Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil. That shouldn't be surprising. I have a high
level of confidence that if you built the antenna just like you modeled
it, you would find the results to closely agree with the model.

Roy Lewallen, W7EL

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.

Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil.

Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem? Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are radiating
a negligible amount, just like the lumped inductance. Why the 40% difference
in current between the two configurations? Is this a characteristic of NEC?
--
73, Cecil http://www.qsl.net/w5dxp



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On Mon, 03 Nov 2003 15:19:44 -0600, Cecil Moore
wrote:

Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals, you
must be modeling it with wires, which should reflect reality quite well.
Look carefully at the currents along the stub and you'll find they're
not equal and opposite on the two conductors. Such a radiating stub *is*
very different from a coil.

Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem?
Ah the quality of sneer review.

Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are radiating
a negligible amount, just like the lumped inductance. Why the 40% difference
in current between the two configurations? Is this a characteristic of NEC?

Cecil, you have two stubs and they are driven antiphase (typical of a
doublet) and through symmetry would have equal antiphase currents when
compared to their opposites, but not necessarily equal currents within
their twin-pair of lines. The sum of ALL currents (and not just the
myopic view of one of two stubs) would suggest exactly what Roy has
offered.

This, of course, returns us to the question of what part of the
radiator radiates. Sadly, the convention of the current pulse (or
maxima, or other equivalent term) trips up discussion just in these
matters. ALL elements radiate, it is only in the far field where
their contributions negate, not literally within the structure.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
Cecil, you have two stubs and they are driven antiphase (typical of a
doublet) and through symmetry would have equal antiphase currents when
compared to their opposites, but not necessarily equal currents within
their twin-pair of lines. The sum of ALL currents (and not just the
myopic view of one of two stubs) would suggest exactly what Roy has
offered.

Sorry, Richard, 1/2 of the loaded dipole turned vertical doesn't
show a trace of horizontal radiation. Sorry about that.

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.
--
73, Cecil http://www.qsl.net/w5dxp



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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

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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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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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.

Roy Lewallen, W7EL

Cecil Moore wrote:
Roy Lewallen wrote:

Because you're seeing different currents at the two stub terminals,
you must be modeling it with wires, which should reflect reality quite
well. Look carefully at the currents along the stub and you'll find
they're not equal and opposite on the two conductors. Such a radiating
stub *is* very different from a coil.


Instead of a knee-jerk defense of your ideas, why don't you actually take
a look at the problem? Those stubs are vertical. EZNEC shows virtually zero
vertically polarized radiation. According to EZNEC, those stubs are
radiating
a negligible amount, just like the lumped inductance. Why the 40%
difference
in current between the two configurations? Is this a characteristic of NEC?

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?
--
73, Cecil http://www.qsl.net/w5dxp



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