Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:

Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are
the same diameter, the same wire in this model.

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99
0.0001 FR 0 0 0 0 15 0
EN

. . .

Is my model above what you suggest?

No. But I did take the time to see what would be necessary to actually
model it. And what I ended up with is identical to a) except that the
wire stub is replaced by the shorted transmission line model, and the
lower wire has become the outside of the coaxial structure so is
increased in diameter. So those are the two differences between a) and
b). As Tom mentioned and I alluded to, there's some interaction

I think that is what I had done, but I used the same diameter top to
bottom.

Here is a revised deck with different diameters:

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 15 0 0 0 0 0 5 0.015
GW 2 30 0 0 5 0 0 15 0.005
GE 1
GN 1
EK
EX 0 1 1 1 0
TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

In the above, the lower conductor is three times the diameter of the
upper conductor. The TL is wired into the lowest segment of the upper
conductor. Again, I have shunted the TL with 10k R to represent loss in a
real TL.

This model does not show in phase currents in upper and lower parts of
the vertical.

between the wire stub and the antenna which doesn't exist between the
ideal transmission line and the antenna, so performance is different.

You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one

I don't think I did that.

transmission line conductor always equals the current out of the
other, so if the two are shorted, no more current can go into or out
of the shorted end. Therefore, any external connection to it looks
like an open circuit since no current will flow through the external
connection.

What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I
have defines only types 1 - 5.

I have been playing with this in EZNEC and 4NEC2. The deck I offered was
from 4NEC2 as my EZNEC files are binaries and couldn't go inline. The EX
6 is an extension for a current source. It is immaterial in this case,
and the 6 can be changed to a 0.

Thanks.
Owen

Owen Duffy wrote:

I think that is what I had done, but I used the same diameter top to
bottom.

Sorry, my mistake when looking at the source. Your model is just as I
described. I apologize for the error.


Here is a revised deck with different diameters:

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 15 0 0 0 0 0 5 0.015
GW 2 30 0 0 5 0 0 15 0.005
GE 1
GN 1
EK
EX 0 1 1 1 0
TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

In the above, the lower conductor is three times the diameter of the
upper conductor. The TL is wired into the lowest segment of the upper
conductor. Again, I have shunted the TL with 10k R to represent loss in a
real TL.

This model does not show in phase currents in upper and lower parts of
the vertical.

I've been running your model without the loss, and I'm seeing currents
in the upper and lower wires which are nearly 180 degrees out of phase.

between the wire stub and the antenna which doesn't exist between the
ideal transmission line and the antenna, so performance is different.

For sure -- maximum gain is about 46 degrees above the horizon.

You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one

I don't think I did that.

You're right, you didn't. My mistake.

. . .

In playing with the model, I noticed something surprising -- length and
Z0 of the transmission line have very little effect on the pattern, even
over wide ranges (5 to 5000 ohm Z0, lengths from essentially zero to one
wavelength). In fact, try removing the transmission line altogether,
leaving the wires connected directly together and look at the pattern.
Then try changing one wire end slightly to break the connection between
them -- again, very little change in the pattern. The fact is that the
junction of the two wires is at a point of very little current, so you
can connect or disconnect them with almost no change. Likewise, you can
insert just about anything (of zero physical size), including an ideal
transmission line of any length, without any real effect. So the
transmission line stub doesn't really do anything significant at all.
What I don't understand yet is exactly why the wire stub does what it
does. It sure doesn't work like the simplified explanations imply.

Roy Lewallen, W7EL

Hi Roy,

Roy Lewallen wrote in
treetonline:

....

Thanks, all noted.

What I don't understand
yet is exactly why the wire stub does what it does. It sure doesn't
work like the simplified explanations imply.

