatever tools he likes. He doesn't work for me,
and (thank God) I don't have to work for him! What he wants me or
others to do is a moot point.

I am concerned about the commonly held but very incorrect view that
current travels through an inductor turn-by-turn, and that a loading
inductor somehow shifts the phase of and/or level of current to "make
up for missing degrees".

My only concern is people not understanding how an inductor and short
antenna actually behaves. That problem is worth attention.

73 Tom

Current through coils
 

"Wes Stewart" wrote :
Maybe part of the myth is that the antenna must be resonant to work.
Nothing could be further from the truth.

Can we agree that if the feedpoint impedance is purely resistive, then
the antenna is resonant?

How about this thought experiment: Assume a too short for resonance
monopole, that has its feedpoint impedance made non-reactive by the
insertion of a "base loading coil." All kinds of arguments, including
this one, arise about what the role of the coil is, what its current
distribution is, how it affects efficiency and so forth.

To minimize these arguments, let's stop calling the inductor a "base
loading coil" and call it part of an "L-network feedpoint matching
network." Now the radiator isn't resonant and the resulting feedpoint
reactance (and resistance) is matched separately with the external
network.

Has the radiator current distribution changed? No. Does the inductor
in the L-network "make up" some number of electrical degrees in the
radiator? Not from my viewpoint.

This can be easily addressed by looking at a G5RV on 40m. On
40m, the feedpoint impedance at the dipole is about 500+j100 ohms.
The parallel twinlead matching section is about 110 degrees long.
The resonant impedance looking into the matching section is about
27+j0 ohms. So the G5RV matching section has indeed made up
about 110 degrees needed by the antenna *system*. The tuned
matching section has increased the electrical length of the antenna
*system* by 110 degrees so its a pretty good match for coax.. The
same goes for an antenna loading coil no matter where it is located.
If the short whip antenna system needs 60 degrees to be matched
(resonated) the coil provides 60 degrees of phase shift.
That's all we are saying when we say the coil replaces 60
of degrees of an antenna system.

And the basic argument is whether a lumped circuit analysis
can be used on a coil when reflections are present. The answer
is NO!

If an antenna system needs 60 degrees to be resonant, it can be
done in any number of ways. You can give it 60 degrees of
transmission line or you can give it 60 degrees of coil. But please
note that the 60 degrees of phase shift in the loading coil has
no effect on the phase of the standing wave current. The phase
of the standing wave current essentially doesn't change all up
and down a 1/2WL dipole. Why would you expect it to change
in a shortened monopole. The fact that it doesn't changing is
essentially meaningless. It doesn't change whether a coil is
present or not.
--
73, Cecil, W5DXP

Current through coils
 

On Thu, 9 Mar 2006 11:03:24 +0000 (UTC), "Reg Edwards"
wrote:

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

Tesla also experimented with over-the-air transmission of utility
power to homes. He might have needed to know how to use a Smith
Chart, or perhaps he needed a straight-jacket.