NEC2 modeling question
 

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I have started modeling my dipole antenna with NEC2, but I've got a
question that doesn't seem to be adequately covered by the
documentation or the examples.

I have a fan dipole with various lengths of wire. The long legs are
significantly bent, and I have modeled them as two wires with
coterminous ends -- this seems to be correct. The shorter legs are
just legs. In the real antenna, the feedpoint is connected to a
length of coax such that the center of the coax is electrically
connected to the western half of the antenna and the shield of the
coax is electrically connected to the eastern half.

I can see how to set the voltage source to the western half of the
antenna, but I do not see how to set the ground to the eastern half of
the antenna without adding a ground wire. Does this really matter?

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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Jack Twilley wrote:
I can see how to set the voltage source to the western half of the
antenna, but I do not see how to set the ground to the eastern half of
the antenna without adding a ground wire. Does this really matter?

Connect the two sides of the antenna together with a wire and install
a source in the middle of that wire. Forget the coax for now. You can
expand on the model later. Something like this:

---------------------+--S--+-----------------------
| |
-----------+ +------------
--
73, Cecil, W5DXP



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I tried what you suggested, and the results on first glance look
better, but I get geometry warnings. Apparently the jumper between
the two sides of the antenna is too short, or something.

Thanks for the tip! I'll keep fussing with it.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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Jack originally wrote:

In the real antenna, the feedpoint is connected to a
length of coax such that the center of the coax is electrically
connected to the western half of the antenna and the shield of the
coax is electrically connected to the eastern half.

Cecil wrote to you:

Connect the two sides of the antenna together with a wire and install
a source in the middle of that wire. Forget the coax for now. You can
expand on the model later. Something like this:

---------------------+--S--+-----------------------
| |
-----------+ +------------

I would just add that the coax can be modeled as shown
below:

---------------------+--S--+-----------------------
| |
-----------+ +------------
| (shield)
|
|
|
|
|
|
|
|
|
|
|
--------
-----
---

Jim, K7JEB

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Sure, I could drop a line down to ground, but the antenna itself
doesn't work that way. It goes down a long coaxial line to the
transmitter, at which point the shield is grounded and the center
conductor is connected to the signal. Does that matter?

This stuff is black magic, I tell you.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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Jack Twilley:
Sure, I could drop a line down to ground, but the antenna itself
doesn't work that way. It goes down a long coaxial line to the
transmitter, at which point the shield is grounded and the center
conductor is connected to the signal. Does that matter?

ASCII art fails me, Jack. You can model the coax
as a wire to ground, but that wire has to have all
the geometric twists and turns that your actual
coax takes getting to the transmitter. The coax
shield is part of your antenna system; it radiates
along with both sides of the dipole. And even at
your transmitter, "ground" isn't really ground.
There is a connecting wire between the transmitter
case and "earth" ground.

In reality, there is no such thing as "ground" with
RF.

The reason your post caught my eye is that I have
the same antenna system you do - coax-fed dipole
with no balun - and had to model my coax line
shield to get "real" EZNEC results.

Jim, K7JEB

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"Jim" == k7jeb remove this K7JEB writes:

Jim ASCII art fails me, Jack. You can model the coax as a wire to
Jim ground, but that wire has to have all the geometric twists and
Jim turns that your actual coax takes getting to the transmitter.

That'll be a royal pain in the butt, to put it kindly.

Jim The coax shield is part of your antenna system; it radiates along
Jim with both sides of the dipole. And even at your transmitter,
Jim "ground" isn't really ground. There is a connecting wire between
Jim the transmitter case and "earth" ground.

Actually, at my station. the water-pipe ground is connected to the
antenna tuner's ground which is connected to the transceiver's ground,
but I get your point.

Jim In reality, there is no such thing as "ground" with RF.

This appears to be one of those subtleties that has slipped past me.

Jim The reason your post caught my eye is that I have the same
Jim antenna system you do - coax-fed dipole with no balun - and had
Jim to model my coax line shield to get "real" EZNEC results.

Interesting. I do actually have two current-mode baluns -- the
feedpoint has eight or nine turns of coax that were wrapped around a
coffee can before being tied down and the coffee can removed, and the
transceiver has three turns of coax through three large rare earth
magnets. How does one model those with NEC2?

Jim Jim, K7JEB

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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I wrote:
ASCII art fails me, Jack. You can model the coax as a wire to
ground, but that wire has to have all the geometric twists and
turns that your actual coax takes getting to the transmitter.

Jack Twilley:
That'll be a royal pain in the butt, to put it kindly.

That's *real* antenna analysis. Think about doing that
for all the wires in a missile silo to assess EMP
vulnerability and you get an idea about how the big
boys use NEC.

Actually, you can fudge it a bit. Any bump or kink
less that 1/10 wavelength in extent can be approximated
with a chord between its endpoints.

In reality, there is no such thing as "ground" with RF.

This appears to be one of those subtleties that
has slipped past me.

Yeah, the best one can do is equipotential surfaces,
with the hope that one of those surfaces is parallel
to that big hunk of lossy dielectric beneath your
feet.

Interesting. I do actually have two current-mode baluns -- the
feedpoint has eight or nine turns of coax that were wrapped around a
coffee can before being tied down and the coffee can removed, and the
transceiver has three turns of coax through three large rare earth
magnets. How does one model those with NEC2?

As fixed inductances in series with the "coax shield" wire.
I make your first balun to be about 10 uH, which is roughly
200 ohms at 3 MHz. Your guess is as good as mine on the
one with the rare-earth magnets. What you want to look for
is reducing the current in the "coax shield" wire to zero.

73,
Jim, K7JEB

On Fri, 09 Apr 2004 11:31:16 -0700, Jack Twilley
wrote:

the
transceiver has three turns of coax through three large rare earth
magnets. How does one model those with NEC2?

Hi Jack,

Magnets do not play any particular role in chokes. Magnetic cores
perhaps, but being magnetic is not in and of itself a remarkable
indicator of efficient choking potential.

Research the archives for ferrites, amidon, type 43 or 75 or 77 (there
are others), beads, and so on as terms.

As for modeling them, they are resistors. You can rate them at about
a Watt to 5 Watts (I've never tested one to destruction). Again, you
need the characteristics to know how much. The typical bead exhibits
about 20 Ohms in the mid to high HF region. The shape of its curve
(the resistance vs. frequency) is variable for any particular type.
Some increase through HF, others decrease, some don't even begin until
VHF.

Beads are additive. Two beads, 40 Ohms, three beads, 60 Ohms and so
on. Passes through a larger core (same material) sees a square law
effect X1, X4, X9 and so on.

73's
Richard Clark, KB7QHC