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.