[TowerTalk] EZNEC Question

Thu Jan 30 03:44:18 EST 2025

Well, I guess I didn't really learn as much as I thought I did.  I took 
your advice and modeled a 450 MHz dipole instead and got a curious 
result.  I split the dipole into 31 wires each 2mm thick and 10 mm long, 
with the feed in the center (wire #16).  I added insulation (Er = 2, 
thickness = 5mm) as balanced pairs, as in #1/#31, #2/#30, #3,#29, etc.  
I found I was getting slightly squirrely results if I only did one side 
of the dipole at a time ... I think the imbalance was swinging the 
result a bit.

Here are the EZNEC Pro/2 results for resonant frequency.  I'm reading 
the numbers off the SWR plot so the accuracy is not better than +/1 at best.

no dielectric loading = 453 MHz
1/31 = 395 MHz
2/30 = 448
3/29 = 449
4/28 = 450
5/27 = 449
6/26 = 448
7/25 = 447
8/24 = 446
9/23 = 446
10/22 = 445
11/21 = 444
12/20 = 443
13/19 = 443
14/18 = 442
15/17 = 441
16 = 453

The end sections clearly show the greatest effect and it is a loading 
effect that decreases the resonant frequency.  This is what I originally 
hypothesized ... that the dielectric loading effect would occur where 
the E-field is highest.  And at first the resonant frequency increases 
as I move the dielectric loading toward the center ...  as expected.  
But to my great surprise, as soon as I get a little further away from 
the ends (at roughly wire #4 going toward the center) some other effect 
takes over that DECREASES the frequency going toward the center, but 
very gradually and starting from a frequency almost equal to having no 
loading at all (448 MHz versus 453 MHz).  And then at the center wire 
(where there is very little E-field) the resonant frequency jumps back 
up to the same as if there was no dielectric loading at all.  I've 
double checked the runs and right or wrong that's what they are.

The current in a half wave dipole follows a half-sine wave profile, but 
the voltage follows a more extreme cotangent-like profile.  The voltage 
increases VERY quickly in a very short distance at the ends of the 
dipole, and my model was done with lossless wires so there really isn't 
a limit to it.  I'm using a 1 amp forcing function for the source at the 
center of the dipole, EZNEC says that the current at the very end 
segment of the very end wire is 0.04 amps.  If I did the math properly 
that works out to be over 30,000 volts per the model.  The next 10mm 
wire closer to the center (wire #2) is already down to about 1,000 volts.

I should probably run the numbers over again with some resistance in the 
wires, but I think that would only moderate the magnitude of the effect 
... not the general shape of the profile.

Going back to the full wave loop I described earlier, I didn't see this 
same kind of result because that model didn't have the same 
granularity.  In that model have 40 cm long wires (7 of them split into 
17 segments) on each side of the loop, and when I look at the current 
distribution on the center wire (#6 in that model) it is clear that the 
insulation for that wire spans enough of the wire that the loading 
effect of the high E-Field in the center segment of wire #6 is swamped 
out by the lesser effect of the adjacent segments.  Instead, I only saw 
the secondary effect similar to what is shown by dipole wires #5 though 
#15 above.

I think I understand WHAT I'm seeing now and the end effect makes sense 
to me, but I still don't understand what is happening as we move toward 
the center of the dipole.

Or maybe EZNEC simply doesn't handle all of that very well and I'm 
letting myself be led around by the nose.

Dave   AB7E

p.s.  At my age (almost 78) understanding something isn't necessarily 
the most important part, although it's frustrating if I don't.  Thinking 
about it, as in keeping my mind active, is probably more valuable.  ;)

>
>
> On Tue, 28 Jan 2025 12:08:57 -0700, David Gilbert <ab7echo at gmail.com> 
> wrote:
>
> I did some more searching and found a similar discussion on the 4NEC2
> reflector from about 3 years ago.  In it was a link to this article by
> Cebic which seems to indicate that dielectric loading of a conductor
> produces an inductive effect.  If that's the case it makes sense that it
> would have more effect at a current maximum than a voltage maximum.
>
> https://hamwaves.com/wire/doc/Insulated%20Wires:%20The%20NEC-2%20Way.html
>
> I also harkened back to the formula for velocity factor in a coaxial
> transmission line, which is V = 1/sqrt(er), where er = the relative
> permittivity.  That certainly describes the equivalent of an inductive
> effect.
>
> Apparently at age 77 I'm not yet too old to learn ... although that day
> might not be too far off.  ;)
>
> Dave   AB7E
>
> >
> >
> > On Mon, 27 Jan 2025 20:33:55 -0700, David Gilbert wrote:
> >
> > I've been using EZNEC Pro/2 to try to model the effects of dielectric
> > loading on a wire antenna ... specifically a full wave loop on 435 MHz.
> > I split the sides of the loop up into multiple wires (each with multiple
> > segments) so I could individually declare a thick dielectric ... i.e.,
> > insulation in the wires table ...  separately for each portion of the
> > loop.  To my surprise the loading effect seems to be greatest at or near
> > current maximums, not at voltage maximums where I would have presumed
> > the electric field would have the most effect.  This has me greatly 
> puzzled.
> >
> > Any thoughts?
> >
> > Dave   AB7E
>
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