"Roy Lewallen" wrote in message
...
Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893 ohms.


I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not considered
small. I'll go back and try it.

My fall-back plan is to make a simple 1/4-wave resonant folded monopole and
feed it with a 1/4-wave length of 75 Ohm coax. There will probably be some
mismatch, but I think it will be tolerable.

Thanks, guys.

John

Roy Lewallen, W7EL wrote:
"By my reckoning, a capacitive reactance of 1893 ohms at 14.2 MHz is 5.9
pF."

Dang me and my sliderule. Neither of us keeps track of decimals very
well. 6 pF can be obtained with wide spacing and high breakdown volts
with small plates. I don`t see much of a hurdle to clear because 6 pF is
a small capacitance.

As a practical matter, Andrew Corporation used another method to tune
its folded monopoles, I believe, because they had d-c continuity. They
supplied these antennas for decades to work at VHF. To move from 10m to
20m brings problems of scale, mechanical and electrical.

The original question said that the open-circuit ground plane has a
35-ohm feedpoint (at some elevation), and a folded ground plane has
about 140 ohms as a feedpoint. Neither ground plane matches the usual
coax at the antenna resonant frequency. Commercial antenna makers
advertise and deliver open-circuit and folded radiator ground plane
antennas which are nearly 50 + j0 ohms feedpoint impedance at a
specified frequency when mounted high and in the clear.

The folded radiator offers more lightning protection than the
open-circuit radiator. The folded radiator contains the ability to
step-up feedpoint impedance in cases where an open-circuit radiator
would have an inconveniently low feedpoint. Most TV yagis, for example,
use a folded dipole as the driven eleement due to the low feedpoint
impedance caused by mutual coupling with the parasitic elements.

Most energy in a lightning strike is at lower frequencies. Tune the
bands during thunderstorm season and notice where the static crashes are
worse, though much of this is due to propagation, some is due to the
shape of the transient. Where the folded antenna loop is small in terms
of wavelength, the loop is nearly a short-circuit and differential
energy is small.

I saw lightning problems solved by replacing open-circuit antennas with
folded-element antennas. As lightning is an interference problem taken
to an extreme, folded elements are also useful in solving some other
interference problems. But there are cautions. A folded dipole has a
resonance where it is only 1/4-wave from tip to tip. Its circumference
is 1/2-wave and resonates. This gives a folded dipole twice as many
resonances as an open-circuit dipole.

I make arithmetic mistakes more frequently when I don`t know for sure
that the number I calculate is reasonable or not. I do know that 20-kV
to 40-kV sparkplug voltage does not ordinarily leap many feet through
the air. I also have a formula for capacitance:

CpF = 0.225 K A / S

CpF = capacitance in pF

K = dielectric constant

A = area of one of the 2-plate capacitor plates
(sq. in.)

S = spacing between the plates in inches

For air, K = 1.0006

For a vacuum, K = 1

6 pF is not much so it should be easy to create.

Best regards, Richard Harrison, KB5WZI

John wrote:

I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not considered
small. I'll go back and try it.
. . .

Because EZNEC uses NEC-2 for calculations, it has the same problems with
small loops that NEC-2 does. It's able to model any kind of antenna that
NEC-2 can, within its segment limitation.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
John wrote:

I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not
considered
small. I'll go back and try it.
. . .

Because EZNEC uses NEC-2 for calculations, it has the same problems with
small loops that NEC-2 does. It's able to model any kind of antenna that
NEC-2 can, within its segment limitation.

Roy Lewallen, W7EL


I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

John wrote:

I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the dielectric.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
John wrote:

I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the dielectric.

Roy Lewallen, W7EL


Okay, great!

I modeled a folded monopole at 434 MHz. Varying the length down from
resonance, the element showed the terminal impedance getting lower in
resistance and become increasingly capacitive just like the unfolded
monopole.

I thought it was supposed to be backwards from the usual unfolded monopole
such that it would go up in resistance and become inductive.?.

John

"John" wrote in message
...

"Roy Lewallen" wrote in message
...
John wrote:

I don't know what NEC-2 is able to do. Does this mean I can model
folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the
dielectric.

Roy Lewallen, W7EL


Okay, great!

I modeled a folded monopole at 434 MHz. Varying the length down from
resonance, the element showed the terminal impedance getting lower in
resistance and become increasingly capacitive just like the unfolded
monopole.

I thought it was supposed to be backwards from the usual unfolded monopole
such that it would go up in resistance and become inductive.?.

John


Did you go down to 217 MHz and below? If not, check it out. Should hit
another resonance at something like 50,000 +j0, and stay inductive below
that.

Tam/WB2TT

John wrote:
. . .
I thought it was supposed to be backwards from the usual unfolded monopole
such that it would go up in resistance and become inductive.?.

Why would it do that?

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
John wrote:
. . .
I thought it was supposed to be backwards from the usual unfolded
monopole
such that it would go up in resistance and become inductive.?.

Why would it do that?

Roy Lewallen, W7EL


Well, you said earlier that the folded monopole could be modeled as an
unfolded monopole with a shorted transmission line in parallel. I thought I
understood. When I modeled the unfolded monopole, I saw it do as usual when
the element was varied in length. But when I included the shorted section of
transmission line and varied it directly with the element, I thought I saw
the terminal reactance go inductive as the length was decreased below
1/4-wave resonance and I thought the terminal resistance went up. So, I was
expecting the same from EZNEC by modeling the folded version.

I guess I'm really lost here.

John