Tom Bruhns wrote:
Cecil, I'm sorry you don't understand that in the presence of
time-varying fields, the potential between two points depends on the
path you take.

I know that, Tom, but we are talking about measuring the RF voltage
between two copper wires one inch apart. The path is well defined.
It is a no brainer. There is no need for obfuscation. The measurement
proves the voltages at the ends of a dipole to be at least a magnitude
higher than the voltage at the feedpoint. Are you not aware of how
the ratio of voltage to current varies over 1/4WL of a wire antenna?
--
73, Cecil http://www.qsl.net/w5dxp



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Tom Bruhns wrote:
Cecil, I'm sorry you don't understand that in the presence of
time-varying fields, the potential between two points depends on the
path you take.

I know that, Tom, but we are talking about measuring the RF voltage
between two copper wires one inch apart. The path is well defined.

That's one path.

It is a no brainer. There is no need for obfuscation.

Universally recognized principles of electromagnetics are obfuscation?

The measurement
proves the voltages at the ends of a dipole to be at least a magnitude
higher than the voltage at the feedpoint.

You changed the geometry. But even if you hadn't, you might be
able to say the changing electrical fields are greater at the ends of
a dipole, but not the voltages, because the voltages
aren't uniquely defined.

Are you not aware of how
the ratio of voltage to current varies over 1/4WL of a wire antenna
--
73, Cecil http://www.qsl.net/w5dxp


There's not much point in arguing with you Cecil, since you don't want
to countenance the more sophisticated ideas of some of the other
posters to explain what's going on at the ends of a dipole. That's
too bad. You'll give some people the impression that things are
as simple as you say they are when things are not simple
at all. If they were, even an old hick like me could become an
engineer, and the job wouldn't pay much at all.
73,
Tom Donaly, KA6RUH

Tdonaly wrote:
Universally recognized principles of electromagnetics are obfuscation?

Complicating a simple measurement task beyond belief is obfuscation.

You changed the geometry. But even if you hadn't, you might be
able to say the changing electrical fields are greater at the ends of
a dipole, but not the voltages, because the voltages
aren't uniquely defined.

When a dipole is bent into an open loop, the relative voltage between
the ends is uniquely defined just like the voltage across a transmission
line is uniquely defined. If I poke two wires through two holes in a
faraday cage and ask you to measure the 10 MHz voltage between them
with 10% accuracy, would you say it can't be done?

You'll give some people the impression that things are
as simple as you say they are when things are not simple
at all.

The measurement may be extremely challenging, but the *concepts* are simple.
All you need to do is note the similarity of the transmission line impedances
on an SWR circle to a wire antenna. If the spacing on a transmission line is
an appreciable percentage of a wavelength, the transmission line will radiate.
That's all a center-fed wire antenna is - a transmission line with large
spacing between the conductors and it radiates.
--
73, Cecil http://www.qsl.net/w5dxp



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Here's an interesting quote from _Transmission Lines, Antennas, and Wave
Guides_, by King, Mimno, and Wing:

"The amplitude of the current is not the same at different points along
a conductor, because electric charge is deposited all along the surface
of the conductor. Superficially it may appear that an antenna consisting
of a straight conductor that is an appreciable fraction of a wavelength
long and with a generator at its center may be looked upon simply as an
open-end transmission line with the parallel conductors bent to lie
along the same axis instead of being parallel to each other. Although
there is considerable similarity between the two cases from the point of
view of the approximate distribution of current, *they are nevertheless
fundamentally different*. The transmission line may be analyzed to a
good approximation in terms of ordinary electric-circuit theory, because
equal and opposite currents are very close together. This is not true of
the antenna and ordinary electric-circuit theory cannot be applied. *It
is fundamentally incorrect to treat a center-driven antenna as though it
were the bent-open ends of a two-wire line.*

Circuits that satisfy the condition for the near zone, either because
they are sufficiently small or because they have equal and opposite
currents everywhere so close together that the currents in widely
separated parts of the circuit exert a negligible effect on one another,
are analyzed correctly by the methods of ordinary electric-circuit
theory. All other circuits must be investigated in terms of
electromagnetism. This nearly always involves a study of the
electromagnetic field as a useful intermediate step in determining
distributions of current and charge."

The above text surrounded by asterisks is printed in italics. The quote
begins on p. 85 of the paperback 1965 Dover reprint. I highly recommend
reading Chapter II, of which the quote is a part.

Roy Lewallen, W7EL

Cecil Moore wrote:
Tdonaly wrote:

Universally recognized principles of electromagnetics are obfuscation?


Complicating a simple measurement task beyond belief is obfuscation.

You changed the geometry. But even if you hadn't, you might be able to
say the changing electrical fields are greater at the ends of a
dipole, but not the voltages, because the voltages aren't uniquely
defined.


When a dipole is bent into an open loop, the relative voltage between
the ends is uniquely defined just like the voltage across a transmission
line is uniquely defined. If I poke two wires through two holes in a
faraday cage and ask you to measure the 10 MHz voltage between them
with 10% accuracy, would you say it can't be done?

You'll give some people the impression that things are as simple as
you say they are when things are not simple
at all.


The measurement may be extremely challenging, but the *concepts* are
simple.
All you need to do is note the similarity of the transmission line
impedances
on an SWR circle to a wire antenna. If the spacing on a transmission
line is
an appreciable percentage of a wavelength, the transmission line will
radiate.
That's all a center-fed wire antenna is - a transmission line with large
spacing between the conductors and it radiates.