Returning to my diagram a), below is an expansion of the detail at the
junction of the stub and vertical sections.



|
|
|
|
|
|
|
| A
B |
---------------------|



--------------------|
|
C |
| D
|
|
|
|
|
|
|
|

It strikes me that if we omit the stub all together, and leave a gap in
its place, we have two unconnected resonant elements, the top half wave,
and the bottom quarter wave with a driving source. The two elements are
field coupled to some extent, and currents will setup in each section out
of phase. NEC models support this, and I think they are correct in doing
so.

Returning now to a) with the stub connected and very close to resonance,
and with reference to the diagram above, for A, B, C and D very close to
the corners, I(A)=I(B) and I(C)=I(D).

If the desired outcome of using the stub is that the upper and lower
sections are in phase, then I(A)~=I(D). That implies common mode current
in the stub, so to cause I(A)~=I(D), the stub must have common mode
current (equal to (I(A)+I(D))/2 per conductor).

If that is true, then reduction of the physical stub to a pure
differential mode TL element is discarding part of what makes it "work".
That implies that replacement of the stub with a two terminal equivalent
impedance, eg by insertion of a load in an NEC segment, or insertion of
one port of a TL network in an NEC segment is an inadequate model.

Am I on the wrong track here?

Owen

On Mar 16, 2:33*pm, Owen Duffy wrote:
Hi Roy,

Roy Lewallen wrote ystreetonline:

...

Thanks, all noted.

What I don't understand
yet is exactly why the wire stub does what it does. It sure doesn't
work like the simplified explanations imply.

Returning to my diagram a), below is an expansion of the detail at the
junction of the stub and vertical sections.

* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *| A
* * * * * * * * * * * *B * * |
* * * * ---------------------|

* * * * *--------------------|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * *C * * |
* * * * * * * * * * * * * * *| D
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|

It strikes me that if we omit the stub all together, and leave a gap in
its place, we have two unconnected resonant elements, the top half wave,
and the bottom quarter wave with a driving source. The two elements are
field coupled to some extent, and currents will setup in each section out
of phase. NEC models support this, and I think they are correct in doing
so.

Returning now to a) with the stub connected and very close to resonance,
and with reference to the diagram above, for A, B, C and D very close to
the corners, I(A)=I(B) and I(C)=I(D).

If the desired outcome of using the stub is that the upper and lower
sections are in phase, then I(A)~=I(D). That implies common mode current
in the stub, so to cause I(A)~=I(D), the stub must have common mode
current (equal to (I(A)+I(D))/2 per conductor).

If that is true, then reduction of the physical stub to a pure
differential mode TL element is discarding part of what makes it "work".
That implies that replacement of the stub with a two terminal equivalent
impedance, eg by insertion of a load in an NEC segment, or insertion of
one port of a TL network in an NEC segment is an inadequate model.

Am I on the wrong track here?

Owen

For what it's worth, I think you're on exactly the right track, Owen.

Some things to ponder: does it make any significant difference if the
stub is, say, 2mm wires spaced 20mm apart or 1mm wires spaced 10mm
apart (that is, the same impedance line, but different physical size),
and does it make any significant difference if the wires are kept in a
plane that includes the antenna elements, or if they are twisted near
their attachment point so they lie in a plane perpendicular to the
antenna wire, or if they are twisted throughout their length? What if
they are coiled in a spiral out from the antenna wire, so their
shorted end lies much closer than a quarter wave from the axis of the
antenna? I don't have any answers to these questions; they just seem
like an interesting and reasonable extension of your original
question.

Cheers,
Tom

On Mar 16, 2:33*pm, Owen Duffy wrote:
Hi Roy,

Roy Lewallen wrote ystreetonline:

...

Thanks, all noted.

What I don't understand
yet is exactly why the wire stub does what it does. It sure doesn't
work like the simplified explanations imply.

Returning to my diagram a), below is an expansion of the detail at the
junction of the stub and vertical sections.

* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *| A
* * * * * * * * * * * *B * * |
* * * * ---------------------|

* * * * *--------------------|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * *C * * |
* * * * * * * * * * * * * * *| D
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|
* * * * * * * * * * * * * * *|

It strikes me that if we omit the stub all together, and leave a gap in
its place, we have two unconnected resonant elements, the top half wave,
and the bottom quarter wave with a driving source. The two elements are
field coupled to some extent, and currents will setup in each section out
of phase. NEC models support this, and I think they are correct in doing
so.

Returning now to a) with the stub connected and very close to resonance,
and with reference to the diagram above, for A, B, C and D very close to
the corners, I(A)=I(B) and I(C)=I(D).

If the desired outcome of using the stub is that the upper and lower
sections are in phase, then I(A)~=I(D). That implies common mode current
in the stub, so to cause I(A)~=I(D), the stub must have common mode
current (equal to (I(A)+I(D))/2 per conductor).

If that is true, then reduction of the physical stub to a pure
differential mode TL element is discarding part of what makes it "work".
That implies that replacement of the stub with a two terminal equivalent
impedance, eg by insertion of a load in an NEC segment, or insertion of
one port of a TL network in an NEC segment is an inadequate model.

Am I on the wrong track here?

Owen

I'm sorry...perhaps I don't understand your notation. Don't you
expect that the current at A will be (rather roughly) out of phase
with the current at D? If I think about a collinear with three half-
wave elements end to end, and drive the center of the center element,
if it's going to act like I want, I'll have high current near the
middle of each element, and those three will be in-phase. Because of
the mutual impedances among the elements, things get a bit funny at
the ends. I suppose there is a fairly large voltage across the gap
between adjacent elements, and therefore there will be moderately high
current near those ends to account for the capacitive current in the
air between them. That's what I'm seeing in the EZNEC model I just
hacked, and it's as I'd expect. The currents near the ends of the
central element are considerably higher than the currents near the
open ends of the outer elements.

(Now to spend a few minutes playing with changing the length of the
stubs through resonance...)

Cheers,
Tom

Hi Tom,

K7ITM wrote in
:

....
I'm sorry...perhaps I don't understand your notation. Don't you

I am taking a convention that the sense of currents in segments is from
bottom to top. That means that I defined all segments in order from bottom
to top.

My notation ~= is to mean approximately equal.

Does that clarify things?

Cheers
Owen

On Mar 16, 11:39*pm, Owen Duffy wrote:
Hi Tom,

K7ITM wrote :

...

I'm sorry...perhaps I don't understand your notation. *Don't you

I am taking a convention that the sense of currents in segments is from
bottom to top. That means that I defined all segments in order from bottom
to top.

My notation ~= is to mean approximately equal.

Does that clarify things?

Cheers
Owen

Yes--and then if they were exactly equal, would that not imply only
transmission line current on the stub? Obviously, they are exactly
equal if you simply connect the ends of the elements together...but
that isn't what gets us to in-phase currents at the centers of each
element (in the case of the symmetrical 3 element design; or the base
current in the bottom quarter wave in phase with the center current in
the top half wave...), and (nearly) equal currents at those current
maxima. To the extent that the currents A and D in your diagram
differ, there is common-mode or "antenna" current on the stub.

Cheers,
Tom

K7ITM wrote in
:

....
Yes--and then if they were exactly equal, would that not imply only
transmission line current on the stub? Obviously, they are exactly

Thinking some more about it, my current thinking is that my analysis was
flawed. I was using the standing wave currents, when I should be using
the travelling wave components.

I suspect that when NEC models the conductor arrangement at my fig a), it
correctly accounts for propagation delay and the phase relationships
compute correctly.

If we replace the stub with a TL element, I suspect that NEC reduces the
TL to a two port network and loads a segment of the vertical with an
equivalent steady state impedance of the s/c stub network. If that is
done, the reduction to a lumped load means that there is zero delay to
travelling waves, and the computed currents (amplitude and phase) in the
vertical will be incorrect. This means that you cannot replace a resonant
stub with a high value of resistance, it doesn't work.

If that is the case, it suggests that NEC cannot model such phasing
schemes using TL elements.

Owen