On Apr 10, 1:22 pm, "Antonio Vernucci" wrote:
I can understand your confusion and doubt. I hope this says it in
clear terms:

In the revised model with the hairpin done as three wires, the
original D.E. single wire was replaced by three wires: a 10cm long
center section which should have the source in its center, and a
"left" and a "right" outer section. Those three sections are three
separate wires. The reason to break the original single wire into
three wires is so that there will be wire "ends" to connect the
hairpin wires to. You can only connect wires at their ends, even
though they have multiple segments. So yes, in the new model the
source is just in the middle of the wire which is the middle section
of the D.E.

Thanks for your clarification that confirms my expectation that I shall put the
generator in the middle of the 10 cm-long center section of the D.E..

Everything works fine and, for a reference 200-ohm resistance, I get an SWR of
almost 1 not too far away from the frequency at which it actually occurs.

If I may take a bit more of your time, I have noted two inconsistencies:

- with the hairpin EZNEC gives an antenna gain of 12.76 dB but, if I just delete
the three hairpin segments leaving everything else unchanged, the gain grows up
to 13.59 dB. I am surprised about that result, as I had understood that, in
EZNEC, the antenna gain has nothing to do with the way the antenna is fed.

- with the hairpin, at the antenna resonant frequency EZNEC shows an impedance
of 200.5 +j2.1 ohm which obviously corresponds to an SWR of almost 1. If I
remove the hairpin, at the same frequency EZNEC shows an impedance of 42.4 - 39
ohm which could seem reasonable, but it is actually not. As a matter of fact, by
applying the appropriate series-to-parallel conversion formula, that impedance
corresponds to the parallel of a 78.2 ohm resistance and a 85.1 ohm capacitive
reactance. Resonating the reactive capacitance with an equal (though opposite)
reactance hairpin, the remaining resistance would be just 78.2 ohm, that is far
away from the 200 ohm EZNEC shows with the hairpin installed.

May I have your opinion?

73

Tony I0JX

Hi Tony,

Hmm...very interesting. I hope some others will check in on this with
ideas too.

My first thought is that the hairpin conductors are really more a part
of the antenna than you may have thought. The current in that 10cm
section of the hairpin, especially, is in a direction parallel to the
D.E. and to all the directors and to the reflector(s). It will couple
to them, and could be changing the gain and impedance some.

I am also surprised by the high resistance you are seeing with the
hairpin wires removed. From what you posted before, and from the 3-
element NBS and the 6-element designs I ran in EZNEC, I would expect a
lower feedpoint resistance than that. If I put a pure inductance in
parallel with your 42.4-j39 ohms, I get the same answer as you did,
that it moves to 78 ohms resistive.

My offer to look at your EZNEC file is still open, of course. Maybe I
could see something in it--I know that I can look and look at
something and not see an obvious problem, even when I know perfectly
well what to look for.

Also--in the model before you split the D.E. into three wires, did you
get a more reasonable feedpoint impedance (with no hairpin, just
looking at the D.E. feedpoint)? I think we were expecting something
like 19-j58 ohms. If you get that, then I would look for the reason
things change (so very much!) when you split the D.E. into three
separate (but connected, end-to-end) wires. I can understand a small
change but not so large a change. Is the total number of segments for
the three wires making up the D.E. still about the same as it was in
the original version of the model, with just one wire for the D.E.?

Cheers,
Tom