Roy Lewallen wrote:
What's the mystery? What's the big deal?

Intel has a lot of problems with buses running in the hundreds of MHz
because of the delay in the conductor path between chips. If I tell them
to install a coil in each path instead of a conductor, the delays will
disappear, right? You're pulling my leg, right?

At frequencies where the delay through a coil is a negligible part of
an AC cycle, the delay can be ignored. At frequencies where the delay
through a coil is not a negligible part of an AC cycle, the delay cannot
be ignored and circuit theory will not yield the correct answers. At the
point where the circuit theory error becomes too great, we must switch
to distributed network analysis.

Consider one foot of wire carrying a 1 GHz signal. The phase shift is
greater than 360 degrees. Can we reduce the phase shift to zero by
installing a coil over that one foot length? You *are* pulling my leg,
right?

All coils have delays. Sometimes those delays are negligible. Sometimes
they are not. The delay through a 75m mobile loading coil is NOT negligible.
That assumption causes errors. The phase delay through a coil is approximately
the same as the section of line replaced by the coil. For an 80m loading coil,
that delay is around 80 degrees or the equivalent of 57 feet of wire. Otherwise,
the forward and reflected currents on the antenna would not have the proper phase.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
Here's an experiment to try.
Take Cecil's model of the vertical with the loading coil. Add a single
horizontal wire, 10 feet long, connected at the top of the loading coil.
That is, make the new wire go from 0, 0, 26 to 10, 0, 26. Notice how
much current there is in the horizontal wire. Notice how much different
the current is in the vertical below the wire compared to above the
wire. Look familiar?

And please note that horizontal wire generates lots of horizontally
polarized radiation where there is none for the horizontal stub alone.
--
73, Cecil http://www.qsl.net/w5dxp



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Huh?

The stub produces just as much horizontally polarized radiation as the wire.

Run your stub vertical model with an elevation plot, and azimuth angle
of 90 degrees. Click FF Tab. Note the magnitude of the horizontal
component -- roughly -30 dBi. Then repeat with the experimental model
with the single horizontal wire.

As I mentioned in my lengthy posting, the radiation from the stub isn't
a large part of the overall field, and this certainly shows it. But it's
certainly enough to disturb the vertical's current. Exactly the same
thing holds for the straight wire. Common mode current is common mode
current. No magic, no mysterious phenomena "not accounted for" by EZNEC.

Roy Lewallen, W7EL

Cecil Moore wrote:

And please note that horizontal wire generates lots of horizontally
polarized radiation where there is none for the horizontal stub alone.

Watch this space for another thrilling episode of the long-running,
nerve-tingling mystery story "The Case of the Missing Third Wire".

Roy and Cecil -- I find your posts most informative -- ignore the flack.
Keep up the good work -- valuable sources

--
73 From The Keyboard
===============
"Roy Lewallen" wrote in message
...
I'm sorry. If I'm bothering the readers, I'll be glad to bow out.

My postings aren't really directed to Cecil -- I know much better than
to imagine that I'll ever change his mind, and I'm a firm believer in
not wasting time on things I can't change.

No, you and the other readers are really the audience, and the whole
reason for the postings. If you and the other readers would rather I
shut up, I'll be more than happy to spend my time at more productive
pursuits.

Just let me know.

Roy Lewallen, W7EL

David Robbins wrote:
Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Cecil Moore wrote:

Roy Lewallen wrote:

Roy Lewallen wrote:

The stub produces just as much horizontally polarized radiation as the
wire.

Not true. The wire produces 2 dB more radiation than the stub. Given
that the stub is located in a high current region compared to the wire,
it is significant how much the stub doesn't radiate. If you replace
the stub with an equal length of single wire, it radiates 4 dB more
than the stub.

Run your stub vertical model with an elevation plot, and azimuth angle
of 90 degrees. Click FF Tab. Note the magnitude of the horizontal
component -- roughly -30 dBi. Then repeat with the experimental model
with the single horizontal wire.

Thanks, Roy, that's an angle I had not looked at. Results are above.

