Richard, it's only a model - and it WORKS very well.

"If you know of a better hole then go to it." - Bruce Bainsfather,
newspaper cartoonist, caption of a cartoon in the trenches, mud and
shrapnel, Belgium, WW1. ;o)
----
Yours, Reg.
=============================


"Richard Harrison" wrote in message
...
Reg Edwards, G4FGQ wrote:
"It has a uniformly distributed radiation resistance according to the
length of the coil form."

That may be the case of "treating" radiation resistance as if uniformly
distributed. Radiation resistance may be defined as the resistance which
would take the same power as that radiated when placed at the
high-current point of the antenna.

While a transmission line of uniform cross section may have uniform
inductance and capacitance per unit length, it is unlikely that an
antenna has uniform capacitance per unit length. Electric field lines of
force have a varying concentration along equal small segments of wire
length. It`s usually a function of distance between wires and this
varies in an antenna because the antenna is meant to radiate.

Variation of capacitance along an antenna causes a variation in surge
impedance along the antenna, but a useful average can be used for
calculations. Straight wire or coiled as in a rubber ducky, an antenna
is subject to this variation in capacitance and surge impedance.

Best regards, Richard Harrison, KB5WZI

(Rhetorical Question)
Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?
===============================

(Rhetorical Answer)
It can't. Are you trying to insinuate I've ever said that it could?

But I'm pleased to see you are getting nearer to understanding how to model
a loading coil in an antenna wire. After all, the concept is simplicitly
itself to any electrical engineer worth his salt. And it's intuitively
obvious it's the only way of going about the job. Your 'cosine formula'
automatically comes out in the wash. As Holmes said to Watson. ;o)

Incidentally, from memory, in the 1950's, in the IEEE "Reference Data for
Radio Engineers" there was an article on short helically-wound antennas,
giving number of turns for a given height of 1/4-wave resonance. The author
was on the right track. But there were errors in the formulae which had not
been arrived at by analytical means.
----
Reg, G4FGQ

Reg Edwards wrote:
(Rhetorical Question)

Given that the coil section resembles a high-Zo transmission line with
reflections, how can the current into the coil section be equal in
magnitude and phase to the current out of the coil section?

(Rhetorical Answer)
It can't. Are you trying to insinuate I've ever said that it could?

Nope, but others have asserted such. What would be a ballpark figure
for the Z0 of a bugcatcher loading coil?

Incidentally, from memory, in the 1950's, in the IEEE "Reference Data for
Radio Engineers" there was an article on short helically-wound antennas,
giving number of turns for a given height of 1/4-wave resonance. The author
was on the right track. But there were errors in the formulae which had not
been arrived at by analytical means.

I just happen to have a 1957 copy from college days. Interestingly, it has
a velocity of propagation formula for the helix that includes axial velocity.
Didn't someone say axial velocity doesn't control the speed of propagation
for a loading coil?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil wrote,

Strangely enough, that won't do it, Jim. A coil equivalent to 1/2WL
reverses the phase of the current such that current is flowing into
both ends at the same time. According to some gurus, that violates
Kirchhoff's laws. But Kraus shows how phase-reversing coils are
used in collinear arrays. Reckon Kraus knows he is violating Kirchhoff's
laws?

For a real-world air-core coil to exhibit identical currents at the
input and output, it would have to be one wavelength long, lossless,
and non-radiating. That's a pretty tall order just to support an
old wives' tale. :-)
--
73, Cecil http://www.qsl.net/w5dxp



You need to go back to school, Cecil. Anyone can stick a coil -
or a capacitor for that matter - between two similar antiresonant circuits
and find a frequency where the two circuits are in phase without resorting
to calling the coil - or capacitor - 1/2WL. There's also a frequency where
the two circuits are 180 deg. out of phase, but current isn't flowing into
two ends of the coil - or capacitor - at the same time. I hope
there's no on this newsgroup gullible enough to take your
fractured circuit theory seriously.
73,
Tom Donaly, KA6RUH

Jim Kelly wrote,


Cecil Moore wrote:

Jim Kelley wrote:
Maybe it's a half wavelength long? ;-)

For a real-world air-core coil to exhibit identical currents at the
input and output, it would have to be one wavelength long, lossless,
and non-radiating. That's a pretty tall order just to support an
old wives' tale. :-)

I happily stand corrected.

73 de jk



Pathetic.
73,
Tom Donaly KA6RUH

Tdonaly wrote:
There's also a frequency where
the two circuits are 180 deg. out of phase, but current isn't flowing into
two ends of the coil - or capacitor - at the same time. I hope
there's no one on this newsgroup gullible enough to take your
fractured circuit theory seriously.

Kraus says: "It is generally assumed that the current distribution
of an infinitesimally thin antenna is sinusoidal, and that the phase
is constant over a 1/2WL interval, changing abruptly by 180 degrees
between intervals."

If I locate a loading-coil such that the phase change node is
in the middle of the coil, current-in will be 180 degrees out of
phase with current-out. In a two-terminal series circuit, if the
net current-in is 180 degrees out of phase with the net current-out,
those two currents are either flowing into the coil at the same time
or flowing out of the coil at the same time 1/2 cycle later.

Consider a 1.5WL helical dipole. The two net currents at 0.4WL and 0.6WL
are flowing in opposite directions.

