Loading Coils; was : Vincent antenna
 

Gene Fuller wrote:
Have you actually read and understood that article? Corum mentions
several times that everything he reduces to the simple formulas applies
only to quarter-wave resonance conditions.

Yes, a mobile 75m bugcatcher antenna is quarter-wave resonant.
It is clear that you have not taken time to understand the
paper. Figure 2 looks just like a loading-coil, stinger, and
top hat which is 1/4WL resonant. Note that the coil is conceptually
replaced with a length of transmission line and that's exactly
how mobile loaded antennas work. Here are the conditions:

At the feedpoint is a piece of transmission line with a Z0 of
4000 ohms and a VF of 0.02 - physical length to be determined.

Attached to that is a piece of transmission line with a Z0 of
400 ohms and a VF of 1.0. This element is 8 feet long.

The frequency of operation is 4.0 MHz. What physical length
of the 4000 ohm line will cause 1/4WL resonance?

If you can solve that problem, you will understand how loaded
mobile antennas work. Hint: the delay through the 4000 ohm
section is NOT 3 nS.

Look at the author's highlight between equations 31 and 32. Look at the
discussion near equation 47. Look at the discussion following equation
60. Read the entire discussion in section 5.

I have done that, Gene. A 75m bugcatcher coil falls within
the specified test conditions and thus the VF equation should
be within ten percent accuracy.

Note that he does not say the characteristic impedance is a constant
that can be deduced from resonance conditions and then applied to
operating conditions. In fact, he says exactly the opposite.

Yes, and I have never stated otherwise. The approach that works
is to take a 1/4WL self-resonant coil and use only a percentage
*at the same frequency*. The VF and Z0 will remain approximately
the same as long as we don't change frequencies.

Here is what can be done. Take a 75m bugcatcher coil and extend
the number of turns until it is self-resonant at 4 MHz indicating
that the coil is 90 degrees long. Measure the VF of the coil at
the 4 MHz self-resonant frequency. Remove those extra turns and
calculate the new electrical length. Hint: That electrical length
will be nowhere near a 3 nS delay (technically impossible).

"It is worth noting that, for a helical anisotropic wave guide, the
effective characteristic impedance is not merely a function of the
geometrical configuration of the conductors (as it would be for lossless
TEM coaxial cables and twin-lead transmission lines), but it is also a
function of the excitation frequency."

That's true - Z0 and VF change with frequency. The solution is
to measure or calculate the Z0 and VF at the chosen frequency
of operation. Problem solved!

I am suspicious of anyone's motives who says he believes in
an impossible 3 nS delay through a huge loading coil while
dismissing an IEEE white paper that suggests otherwise.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Gene Fuller wrote:
Cecil Moore wrote:
Tom Donaly wrote:
What is the characteristic impedance of Tom's coil?

A few thousand ohms. Use equation 50 at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

What's your formula for the velocity factor of Tom's coil? Is it from
the same Tesla coil crackpot you quoted in previous posts?

Do you reject all IEEE white papers? The formula
is equation 32.

Cecil,

Have you actually read and understood that article? Corum mentions
several times that everything he reduces to the simple formulas applies
only to quarter-wave resonance conditions.

Look at the author's highlight between equations 31 and 32. Look at the
discussion near equation 47. Look at the discussion following equation
60. Read the entire discussion in section 5.

Note that he does not say the characteristic impedance is a constant
that can be deduced from resonance conditions and then applied to
operating conditions. In fact, he says exactly the opposite.

"It is worth noting that, for a helical anisotropic wave guide, the
effective characteristic impedance is not merely a function of the
geometrical configuration of the conductors (as it would be for lossless
TEM coaxial cables and twin-lead transmission lines), but it is also a
function of the excitation frequency."

I have no comment on the validity of the Corum analysis. He makes a lot
of approximations and simplifications which may or may not be completely
correct. However, it is clear that you are mis-quoting him.