As I mentioned in my lengthy posting, the radiation from the stub isn't
a large part of the overall field, and this certainly shows it. But it's
certainly enough to disturb the vertical's current. Exactly the same
thing holds for the straight wire. Common mode current is common mode
current. No magic, no mysterious phenomena "not accounted for" by EZNEC.

What EZNEC doesn't account for is the phase delay through a bugcatcher
coil which is an appreciable percentage of a wavelength. EZNEC is incapable
of modeling a bugcatcher coil. The only coil that EZNEC is capable of modeling
is one that does not and cannot exist in reality.

Therefo One cannot use EZNEC to try to prove the current is the
same at both ends of a bugcatcher coil which is what kicked off
this entire discussion.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
Watch this space for another thrilling episode of the long-running,
nerve-tingling mystery story "The Case of the Missing Third Wire".

Reg, you have a black box in the middle of a transmission line with
a high SWR. You measure the current into the box and current out of
the box. You measure 1 amp at 0 degrees going in and 1 amp at 180 degrees
going out. This means that both currents are flowing into the box at
the same time. There is no third wire. What's in the box?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, W5DXP wrote:
"What EZNEC doesn`t account for is the phase delay through the
bugcatcher coil which is an appreciable percentage of a wavelength."

It needs to have enough delay to replace that missing from the resonant
length of the antenna as ON4UN shows in his Fig 9-22.

Inductors are retardation coils.. They delay current change much as a
flywhell inhibits change in rotation. When voltage is applied to an RL
circuit, current is a function of time. If reactance in a circuit is
low, instantaneous current is almost in-phase with instantaneous applied
voltage. There is little if any delay of current in response to applied
voltage. If inductance in a circuit is high, delay of current is high.
Terman gives an example of inductive delay on page 643 of his 1955
edition:

"Another type of artificial line, suitable for low power operation,
consists of a coaxial cable in which the inner conductor is a continuous
coil of small wire wound on an insulating cylindrical core. This greatly
increases inductance per unit length of line with corresponding
reduction in velocity and increase in time delay per unit length. It is
possible with such a line to obtain a round-trip transit time of as much
as 1 microsecond in a length of two feet.

Best regards, Richard Harrison, KB5WZI

What's in the box?

Reg with whatever he is drinking :-)

Richard Harrison wrote:

Cecil, W5DXP wrote:
"What EZNEC doesn`t account for is the phase delay through the
bugcatcher coil which is an appreciable percentage of a wavelength."

It needs to have enough delay to replace that missing from the resonant
length of the antenna as ON4UN shows in his Fig 9-22.

This is true in order to get the feedpoint voltage and current in phase,
i.e. in order to resonate the antenna.

The voltage needs 180 degrees round trip delay from the feedpoint to
the end of the antenna and back.

The current needs 360 degrees round trip delay from the feedpoint to
the end of the antenna and back. 180 degrees of that comes from the
reversal of direction at the open end.

Can someone please measure the delay through a 75m bugcatcher coil?
I simply do not believe it is zero, i.e. faster than light.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, W5DXP wrote:
"Reg, you have a black box in the middle of a transmission line with a
high SWR. You measure the current into the box and out of the box, You
measure 1 amp into the box and out of the box, You measure 1 amp and 0
degrees going in and 1 amp at 180 degrees going out. This means that
both currents are flowing into the box at the same time. There is no
third wire. What`s in the box?"

A phase inverter.

You could have a center-tapped coil in the box. One end and the center
could take the input. The other end and the center could provide an
output 180-degrees out of phase with the input.

This requires a minimum of three terminals but only 2 wires in and 2
wires out. If 2 directions of travel are allowed on a pair, an open or a
short reverses the direction (phase) of the reflected wave.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Cecil, W5DXP wrote:
"Reg, you have a black box in the middle of a transmission line with a
high SWR. You measure the current into the box and out of the box, You
measure 1 amp into the box and out of the box, You measure 1 amp and 0
degrees going in and 1 amp at 180 degrees going out. This means that
both currents are flowing into the box at the same time. There is no
third wire. What`s in the box?"

A phase inverter.