Consider something even more bizarre. If the coil is exactly 1/2WL and
each end is located at a current node, assuming the forward current is
equal to the reflected current (Kraus' assumption) then zero net current
is flowing in and out of both ends of the coil even though there is a
current maximum point in the middle of the coil. This is how Kraus'
phase-reversing coil works in his collinear array antenna.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

snip

Kraus says: "It is generally assumed that the current distribution
of an infinitesimally thin antenna is sinusoidal, and that the phase
is constant over a 1/2WL interval, changing abruptly by 180 degrees
between intervals."

Kraus says something quite similar in the second edition of "Antennas". (I
believe you are quoting from the third edition, which was co-authored by someone
else.)

However, Kraus is merely being careless with terminology. (It is likely that he
did not fully anticipate that he would be quoted out of context.) If one studies
the accompanying diagrams it is clear that Kraus is simply referring to the
standard functional form of a sinusoidal curve. For reasons not clear to me he
decides to call the natural progression from positive to negative as the sine
function passes through zero an abrupt 180 degree phase change. This is
misleading at best.

A true phase change would be, for example, an abrupt transition from +1 to -1 in
the sine function. This sort of phase change is used in numerous communication
schemes, such as PSK31.

snip

Consider something even more bizarre. If the coil is exactly 1/2WL and
each end is located at a current node, assuming the forward current is
equal to the reflected current (Kraus' assumption) then zero net current
is flowing in and out of both ends of the coil even though there is a
current maximum point in the middle of the coil. This is how Kraus'
phase-reversing coil works in his collinear array antenna.

Why is this even the least bit bizarre? Your favorite example of an ideal
transmission line with a perfectly reflecting termination shows exactly the same
thing. Are you suggesting that any node on an ideal standing wave cuts off
everything further downstream? If so, then you might want to consider Tom's
suggestion and head back to school.

73,
Gene
W4SZ

Gene Fuller wrote:
Consider something even more bizarre. If the coil is exactly 1/2WL and
each end is located at a current node, assuming the forward current is
equal to the reflected current (Kraus' assumption) then zero net current
is flowing in and out of both ends of the coil even though there is a
current maximum point in the middle of the coil. This is how Kraus'
phase-reversing coil works in his collinear array antenna.

Why is this even the least bit bizarre? Your favorite example of an
ideal transmission line with a perfectly reflecting termination shows
exactly the same thing. Are you suggesting that any node on an ideal
standing wave cuts off everything further downstream?

No, no, no. I'm saying that if Tom finds current flowing into both ends of a
coil at the same time to be a bizarre thought, then a coil with no current
flowing into the ends at all, even though current is maximum at the center
of the coil, would be an even more bizarre thought *FOR HIM*. Tom seems to
have a sacred cow that he doesn't want to barbecue.

Assume a 180 degree phase shifting coil with a current node at each end
as I described above. Also assume one misses the current nodes by 6 degrees
and that the maximum current loop is 1 amp. The current at one end of the
coil will be ~0.1 amp at zero degrees while the current at the other end
of the coil is ~0.1 amp at 180 degrees. That means current is flowing into
both ends of the coil at the same time and then flowing out of both ends
at the same time 1/2 cycle later. Tom calls anyone who believes that
"gullible" and that tells me I should go back to school. I am merely
demonstrating the laws of physics operating outside of Tom's sacred cow box.
--
73, Cecil http://www.qsl.net/w5dxp



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Tdonaly wrote:

Jim Kelly wrote,


Cecil Moore wrote:

Jim Kelley wrote:
Maybe it's a half wavelength long? ;-)

For a real-world air-core coil to exhibit identical currents at the
input and output, it would have to be one wavelength long, lossless,
and non-radiating. That's a pretty tall order just to support an
old wives' tale. :-)

I happily stand corrected.

73 de jk



Pathetic.
73,

Pathetic, best regards?

Please elaborate.

73, Jim AC6XG

Gene Fuller wrote:
Cecil Moore wrote:
Kraus says: "It is generally assumed that the current distribution
of an infinitesimally thin antenna is sinusoidal, and that the phase
is constant over a 1/2WL interval, changing abruptly by 180 degrees
between intervals."

Kraus says something quite similar in the second edition of "Antennas".
(I believe you are quoting from the third edition, which was co-authored
by someone else.)

However, Kraus is merely being careless with terminology. (It is likely
that he did not fully anticipate that he would be quoted out of
context.)

It is within the context of physics. It is only out of context
when the context is sacred cows and old wives' tales.

If one studies the accompanying diagrams it is clear that
Kraus is simply referring to the standard functional form of a
sinusoidal curve. For reasons not clear to me he decides to call the
natural progression from positive to negative as the sine function
passes through zero an abrupt 180 degree phase change. This is
misleading at best.

Kraus is merely following convention. The sign of the real part of the
current at 89 degrees is positive. The sign of the real part of the current
at 91 degrees is negative. A positive sign indicates current flowing in
one direction. A negative sign indicates current flowing in the opposite
direction. Since there are only two possible directions in a wire, those
two directions are 180 degrees apart, by definition.

A true phase change would be, for example, an abrupt transition from +1
to -1 in the sine function. This sort of phase change is used in
numerous communication schemes, such as PSK31.

A true phase change would also be, a smooth transition from +0.001 through
zero to -0.001. When current equals zero at a standing wave node, the phase
of the real component of current on each side of that zero is 180 degrees
different. For the real component of the current, a 180 degree phase reversal
occurs between 89 degrees and 91 degrees. Cos(89) = +0.017, Cos(91) = -0.017
--
73, Cecil http://www.qsl.net/w5dxp



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