73,
Gene
W4SZ

The Corum duo model their Tesla coil as "an isotropically conducting
cylindrical boundary." Later, they call it a "helically disposed surface
waveguide." Later, they write, "Further, the Tesla coil passes to a
conventional lumped element inductor as the helix is electrically
shortened." Do the first two quotes resemble a description of a
typical ham antenna loading coil? Has anybody here used a Tesla coil
to load an antenna? The Corums also state in one part of their paper
that their method of analysis is "fraught with danger." Indeed.
Cecil's misuse of the formulas certainly proves that.
Many people over the years have done just fine loading their antennas
with lumped inductors. There's no need to put a "helically disposed
surface waveguide" on a mobile antenna, and if someone thinks that
modeling a coil as "an isotropically conducting cylindrical boundary"
actually turns that coil into an isotropically conducting cylindrical
boundary, that someone should seek help.
73,
Tom Donaly, KA6RUH

Loading Coils; was : Vincent antenna
 

John Smith wrote:
I found it the same in institutions of higher learning--surest way to a
low grade was/is to recognize an instructors mistake(s) ...

The quickest way to get ploinked on this newsgroup
is to catch a guru in a severe technical blunder. The
guru gang then bands together to punish the technically
correct upstart who dares to question their authority.

Trouble is, those reincarnations of Galileo's judges
wind up hoodwinking the naive and uninitiated. "If
________ says it, it must be a fact."
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

On Thu, 29 Nov 2007 16:02:30 GMT, Cecil Moore
wrote:
You and Art seem intent on collecting on a bet, or a debt, or
otherwise mooching validation, because if you two had such
dead-to-rights positions, they wouldn't require exhumation from the
grave to prop the corpses on soap box pedestals as resurrected proof.

On the contrary, Richard, old wives' tales sometimes
die hard. It's like water wearing away a stone.

More mooching validation. You guys could collect more nickels if you
learned to doff your caps instead of engaging in your incessant
muttering.

Loading Coils; was : Vincent antenna
 

Jim Kelley wrote:
Cecil Moore wrote:
W8JI's mistake was using standing wave current to try
to measure that delay.

It's not at all apparent that that was his mistake.

The description of his test setup is on his web page.
Did he use standing-wave current to try to measure the
delay through a loading-coil? Yes, it is obvious that
he did. He should have loaded the circuit with the
characteristic impedance of his loading coil. That
would have reduced the reflections to the point that
the actual delay could be measured.

Lacking any sort of description of the stimulus or
of the instrument, it's not clear to me what W8JI's test unit is
actually measuring.

Second, show how those measurements are supported by the
underlying principles, and are predicted by the associated
mathematics. Without those things, you may as well go shout
it at cars.

But since W8JI's measurements are NOT "supported by the
underlying principles", by your own assertions, he indeed
seems to be "shouting it at cars" on his web page. I am
merely objecting to a technical absurdity, e.g. a 3 nS
delay through a foot long loading coil. The standing-
wave current phase shift through a coil bears no
relationship to the delay through a coil.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Cecil Moore wrote:
Gene Fuller wrote:
Have you actually read and understood that article? Corum mentions
several times that everything he reduces to the simple formulas
applies only to quarter-wave resonance conditions.

Yes, a mobile 75m bugcatcher antenna is quarter-wave resonant.
It is clear that you have not taken time to understand the
paper. Figure 2 looks just like a loading-coil, stinger, and
top hat which is 1/4WL resonant. Note that the coil is conceptually
replaced with a length of transmission line and that's exactly
how mobile loaded antennas work. Here are the conditions:

At the feedpoint is a piece of transmission line with a Z0 of
4000 ohms and a VF of 0.02 - physical length to be determined.

Attached to that is a piece of transmission line with a Z0 of
400 ohms and a VF of 1.0. This element is 8 feet long.

The frequency of operation is 4.0 MHz. What physical length
of the 4000 ohm line will cause 1/4WL resonance?

If you can solve that problem, you will understand how loaded
mobile antennas work. Hint: the delay through the 4000 ohm
section is NOT 3 nS.

Look at the author's highlight between equations 31 and 32. Look at
the discussion near equation 47. Look at the discussion following
equation 60. Read the entire discussion in section 5.

I have done that, Gene. A 75m bugcatcher coil falls within
the specified test conditions and thus the VF equation should
be within ten percent accuracy.

Note that he does not say the characteristic impedance is a constant
that can be deduced from resonance conditions and then applied to
operating conditions. In fact, he says exactly the opposite.