You could have a center-tapped coil in the box. One end and the center
could take the input. The other end and the center could provide an
output 180-degrees out of phase with the input.

Yep, that's one answer. Another answer is a piece of low-loss transmission
line that shifts the phase by 180 degrees, i.e. 1/2WL of transmission line.

Point is that unequal currents at the input and output of a black box are
easy to achieve and do not violate Kirchhoff's laws. Although physically
small, this black box does not meet the definition of a lumped circuit.

A bugcatcher coil on a 75m mobile antenna also does not meet the
definition of a lumped circuit.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
Richard Harrison wrote:
The purpose of a loading coil in a short loaded vertical antenna is
often to add to the existing degrees of antenna length to reach a
resonant length of 90-degrees, as shown in Fig 9-22 of ON4UN`s "Low-Band
DXing", and included on Yuri`s web pages.

In order for a current maximum to exist at the feedpoint of a shortened
(less than 1/4WL) vertical, the forward current must undergo a phase
shift of 90 degrees, followed by the 180 degree phase shift from being
reflected by an open circuit, followed by another 90 degree phase shift
in the reflected current wave. An 8 foot whip gives about 11 degrees of
phase shift end to end on 75m for a total of 22 degrees. If the coil
causes no phase shift, where does the other 338 degrees of phase shift
come from?

Some people thought I was disagreeing with Richard. I wasn't. I was
agreeing with him and adding another reason why he is right. Incidentally,
the 338 degrees above should have been 158 degrees. I forgot to subtract
the 180 degree current phase reversal at the end of the standing-wave
antenna. Since the coil is the only other thing in the circuit, it
must necessarily contribute that 158 degrees, 79 degrees in each
direction.
--
73, Cecil http://www.qsl.net/w5dxp



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I hope the readers will forgive me if I considered the radiation of the
two cases to be equal, not worrying about a couple of dB difference in
the range of -30 dBi. Actually, I know of no way to ascertain the total
radiation from the stub or wire alone, since it occurs at all azimuths
and elevations, producing both horizontal and vertical components, and
adds to and modify's the vertical's pattern. What I meant to say was
that the radiation characteristics are certainly very similar, and both
have the same general effect on the vertical's current distribution.
And, both for exactly the same reason. Cecil's earlier statement that
the wire radiates while the stub does not is certainly and demonstrably
not true, and the 2 dB difference in field strength isn't at all
evidence that one radiates more total energy than the other.

As for the statement that "EZNEC doesn't account for is the phase delay
through a bugcatcher coil", that's entirely true. As I've said several
times now, an EZNEC coil "load" is a lumped element model, which has
equal currents at its two terminals. A coil with significant physical
length doesn't behave like a lumped inductor, and therefore not like the
EZNEC model. I believe, but have no proof, that approximating a lengthy
coil with a combination of wire and load models will produce reasonable
results, but that's the best you can do with NEC based programs like
EZNEC. (Or with MININEC-based programs for that matter.)

Anyone who attempts to model a lengthy coil as a lumped "load" component
won't get results that closely model reality, for the same reason that
anyone who attempts to model a long wire as a short wire will be
disappointed. Neither should be a surprise.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

The stub produces just as much horizontally polarized radiation as the
wire.


Not true. The wire produces 2 dB more radiation than the stub. Given
that the stub is located in a high current region compared to the wire,
it is significant how much the stub doesn't radiate. If you replace
the stub with an equal length of single wire, it radiates 4 dB more
than the stub.

Run your stub vertical model with an elevation plot, and azimuth angle
of 90 degrees. Click FF Tab. Note the magnitude of the horizontal
component -- roughly -30 dBi. Then repeat with the experimental model
with the single horizontal wire.


Thanks, Roy, that's an angle I had not looked at. Results are above.

As I mentioned in my lengthy posting, the radiation from the stub
isn't a large part of the overall field, and this certainly shows it.
But it's certainly enough to disturb the vertical's current. Exactly
the same thing holds for the straight wire. Common mode current is
common mode current. No magic, no mysterious phenomena "not accounted
for" by EZNEC.


