dykesc wrote in news:c0bab953-135b-455c-9a3f-
:

I'm going to 450 ohm window line to my OCF Dipole. Obviously, as
confirmed by EZNEC, without some kind of choking action at the
feedpoint, my feedline will be radiating. What kind of 1:1 choke could
I use on the 450 ohm line at the antenna feedpoint? I could build a
1:1 choke with the right toroid but not sure if I can maintain the 450
ohm line Z0 through the choke.

I differ with some of the other advice that you have.

I suggest that the "appropriate" common mode impedance is independent of
differential Zo. I refer to this as Rule 500, as it usually requires the
magnitude of common mode impedance to be 500 ohms or greater for use on a
50 ohm line.

Rule 500 is born out of a lumped circuit analysis, and is less relevant
to the antenna scenario with its coupled conductors. Like most Rules of
Thumb, it is spruiked without stating or often knowing the underlying
assumptions and is then RoT.

You could create a model of your antenna, including the common mode
current path and insert the common mode impedance as a load in the
relevant conductor in the model, and explore the effect with different
choke impedances and frequencies etc.

My article at http://www.vk1od.net/balun/W2DU/index.htm shows that done,
in that case with a W2DU balun. Several other articles on the site model
the common mode impedance of ferrite cored baluns, so they might give you
some ball park figures to try (eg
http://www.vk1od.net/balun/Guanella/G.1-1.htm).

The model also allows you to explore the importance of a shunt path to
ground outside the shack as an element of mitigating common mode current.

You may also come to the conclusion that Rule 500 is not applicable to
the scenario.


Owen

dykesc wrote:

Ok Roy I added the RF ground counterpoise as you stated above, but it
would appear to me all I did was add another antenna wire at the
source end of my model. I did ground one end of the RF ground
counterpoise, but I'm having a tough time seeing how the source is not
just seeing this as another antenna wire. Maybe that's the way it
should be, but in the physical world the counterpoise is only
connected to transmitter ground and my transmitter is not trying to
drive it through the center conductor.

73
Dykes (AD5VS)


Sorry, I don't understand. EZNEC has no idea of what you consider to be
an "antenna", "counterpoise", or "ground" (except when connected to a
perfect or MININEC-type ground plane). A conductor is a conductor, just
as in the real world. It sounds to me like the model matches your setup
-- the terminal of the source connected to the long wire is the
transmitter "ground", and the other source terminal is the transmitter
"hot". But if the suggestion I made doesn't match the physical
configuration of your setup, then it needs to be modified so it does.
Unless other readers are interested, contact me by email and try again
at describing the physical setup if you still need help with it.

Roy Lewallen, W7EL

dykesc wrote:
... but it
would appear to me all I did was add another antenna wire at the
source end of my model.

Welcome to the world of RF. On a long "ground" wire,
the only place that is at "ground" is where it is
grounded and at 1/2WL intervals. With standing waves
on that wire, there are points at which the impedance
(and voltage) is sky high - yes, on the "ground" wire.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Jun 26, 10:31*am, Cecil Moore wrote:

Welcome to the world of RF. On a long "ground" wire,
the only place that is at "ground" is where it is
grounded and at 1/2WL intervals. With standing waves
on that wire, there are points at which the impedance
(and voltage) is sky high - yes, on the "ground" wire.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Thanks for the reply Cecil. I am aware of the physics of RF on
longwires. What I am having trouble with is how to include a longwire
RF energy sink at the source end of my antenna model. These are
normally 1/4 wavelength at the operating frequency (or band midpoint
for a best fit) and are open circuit on the far end resulting in a low
impedance path for stray rf energy as part of the station ground
configuration. Look back at Roy's reply on how to implement this in a
model. The way he suggested would appear to have the source driving
the 1/4 wave ground wire as an end fed element. Just doesn't look
right to me. The source should only see this wire on the ground side
of the model.

73
Dykes (AD5VS)

On Fri, 26 Jun 2009 22:07:13 -0700 (PDT), dykesc
wrote:

What I am having trouble with is how to include a longwire
RF energy sink at the source end of my antenna model.

There are any number of alternatives, each matching the situation of
"someone." They vary by rather simple to rather elaborate estimations
of "ground." At least one can be found in the generous help manual.

The trick is how well you know your "ground's" characteristic to
sufficient accuracy. Chances are you (given you have an ad hoc
design) you barely know. This, of course, translates into a model of
equal imprecision.

Myself, I lay out several dozen radials over an average earth and
connect to it. If it is a simple dipole, I lay out a ground mat below
the antenna (it does make a difference).

73's
Richard Clark, KB7QHC

dykesc wrote:
Look back at Roy's reply on how to implement this in a
model. The way he suggested would appear to have the source driving
the 1/4 wave ground wire as an end fed element.

Is it possible for you to publish a graphic of your
antenna system or share your EZNEC file? I'm not sure
that anyone really understands your text description.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Jun 27, 9:07*am, Cecil Moore wrote:

Is it possible for you to publish a graphic of your
antenna system or share your EZNEC file? I'm not sure
that anyone really understands your text description.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Cecil,

I sent you am e-mail with the EZNEC file attached. Don't know how to
make it public here.

Thanks for looking at the file for me.

73
Dykes (AD5VS)

dykesc wrote:
I sent you am e-mail with the EZNEC file attached. Don't know how to
make it public here.

Do you mind if I upload it to my web page so
everyone can take a look?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Jun 27, 5:17*pm, Cecil Moore wrote:
dykesc wrote:
I sent you am e-mail with the EZNEC file attached. Don't know how to
make it public here.

Do you mind if I upload it to my web page so
everyone can take a look?
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Please do Cecil. Thanks.

Dykes (AD5VS)

dykesc wrote:
On Jun 27, 5:17 pm, Cecil Moore wrote:
Do you mind if I upload it to my web page so
everyone can take a look?

Please do Cecil. Thanks. Dykes (AD5VS)

OK, Dykes, anyone interested in downloading your
EZNEC file can do so from:

http://www.w5dxp.com/ocfv.EZ

Roy, w7el, offered to help. Did you send your .EZ
file to him by email?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com