It doesn't sound like a complicated model. If that's the case, it
wouldn't take long to enter it manually. So in EZNEC, click Outputs,
then Show Description. Copy the description and paste it into a posting.
After I make an EZNEC file out of it, I'll upload it to my web site so
everyone with EZNEC can see what we're talking about.

In general, I strongly prefer that people send me an .EZ file instead of
a text description, both to save time and to reduce the probability of
an error.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Are you referring to something you emailed me? If so, I haven't
received it. I'll be glad to look at it when I do.


Yep, I've emailed it twice to you. I had to email them three times to
Yuri before he received it. Apparently, some of the new internet filters
are deleting emails.

Would you agree to cease the ad hominem attacks until you have actually
viewed the problem?

Cecil and his confederates will soon be taking into account the difference
in current, and the forward and backward reflections, between the ends of
the coil in a Pi-match network between the plate and 50-ohm output socket of
the common or garden 807 linear power amplifier. The Pi-match network
behaves as a 1/4-wave transmission line impedance matching transformer. Keep
going Cec!

And it has not yet been sorted out whether or not a congugate match exists.

There's a long way still to go. ;o) ;o)
---
Reg.

On Mon, 03 Nov 2003 12:20:02 -0600, Cecil Moore
wrote:

|Wes Stewart wrote:
| On Mon, 03 Nov 2003 09:26:05 -0600, Cecil Moore
| wrote:
|
| |Roy Lewallen wrote:
| | If you could build an antenna from
| | straight conductors and lumped inductors, the result would be very close
| | to EZNEC's predictions.
| |
| |Hard to prove since lumped inductors are impossible in reality. Why
| |does EZNEC show so much difference between lumped inductors and stub
| |inductors?
|
| I see no such difference in my model.
|
|There shouldn't be a lot of difference. I have modeled two short dipoles,
|one loaded with a lumped inductive reactance and one modeled with the
|same reactance using an inductive stub. EZNEC reports the following:
|
| Inductance lumped j335 10'stub
|
|current in segment just before the coil .8374 amp .8384 amp
|
|current in segment just after the coil .7971 amp .5642 amp
|
|The relative difference just before the coil is quite small, 0.12%.
|
|The relative difference just after the coil is quite large, 41.28%.
|
|There just cannot be that amount of difference between a coil and a
|stub.

If you use the ideal transmission line model, there is *zero*
difference between an ideal inductor and a transmission line stub.

You are comparing a mess of wire with a ideal lumped inductor. Apples
and oranges.

If you really want to model this stuff accurately take a few hundred
$K out of your next retirement check and buy a high frequency
structure simulator.

www.hfss.com

Roy Lewallen wrote:
It doesn't sound like a complicated model. If that's the case, it
wouldn't take long to enter it manually.

There may be a bug in Netscape 7.1. The "reply all" function may not
work right. I'll send you an email direct.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:

Cecil and his confederates will soon be taking into account the difference
in current, and the forward and backward reflections, between the ends of
the coil in a Pi-match network between the plate and 50-ohm output socket of
the common or garden 807 linear power amplifier. The Pi-match network
behaves as a 1/4-wave transmission line impedance matching transformer. Keep
going Cec!

Reg, I cannot believe you would side with the people who claim there is zero
delay through a p-net coil. Do you also believe in faster than light
propagation of signals? If there is a delay in one foot of copper wire,
don't you think it is reasonable to expect a delay in 20 feet of copper
wire formed into a coil?

Is the number of degrees on a Smith Chart between 0+j0 ohms and 0+j50 ohms
just a figment of my imagination?
--
73, Cecil http://www.qsl.net/w5dxp



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Wes Stewart wrote:
You are comparing a mess of wire with a ideal lumped inductor. Apples
and oranges.

Not my fault. Why does EZNEC treat these two inductances so differently?

If you really want to model this stuff accurately take a few hundred
$K out of your next retirement check and buy a high frequency
structure simulator.

No thanks, Wes. I can live without that. But let me ask you a question.
If there is a one degree delay through one foot of copper wire, how
can there be a zero delay through 20 feet of wire coiled into a one
foot coil? Given the pressure on the electrons, one would expect
20 times the delay through the coil as through the one foot wire.
--
73, Cecil http://www.qsl.net/w5dxp



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On Mon, 03 Nov 2003 16:36:59 -0800, Roy Lewallen
wrote:

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

Roy:

You and a handfull others are on my "must read" list. Keep up the good
work!

Jack K8ZOA

On Mon, 03 Nov 2003 19:55:33 -0600, Cecil Moore
wrote:

|Wes Stewart wrote:
| You are comparing a mess of wire with a ideal lumped inductor. Apples
| and oranges.
|
|Not my fault. Why does EZNEC treat these two inductances so differently?

Uh... maybe because they're different?
|
| If you really want to model this stuff accurately take a few hundred
| $K out of your next retirement check and buy a high frequency
| structure simulator.
|
|No thanks, Wes. I can live without that. But let me ask you a question.
|If there is a one degree delay through one foot of copper wire, how
|can there be a zero delay through 20 feet of wire coiled into a one
|foot coil? Given the pressure on the electrons, one would expect
|20 times the delay through the coil as through the one foot wire.

