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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