What EZNEC doesn't account for is the phase delay through a bugcatcher
coil which is an appreciable percentage of a wavelength. EZNEC is incapable
of modeling a bugcatcher coil. The only coil that EZNEC is capable of
modeling
is one that does not and cannot exist in reality.

Therefo One cannot use EZNEC to try to prove the current is the
same at both ends of a bugcatcher coil which is what kicked off
this entire discussion.

This is misleading.

The Rule is that the sum of currents on *all* the box's conductors has
to add to zero. If the box has only two terminals, the sum of the two
has to be zero -- the only way to get around that would be to put Cecil
into the box and have him suck coulombs just as fast as he can. If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero. Provided, of course, that the box is very
small in terms of wavelength, and we're measuring over the long term.
It's ok to suck up and store charge for a while -- but not forever.

When you put even a third terminal on the box, you have a lot more
choices as to what you put into it -- an autotransformer, for example.
Then you can find any number of gee, whiz, Mr. Science, absolutely
wonderful things about the voltages and currents to dazzle the
technically uncertain. The four terminals of Cecil's box provide even
more opportunities to amaze. But one thing you can take to the bank,
folks: the sum of the currents on all the terminals better add to zero.
Unless, of course, Cecil is in the box.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Harrison wrote:

Cecil, W5DXP wrote:
"Reg, you have a black box in the middle of a transmission line with a
high SWR. You measure the current into the box and out of the box, You
measure 1 amp into the box and out of the box, You measure 1 amp and 0
degrees going in and 1 amp at 180 degrees going out. This means that
both currents are flowing into the box at the same time. There is no
third wire. What`s in the box?"

A phase inverter.

You could have a center-tapped coil in the box. One end and the center
could take the input. The other end and the center could provide an
output 180-degrees out of phase with the input.


Yep, that's one answer. Another answer is a piece of low-loss transmission
line that shifts the phase by 180 degrees, i.e. 1/2WL of transmission line.

Point is that unequal currents at the input and output of a black box are
easy to achieve and do not violate Kirchhoff's laws. Although physically
small, this black box does not meet the definition of a lumped circuit.

A bugcatcher coil on a 75m mobile antenna also does not meet the
definition of a lumped circuit.

Roy Lewallen wrote:
If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG

Roy Lewallen wrote:
I hope the readers will forgive me if I considered the radiation of the
two cases to be equal, not worrying about a couple of dB difference in
the range of -30 dBi.

Actually, a more fair comparison is to replace the stub with a wire and
move the rest of the antenna over ten feet so it goes up 25 feet, zigs
to the side by ten feet, and then goes up another 25 feet from there.
That makes the current in the stub and the current in the horizontal
section approximately equal. With that configuration, the radiation
from the horizontal section is 12 dB greater than from the stub, i.e.
24 times as great. Seems the stub works pretty well after all.

Anyone who attempts to model a lengthy coil as a lumped "load" component
won't get results that closely model reality, for the same reason that
anyone who attempts to model a long wire as a short wire will be
disappointed. Neither should be a surprise.

What kicked off this discussion in the first place is that someone claimed
to know the current below and above a bugcatcher type coil based on modeling
loads in EZNEC. Presumably, he was indeed in for a surprise.
--
73, Cecil http://www.qsl.net/w5dxp



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Read again the fourth sentence of the posting you quoted.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:

If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.


What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG

Roy Lewallen wrote:
The Rule is that the sum of currents on *all* the box's conductors has
to add to zero. If the box has only two terminals, the sum of the two
has to be zero -- the only way to get around that would be to put Cecil
into the box and have him suck coulombs just as fast as he can. If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.

That's just the point, Roy. The two terminals don't have to sum to zero
for a distributed network problem, like 1/2WL of coax coiled up inside
that black box.

But one thing you can take to the bank,
folks: the sum of the currents on all the terminals better add to zero.
Unless, of course, Cecil is in the box.

Are you saying I cannot coil up 1/2WL of coax inside a black box and
observe current flowing into both terminals for 1/2 cycle and current
flowing out of both terminals for 1/2 cycle? That would be quite a
revelation.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:
Read again the fourth sentence of the posting you quoted.