Roy Lewallen wrote:
Here's an interesting quote from _Transmission Lines, Antennas, and Wave
Guides_, by King, Mimno, and Wing: *It
is fundamentally incorrect to treat a center-driven antenna as though it
were the bent-open ends of a two-wire line.*

Funny, I thought Maxwell's equations worked for either case. Did you also
know that *It is fundamentally incorrect to treat a wolf like a dog.*

If you slowly increase the spacing and angle between the two conductors
of a transmission line, at exactly what spacing and angle does it magically
cease being a transmission line and become an antenna requiring a completely
different treatment? Please be specific as to the exact spacing and angle at
which it becomes "fundamentally incorrect" to treat the configuration as a
transmission line. Incidentally, I don't usually use circuit theory for
transmission lines.

Your current distribution curve displayed by EZNEC for a 1/2WL dipole
looks exactly like Fig 1, page 2-2, ARRL Antenna Book, 15th edition. I thought
you or Walt probably wrote that section. If you had displayed both the current
and the voltage distribution in EZNEC, what would it look like for a 1/2WL dipole?

We can easily measure the impedance at any point along a 1/2WL section of
transmission line with reflections or the feedpoint impedance at any point
along a 1/2WL dipole. As you know, those impedances are the ratio of net
voltage to net current. At the impedance minimum on a transmission line,
the voltage is minimum and the current is maximum. Same for a feedpoint
impedance minimum on an antenna. At the impedance maximum on a transmission
line, the voltage is maximum and the current is minimum. Same for a feedpoint
impedance maximum on an antenna. We can deduce the ratio of the voltage to
the current from the feedpoint impedance - or is that one of the rules of
physics that a transmission line obeys but an antenna disobeys?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
Roy Lewallen wrote:

Here's an interesting quote from _Transmission Lines, Antennas, and
Wave Guides_, by King, Mimno, and Wing: *It is fundamentally incorrect
to treat a center-driven antenna as though it were the bent-open ends
of a two-wire line.*


Funny, I thought Maxwell's equations worked for either case. Did you also
know that *It is fundamentally incorrect to treat a wolf like a dog.*

If you slowly increase the spacing and angle between the two conductors
of a transmission line, at exactly what spacing and angle does it magically
cease being a transmission line and become an antenna requiring a
completely
different treatment? Please be specific as to the exact spacing and
angle at
which it becomes "fundamentally incorrect" to treat the configuration as a
transmission line. Incidentally, I don't usually use circuit theory for
transmission lines.

That question is a lot like asking for the exact speed an object has to
be moving before non-relativistic becomes invalid, or how small before
quantum theory has to be used.

I maintain that the authors of that book know more than you do about the
topic by at least an order of magnitude -- more likely about three. If
you really want to know the answer to your silly question, you should
study what they've written and try to understand it, rather than posting
it as a question to me on this newsgroup.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:
I maintain that the authors of that book know more than you do about the
topic by at least an order of magnitude -- more likely about three.

That doesn't prove you understand what they said.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
I maintain that the authors of that book know more than you do about the
topic by at least an order of magnitude -- more likely about three. If
you really want to know the answer to your silly question, you should
study what they've written and try to understand it, rather than posting
it as a question to me on this newsgroup.

Hmmmmmm, perhaps you and your authors would understand the subject better
by reading and understanding a good book on quantum electrodymanics. You
will not fully understand fields until you accept the fact that photons
can have four, not two, polarizations. I maintain that the developers of
quantum electro-dynamics know more than you and your authors combined.
You have earlier rejected the latest physics theories. I doubt that you
will understand fields until you fully understand virtual photons.

(See, two can play your silly argumentum ad verecundiam game.) :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote in message ...
Here's an interesting quote from _Transmission Lines, Antennas, and Wave
Guides_, by King, Mimno, and Wing:

"The amplitude of the current is not the same at different points along
a conductor, because electric charge is deposited all along the surface
of the conductor. Superficially it may appear ...

Here's another interesting quote from exactly the same source, page
71, the very first two paragraphs of King's "Antennas" chapter.

CHAPTER II
ANTENNAS

Electric Circuit Theory and Electromagnetic Theory. -- In order to
understand the behavior of antennas and of electric circuits at
ultra-high frequencies, it is essential to recognize that phenomena of
a vastly more general nature are involved than are encountered in
conventional electric networks. Attention is seldom called to the fact
that electric-circuit theory which proceeds from Kirchhoff's laws is a
highly specialized form of a more general theory. In some respects,
the situation is like that in mechanics, in which the simple law of
gravitation due to Newton may be looked upon as a special case of a
more general law formulated in the theory of relativity. Much as
Newtonian mechanics is adequate for the mechanical engineer, ordinary
electric-circuit theory is accurate for the requirements of electrical
power engineering and for many requirements in communications. But
even as Newton's laws of motion are inadequate in dealing with atomic
phenomena and some astronomical problems, so ordinary electric-circuit
theory fails when applied to antennas and to most circuits that are to
be used at ultra-high frequencies. The reason is that the conditions
that limit the generality of Newton's laws on the one hand, or the
theorems of electric-circuit theory on the other, are not satisfied.
For those who have assumed that Kirchhoff's laws are perfectly
general, a series of surprises is in store. They may, in fact, feel
like Alice when the Red Queen was annoyed by her reluctance to believe
"six impossible things before breakfast." But presently they may
return through the Looking Glass and discover that they have been
living in the one-dimensional Wonderland of electric-circuit theory
and that Nature is as simple as this suggests only in sufficiently
small spaces.

It is difficult to understand the structure of general
electromagnetism without first learning the appropriate symbolism,
that of mathematics. But if one is willing to accept some things on
faith and to meet others with an open, perhaps even an adventurous
mind, a degree of familiarity with many electromagnetic phenomena can
be acquired from a qualitative discussion.