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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