It would be nice just to repeat it so 5000 readers don't have to
go searching for it.

In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground. To what is EZNEC referencing voltage measurements
on an antenna in free space?
--
73, Cecil http://www.qsl.net/w5dxp



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Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:

If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.


What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG

Okay. I read it. I'll try asking the question another way.

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

73, Jim AC6XG

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

73, Jim AC6XG

Jim Kelley wrote:

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

Sorry to be obtuse, Roy. The point is only that antenna circuits
obviously present a problem to the assumption that such two terminal
black boxes will necessarily have equal currents at both terminals.

A solenoid should produce a field in the direction of its axis, should
it not?

73, Jim AC6XG

Jim Kelley wrote:
What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

The two-terminal black box must have the same zero dimensions
as the inductance. :-)
--
73, Cecil http://www.qsl.net/w5dxp



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You also need to go back and read the fourth sentence of my posting.

Sheesh.

Roy Lewallen, W7EL

Cecil Moore wrote:
. . .
Are you saying I cannot coil up 1/2WL of coax inside a black box and
observe current flowing into both terminals for 1/2 cycle and current
flowing out of both terminals for 1/2 cycle? That would be quite a
revelation.

Jim Kelley wrote:

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

For a 1/4WL vertical, that would be true even if the path to ground
is through displacement currents. But, what about free space?
--
73, Cecil http://www.qsl.net/w5dxp



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On 3-Nov-2003, Wes Stewart wrote:
....
Whatever we were doing required the use of a "magic" T. I, the ever
inquisitive student, asked; "Doc, how does a magic T work?"

Doc, former professor and the author of "Intermediate Mathematics of
Electromagnetics", replied,

"It's magic."

In the US Navy, we usually explained that such devices worked on the principle of
PFM. First word is Pure, Last word is Magic

-ken-

It would be even nicer if readers would read it.

An antenna could be regarded to have any number of terminals you'd like.

EZNEC does not make voltage measurements. Please go back and read the
lengthy recent thread regarding the meaning of voltage and voltage
measurement. I recall you were a major contributor. But it's really not
necessary to re-post it. Please don't.

EZNEC does show the voltages across sources and loads, both of which are
two-terminal lumped components.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.


It would be nice just to repeat it so 5000 readers don't have to
go searching for it.

In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground. To what is EZNEC referencing voltage measurements
on an antenna in free space?

Assume the box is non-conducting. There would be a current difference
between the two terminals unless you shrunk the length of antenna it
contains to a vanishingly small length. In the context of the antenna,
even a few feet of an antenna is a significant fraction of a wavelength.
Put the box around a few feet of a power transmission line carrying 60
Hz current, and you won't be able to measure any difference between the
current going in and coming out. In that situation, a few feet isn't a
significant part of a wavelength.

If you use a conducting box, you'll end up with current on the outside
of the box that gets into the problem. Its value depends on coupling to
the wire inside and to the antenna outside, so it gets stickier than I
want to deal with.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:

Read again the fourth sentence of the posting you quoted.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


If that
two-terminal box contains an inductor, then the current out has to equal
the current in -- that's the only way the sum of currents at the two
terminals can sum to zero.


What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the current become the same at both
ends?

73, Jim AC6XG


Okay. I read it. I'll try asking the question another way.

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?

73, Jim AC6XG

Cecil Moore wrote:

Jim Kelley wrote:

Cecil Moore wrote:
In any case, it appears to me that an antenna is not a two terminal
network. It appears to be a three or four terminal network with a
virtual ground.

The only way I can see to model it and get these particular results
would be to have it as a device which increases in impedance at every
point along its length, and which has a current path to ground at each
of an infinite number of such points.

For a 1/4WL vertical, that would be true even if the path to ground
is through displacement currents. But, what about free space?

Evidently there's some kind of an equivalence between electric current
and the electric smoke liberated by the antenna. ;-)

73, Jim AC6XG

I'm not sure why anyone would think that you can treat an antenna, or a
loading coil of significant length, as a lumped element and expect to
get anything resembling accurate results. Who in the world is proposing
such a thing? Or is this something to be attributed to the "old wives"
and "gurus", so we can then show how much smarter we are by pointing out
how stupid it is?

Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?

Yes, a solenoid produces a local (near) field in the direction of its
axis. The far field that remains depends on the size and aspect ratio of
the solenoid. Hence, we have solenoidal antennas that radiate primarily
axially and those which radiate primarily radially. It's not clear to me
how this bears on the topic.

Jim Kelley wrote:

Jim Kelley wrote:

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?


Sorry to be obtuse, Roy. The point is only that antenna circuits
obviously present a problem to the assumption that such two terminal
black boxes will necessarily have equal currents at both terminals.

A solenoid should produce a field in the direction of its axis, should
it not?

73, Jim AC6XG

Roy Lewallen wrote:

Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?

The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:
Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?


The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG

No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG

Roy Lewallen wrote:

no, you can't put a box around anything having any length and expect
the current in to equal the current out. And why should this be
surprising to anyone?

Possibly because radio amateurs are not taught well about what a "lumped
component" is. All lumped components are defined as having zero (or
negligible) physical dimensions relative to the wavelength at the
operating frequency. Similarly, lumped networks are defined as having
zero (or negligible) lead lengths relative to the operating wavelength.

In this idealized case, the current into and out of the two terminals of
a lumped inductor is always exactly the same. This remains true even if
that component is embedded into an antenna where current variations
along the length of the conductor do exist.

Because all practical components and networks have some finite physical
size, lumped-component behaviour is never absolutely perfect. In
principle, any real component must also show some "antenna-like"
behaviour, which does allow some variation of current between its
terminals... but in practice this effect is usually very small indeed.
For example, for physically small components the lumped-component
approximation works well in circuit simulations at frequencies up to
several GHz. (You may have to simulate each component as a small network
in order to account accurately for self-capacitance, self-inductance and
loss resistance, but these are still networks of idealized lumped
components.)


Yes, a solenoid produces a local (near) field in the direction of its
axis. The far field that remains depends on the size and aspect ratio
of the solenoid. Hence, we have solenoidal antennas that radiate
primarily axially and those which radiate primarily radially. It's not
clear to me how this bears on the topic.

Quite a lot, I think. At one extreme, a loading coil may be so small
that it behaves as a near-perfect lumped inductor. Such an inductor will
not radiate, and will have almost zero difference in current between its
two terminals. Those two properties - lack of radiation and no
difference in terminal currents - are locked together.

At the other extreme, you may have a long, skinny loading coil that has
significant antenna-like properties, radiating at right-angles to its
length like a "rubber-duck" (more formally known as a normal-mode
helix). In this case the coil does form part of the radiating structure,
so you do expect to see a variation in current along the length of the
coil, and hence a difference between the currents at its two ends. Once
again, the two properties of radiation and current variation are locked
together.

This brings us back to the question of practical loading coils, and how
much radiation (and therefore current variation along the length) we can
expect. I haven't ever tried to work it out, but my guess is that a
fairly short "square" coil that has been optimized for high Q is not
going to radiate much, and that we therefore shouldn't expect a large
difference in current between its two ends.

Let's see now... a 3.5MHz loading coil that is as much as 10 inches long
would scale down to 0.010 inches at 3.5GHz... at that frequency I'd
expect to be able to model such a tiny inductor very accurately as a
small network of lumped components with no radiating properties.

On the other hand, people who mistakenly believe that even an ideal
lumped inductor can have a difference between the currents at its two
terminals are rather unlikely to be convinced.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Tue, 04 Nov 2003 15:37:11 -0800, Roy Lewallen
wrote:

[lots of good stuff snipped]
|
|And with that, I'm outta here. Hope my postings have been helpful.

Thanks, Roy. I'm surprised you stuck around this long. Your posts
are always helpful.

73 Wes N7WS

Roy,

Let me first apologize for having put the burr under your saddle. This
is not my intent. The intent is to try to stimulate some additional
thinking on the subject, and perhaps apply some other things we also
know about these devices. Your contributions here are obviously
invaluable to the entire group. Of course they are helpful. Having
recognized that, I hope there are no subjects which are off limits to
debate - or opinions that are deemed beyond reproach. Yuri has
introduced an interesting subject; one that appears to have two
disparate points of view.