Yes, and I have never stated otherwise. The approach that works
is to take a 1/4WL self-resonant coil and use only a percentage
*at the same frequency*. The VF and Z0 will remain approximately
the same as long as we don't change frequencies.

Here is what can be done. Take a 75m bugcatcher coil and extend
the number of turns until it is self-resonant at 4 MHz indicating
that the coil is 90 degrees long. Measure the VF of the coil at
the 4 MHz self-resonant frequency. Remove those extra turns and
calculate the new electrical length. Hint: That electrical length
will be nowhere near a 3 nS delay (technically impossible).

"It is worth noting that, for a helical anisotropic wave guide, the
effective characteristic impedance is not merely a function of the
geometrical configuration of the conductors (as it would be for
lossless TEM coaxial cables and twin-lead transmission lines), but it
is also a function of the excitation frequency."

That's true - Z0 and VF change with frequency. The solution is
to measure or calculate the Z0 and VF at the chosen frequency
of operation. Problem solved!

I am suspicious of anyone's motives who says he believes in
an impossible 3 nS delay through a huge loading coil while
dismissing an IEEE white paper that suggests otherwise.


Cecil,

First, this is NOT an IEEE white paper. It appears to be a simple
conference proceedings paper.

Second, your analysis is utter rot! Are you suggesting that if the coil
can be made resonant at some frequency, and then you cut it in half,
that it still behaves the same?

Corum does not say anything like that, and you shouldn't either.

Shame on you!

73,
Gene
W4SZ

Loading Coils; was : Vincent antenna
 

Tom Donaly wrote:
Many people over the years have done just fine loading their antennas
with lumped inductors.

That's not the point of this discussion, Tom. The
only question that needs to be answered here is:
Can a 2" dia, 100 T, 10" long loading coil have
a delay of 3 nS through it at 4 MHz? Do you support
such a technical absurdity? The Corum IEEE white
paper suggests that delay is in error by a magnitude.

All of the boundary test conditions given in Corum's
IEEE white paper are satisfied by a 75m bugcatcher
loading coil. There is no reason to believe that
the underlying principles of physics do not apply.
In fact, the diagram of the 1/4WL resonant system
looks exactly like a base loading coil, stinger,
and top hat as is used for 75m mobile operation.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Richard Clark wrote:
More mooching validation. You guys could collect more nickels if you
learned to doff your caps instead of engaging in your incessant
muttering.

The same could have been said of Galileo. Do you
suggest that technical absurdities go unchallenged?
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Gene Fuller wrote:
Second, your analysis is utter rot! Are you suggesting that if the coil
can be made resonant at some frequency, and then you cut it in half,
that it still behaves the same?

No, it behaves approximately like half of the original
coil tending to have approximately the same Z0 and VF as
the original coil. The phase shift through the coil will
tend to be approximately 1/2 of the original phase shift -
not exact because of end effects.

Let's say we have a 1/4WL helical antenna with an obvious
phase shift of 90 degrees. If we cut that helical in half,
it is likely to have a phase shift of approximately 45
degrees, nowhere near the 4.5 degrees that W8JI has
"measured".

If we add a stinger to the above half-coil, we will have
a base-loaded antenna. The phase shift will be relatively
close to 45 degrees at the same frequency. The stinger
contributes another few degrees. The impedance discontinuity
between the coil and stinger contributes the rest of the
90 degrees of electrical length.
--
73, Cecil http://www.w5dxp.com

Loading Coils; was : Vincent antenna
 

Cecil Moore wrote:

The description of his test setup is on his web page.
Did he use standing-wave current to try to measure the
delay through a loading-coil? Yes, it is obvious that
he did.

From what is written there it's not possible to know exactly what he
measured. Why don't you take some measurements yourself if you feel
that strongly about it? And please stop shouting at cars. :-)

The standing-
wave current phase shift through a coil bears no
relationship to the delay through a coil.

I have no idea what 'standing wave current phase shift' is supposed to
mean. Standing waves obviously don't propagate, so naturally there
wouldn't be a propagation delay associated with them. Hopefully you
understand that radiating RF currents don't stand, they travel [1].

73, ac6xg


[1] Contrary to unpopular misconception, the reflected wave doesn't
actually affect the forward wave. Forward and reflected currents both
radiate equally well at every point along a radiator. What we see is
the net result of the fields that are present.