On Jun 25, 1:17*pm, Roy Lewallen wrote:
There can be substantial radiation from an OCF dipole feedline even with
an effective choke in place. The asymmetrical positioning of the
feedline relative to the antenna results in a net induced common mode
current, while a choke only stops the current at one point. Effective
reduction of induced current can require two chokes placed about a
quarter wavelength apart, which isn't practical for a multi-band
antenna. In practice, you might have to fiddle with the feedline length
(since some lengths will result in more current than others on
particular bands) in addition to using two or more chokes in order to
drop the feedline radiation to a tolerable level. I think that with an
OCF dipole you've just about got to expect some feedline radiation.

Roy Lewallen, W7EL

Thanks Roy. You've pretty much confirmed what everyone else is saying
regarding feedline radiation. Hadn't thought about the induced
contributor from the asymmetrical configuration. So a 90 degree
feedline takeoff in this situation would still result in induced
current it would appear. I added a wire simulating the radiating
feedline conductor in my EZNEC model. It appears from the FF patterns
it may be helping in some directions.

While I've got you, is there any way in EZNEC to add an RF ground
counterpoise at the source end of my model? I have a long wire at my
common equipment ground point to better enable RF grounding on my
equipment and would like to see if EZNEC can tell me anything about
its effectiveness. I can't figure out how to tie the wire in to only
the ground side of a source. I read up on split sources but still
can't be sure I'm doing this correctly.

73
Dykes (AD5VS)

dykesc wrote:

Thanks Roy. You've pretty much confirmed what everyone else is saying
regarding feedline radiation. Hadn't thought about the induced
contributor from the asymmetrical configuration. So a 90 degree
feedline takeoff in this situation would still result in induced
current it would appear.

Yes, that's correct. And sometimes a single common mode choke (current
balun) can actually make it worse by forcing near-zero current at a
point favorable to the current distribution.

I added a wire simulating the radiating
feedline conductor in my EZNEC model. It appears from the FF patterns
it may be helping in some directions.

It may well be. But in order to really know what the common mode current
will be, you'll need to accurately model the entire path to the Earth,
because that path (length and orientation) determines the common mode
current distribution.

While I've got you, is there any way in EZNEC to add an RF ground
counterpoise at the source end of my model? I have a long wire at my
common equipment ground point to better enable RF grounding on my
equipment and would like to see if EZNEC can tell me anything about
its effectiveness. I can't figure out how to tie the wire in to only
the ground side of a source. I read up on split sources but still
can't be sure I'm doing this correctly.

The way to do it is to put your (conventional, non-split) source on the
end segment of the counterpoise wire, and connect the transmission line
end to the same segment, rather than using a virtual segment for the
interconnection.

Roy Lewallen, W7EL

On Jun 25, 3:13*pm, Roy Lewallen wrote:
dykesc wrote:

While I've got you, is there any way in EZNEC to add an RF ground
counterpoise at the source end of my model? I have a long wire at *my
common equipment ground point to better enable RF grounding on my
equipment and would like to see if EZNEC can tell me anything about
its effectiveness. I can't figure out how to tie the wire in to only
the ground side of a source. I read up on split sources but still
can't be sure I'm doing this correctly.

The way to do it is to put your (conventional, non-split) source on the
end segment of the counterpoise wire, and connect the transmission line
end to the same segment, rather than using a virtual segment for the
interconnection.

Roy Lewallen, W7EL

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)

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)