At this point, in my view, the side asserting that there is no
difference in current from one end of an inductor to the other hasn't
defended its position as well as the other side. The point you make for
the torroid is well taken I think. Flux from this type of coil is well
confined to the core of the inductor.

But torroidal coils are by design, a unique case. I don't think the
same case can be made for the helix, or a loopstick type coil for
example. These coils do radiate quite well along their axis if nowhere
else, and might therefore be expected to behave in a fashion not unlike
other radiators, i.e. impedance and hence, current, would vary with
position. Since air core coils are ubiquitous in antenna construction,
I don't think it's unreasonable to discuss their performance, and
consider the findings Yuri has presented as being both reasonable and
viable.

73, Jim AC6XG


Roy Lewallen wrote:

No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG

Thanks for the very cogent and informative posting, as yours always are.

One thing you said really rang a bell, and made me consider something
I'd never thought much about before. If a coil is radiating
significantly, the Q will of course be necessarily poor due to the
energy "lost" by radiation. Yet this "loss" won't be detrimental to the
antenna performance. Tom Rauch has pointed out that loading inductor
loss is very often insignificant compared to ground loss in a typical HF
mobile system. Maybe the presumed "loss" of some coils is not as bad as
it appears in other antenna applications, either.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
. . .
This brings us back to the question of practical loading coils, and how
much radiation (and therefore current variation along the length) we can
expect. I haven't ever tried to work it out, but my guess is that a
fairly short "square" coil that has been optimized for high Q is not
going to radiate much, and that we therefore shouldn't expect a large
difference in current between its two ends.
. . .

Thanks for your down to earth input Roy
I haven't entered this thread because I was
very confused how the group was dealing with
inductance since what is important to me is
the field around it that permits effective,
efficient coupling of different circuits.
When looking for a lossless system ,coupling
by either capacitance or inductance is necessary
together with the all important HIGH Q.
And if one were to dwell on the wire used as a
missing part of a radiator alone and disregarding
the ebb and flow of the enclosing field
just blows my mind.
Interesting that you spoke of lumped loads and
distributed loads in terms of modeling, many people
here would learn a lot by starting of with a T or pi
type matching system e.t.c. all of which use lumped loads,
and then manipulate these same lumped loads
into distributed loads in multiple coupled circuits
in a form of many coupled distributed load ircuits to
produce a radiator that can maintain a constant input
impedance.
After playing with such circuits to form a combination
radiating lossless circuit( reverse of complex circuit resolving)
the idea of missing radiator lengths would quickly
disappear, as it becomes noticable that the energy field equates
to the actual length of the inductance and not to a slinky style stretch.
There again, as a total amateur with respect to electrical
thing a me jigs I could be adding to the mental riots of those who
are partaking in this never ending gymnastics.If so I will now
sneak quietly out of this conference room before Tom arrives and
put everybody in their place as only he can do.
Regards
Art



Roy Lewallen wrote in message ...
No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG

If reactance can be seen as a "{missing" part
of a radiator how should we view what a
capacitor represents? Grin
Art


Cecil Moore wrote in message ...
w4jle wrote:
Current through a coil in an antenna.

If we feed an antenna at the current point, the current decreases as the
voltage increases along the antenna element from feed point to end..

That being said, a coil replacing a segment of an antenna (in order to
physically shorten it) will exhibit the same properties (relating to
currents) as the segment it replaced.

Yep, if the feedpoint impedances are the same and both are lossless,
that has to be true.

Here's a repeat of a diagram I drew earlier.

-----y----------x-----FP-----x----------y----- 1/2WL dipole

-----coil-----FP-----coil----- loaded dipole

Assume the physical length of the loaded dipole is 1/4WL.

Each coil replaces the section between 'x' and 'y'. The currents
at 'x' and 'y' are quite different, being 1/8WL apart.

Consider an 8 foot center-loaded 75m mobile antenna. 87% of the
electrical length of the antenna is in the coil.