When I was working my way to engineering I was a senior associate
engineer working with a PhD who had tired of the politics in the
university environment, given up his tenure, and come to work for
Hughes. We were putting together a bench top setup to evaluate
something or the other in the mixers from the early version of the
Phoenix missile. The front end of this thing, from the waveguide slot
planar antenna back the the mixer inputs, was all waveguide.

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."

I'll choose this one to respond to, since it talks directly about the
models.

Cecil has been kind enough to provide me with the models, and given me
permission to post them on my web site. You can download them from
ftp://eznec.com/pub/ as Cecils_Models.zip, and run them with the
standard version of EZNEC. If you only have the demo version, a
reduced-segment model of the verticals at least should work well enough
to illustrate the subject matter.

I looked particularly at the two models of a vertical, since they're the
simplest. They're both 51 feet high, on 75 meters. One has a single
lumped "load" of +j335 ohms between 25 and 26 feet from the ground, and
the other has a one-foot-spaced horizontal stub protruding horizontally
from the same point.

One point that seems to be drawing attention, if not to say some
creative theories, is that the current at the bottom terminal of the
stub isn't equal to the current at the top terminal. They are, EZNEC
reports, 0.846 and 0.581 amps respectively (at the stub end segments).
EZNEC shows them to have very little phase shift along the stub, and
very nearly 180 degrees out of phase on the two wires. (People looking
at the model should be aware that stub wires 2 and 4 are defined with
end 1 of one opposite end 2 of the other. So the phase angles reported
by EZNEC are referenced in opposite physical directions. EZNEC reports
the phase angles of the currents on the two wires as being nearly
identical. So that means that the currents are flowing in phase in
opposite directions -- or nearly exactly out of phase if you define
positive as the same direction for both wires.)

The fact that the currents at the stub terminals aren't the same means
that there can be no doubt that the stub is radiating. The difference
constitutes a common mode current. Because the currents are almost
exactly out of phase, we can simply subtract them to find the common
mode current. At the antenna end of the stub, it's about 0.27 amp. At
the output end, it's zero (EZNEC reports a 0.03 amp difference for the
segments nearest the short). Taking a simple-minded average, we can say
it's very roughly 0.15 amp. This is the equivalent single-wire common
mode current. That is, it will radiate as though that amount of current
were flowing on a single conductor of the same length.

Field strength is proportional to the current flowing on a conductor,
and the length of the conductor. It's not at all valid (using this sort
of analysis at least) to apportion radiation to being so much from this
part of the antenna or so much from that. For example, the field from
one part of the antenna can interfere with the field from others,
resulting in little or no contribution from those sections in certain
directions, or maybe in nearly all directions. But to get an idea of the
potential radiation from the stub, we can look at the 0.15 amp
approximate average current flowing along the ten foot stub, and compare
it to the roughly 0.5 amp average over 51 feet for the vertical itself.
From that, we see it probably won't be a big contributor to the total
field. But that's not at all a criterion for imbalancing the stub
current -- which does affect the feedpoint impedance and potentially the
pattern. In fact, the stub can cause more disturbance by modifying the
current in the main radiator than by its own radiation. That's
definitely true in something like a collinear with phasing stubs.

The stub common mode current (that it, the imbalance between currents at
the stub terminals) is due to mutual coupling between the stub
conductors and the vertical portion of the antenna. It shouldn't be a
surprise, and it doesn't require any new theory, reflected currents or
powers, or hocus-pocus to explain. It's exactly the same phenomenon that
induces current in a Yagi parasitic element, and countless other
familiar everyday examples. And EZNEC does the calculation exactly the
same (from very basic principles) for all conductors -- it doesn't know
or care if you regard some of them as being a "stub".

I'll make a prediction here without having actually tried it. So here's
a chance to show just how full of BS I am. Convert the model to a dipole
of double the length, in free space, but with a stub on only one side.
Move the stub inward toward the center. As you do, I predict that the
currents will become more and more balanced. That is, the currents on
the two terminals of the stub should become more and more equal. Why?
Because as you get it closer to the center, the mutual coupling from the
two halves of the antenna to the stub becomes more equal. Exactly at the
antenna center, they cancel out. At that point, you can replace the stub
with a lumped inductor and find no change. I restricted this to one stub
because if there were two, coupling from one stub to the other would
create imbalance even near the center.

To answer an earlier question of whether you should expect a ten foot
stub to behave like a lumped inductor, the answer is, of course not. At
least not if it's in the field of other current-carrying conductors.

What's the mystery? What's the big deal?

Roy Lewallen, W7EL


Cecil Moore wrote:
. . .

Roy suggested the stubs might be radiating. EZNEC says they are not.
You can add two inches of vertical wire to the ends of the antenna
and see the red vertical radiation. The stubs are 0.04WL, #14 wire,
and 6 inch spacing. How much could they radiate on 75m? The difference
in current is not due to radiation. It is due to the phase shift
between Ifwd and Iref through the stub.

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?

We can work our magic without either an inductor or a stub.

Roy Lewallen, W7EL