OCF Dipole and Window Line Choke
 

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.

OCF Dipole and Window Line Choke
 

dykesc wrote:
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.

It is very difficult to choke 450 ohm window line.
I would suggest a husky 1:1 current-choke-balun at
your window-line/coax junction. The Carolina Windom
advertises feedline radiation as a benefit/feature.

With an OCF, fighting feedline radiation is usually a
losing battle.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

OCF Dipole and Window Line Choke
 

On Thu, 25 Jun 2009 08:34:00 -0700 (PDT), dykesc
wrote:

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.

No, you cannot, but is that important? Select a product that works
and connect the line to it, and the antenna to the other side. Of
course "works" means: offers sufficient Z in relation to the load and
the line. "Sufficient" means: 10 times Z.

Going further, what is the point of 450 Ohm (sic) Window line in this
application anyway? An OCF would only demand a 200 - 300 Ohm line
depending on where the off-center is off center.

Given there are an infinite off centers to every center, that makes
for a lot variation in drive point Z. Have you selected the optimum,
or just going for an ad hoc design? Given your 450 Ohm desire for an
antenna reputed to be 200 Ohms, it would seem you are guessing.

As you report you have EZNEC, the solution is at hand. Put up the
longest wire you can run. Place the source at the center of 101
segments. Set the alternative SWR source Z to 100 Ohms. Run a full
sweep from 3 to 30 MHz at 10 kHz increments. On a separate chart of
about 10 columns, make notes of each dip in SWR (about 4) in columns
that are spaced equal to the dip's relation to the full 3-30 sweep.
Now move the source 5% and repeat the steps. Keep moving the source
towards the end by 5% steps, again repeating the steps.

I've done this and know where the "best" offset is, but telling would
cheat you of your investment in EZNEC.

73's
Richard Clark, KB7QHC

OCF Dipole and Window Line Choke
 

dykesc wrote:
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'd let the feedline radiate. It's a part of the package.

Also, is there a reason that you wouldn't use coax instead of window
line? Your impedance on amateur frequencies should be such that a 4:1 or
6:1 balun at the (off) center point then a run of coax should work well.

- 73 de Mike N3LI -

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

On Jun 25, 12:29*pm, Michael Coslo wrote:
dykesc wrote:
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'd let the feedline radiate. It's a part of the package.

Also, is there a reason that you wouldn't use coax instead of window
line? Your impedance on amateur frequencies should be such that a 4:1 or
6:1 balun at the (off) center point then a run of coax should work well.

* * * * - 73 de Mike N3LI


Hi Mike

I modeled a 4:1 at the feedpoint in EZNEC. I would need 100 feet of 50
ohm line. Loss on RG-58 at 3.7 Mhz is 3.25 db. At 28.3 Mhz loss is
4.7db. Those are "best swr" frequencies for those bands. Operation on
either side of them results in even greater loss. Similar results on
other bands. RG-8 cuts the losses in half but the variation in SWR I
mentioned makes that a still unacceptable option for me. No amplifier
here so conservation of power delivered to the antenna is important to
me.

73
Dykes (AD5VS)

OCF Dipole and Window Line Choke
 

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)

OCF Dipole and Window Line Choke
 

On Jun 25, 11:14*am, Richard Clark wrote:
Ohm (sic) Window line in this

As you report you have EZNEC, the solution is at hand. *Put up the
longest wire you can run. *Place the source at the center of 101
segments. *Set the alternative SWR source Z to 100 Ohms. * Run a full
sweep from 3 to 30 MHz at 10 kHz increments. *On a separate chart of
about 10 columns, make notes of each dip in SWR (about 4) in columns
that are spaced equal to the dip's relation to the full 3-30 sweep.
Now move the source 5% and repeat the steps. *Keep moving the source
towards the end by 5% steps, again repeating the steps.

I've done this and know where the "best" offset is, but telling would
cheat you of your investment in EZNEC.

73's
Richard Clark, KB7QHC

You're right Richard. I am working with an "ad-hoc" design. I'll try
your method, but since I've only got 450 ohm line to work with, other
than lossy coax, I'll use 450 as the alternate Z0.

73
Dykes (AD5VS)

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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)

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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)

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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)

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

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)

OCF Dipole and Window Line Choke
 

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

OCF Dipole and Window Line Choke
 

On Jun 28, 9:26*am, Cecil Moore wrote:
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

Thanks for making the file public Cecil. I'll send a copy to Roy. He
said he would be tied up with field day activities through the
weekend.

73
Dykes (AD5VS)

OCF Dipole and Window Line Choke
 

dykesc wrote in news:447c037c-c3e4-4999-9eb1-
:

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

I had a look at your model and couldn't make head and tail of it.

Here is a simple model of a Carolina Windom, well to the extent that it
can be guessed from the smoke and mirrors used to describe and promote
it... http://www.vk1od.net/files/CW01.EZ .

The model attempts to represent a voltage balun at the dipole feed point,
a vertical coax feedline to a common mode choke (2000+j2000) and then the
coax outer is grounded via a moderately good station ground (directly
under the feedpoint). The power lost in the ground connection and the
choke can be found from the reports. The model does not attempt to
represent the differential mode transmission line, though it could be
done. The CW's voltage balun at the feedpoint is modelled as a pair of
equal voltage sources, with the common mode path connected between them.
I have treated the voltage balun as having zero common mode impedance, it
is probably a few ohms, but insignificant in this model.

As Roy I think already advised you, you have to model the entire common
mode path to ground and my example above is a simple one, but it
demonstrates a technique.

You will see the currents flowing in the three conductors (each side of
the dipole and the coax outer), and their contribution to / effect on the
pattern.

Others may disagree with my representation of the Carolina Windom.

Owen

OCF Dipole and Window Line Choke
 

On Jun 28, 4:51*pm, Owen Duffy wrote:

I had a look at your model and couldn't make head and tail of it.

Owen, maybe the following description will help with understanding my
model.

The antenna is an OCF Dipole in a Vee configuration. Wires 1 and 3 are
the short and long sides respectively. Wire 4 is just a short
connector at the apex. The antenna is fed with 100 feet of 450 ohm
line which transitions through a 4:1 transformer to 5 feet of 50 ohm
coax and then to the source. Wire 6 is an attempt to simulate the
unbalance radiation from the 450 ohm line. Wire 7 is an approximate
80m 1/4 wave RF "sink" on the ground side of the source end of the
model and is not meant to be a main element radiator so its position
relative to the other elements is immaterial.

Also, as you can see, I have wire 6 grounded which isn't actually the
case in the physical world. It is meant to simulate radiation from the
450 ohm line which actually ties into the 200 ohm (ungrounded high Z
side) of the transformer. I guess I figured grounding it was the right
thing to do since the EZNEC Help file implies the need for that to
simulate coax radiation. I do have wire 6 positioned correctly
relative to the other antenna elements, so its effect on the antenna
pattern should be accurate.

Don't let short wire 5 which looks out of position throw you. It just
serves as a termination junction between the 50 ohm side of the
transformer and the short run of coax. There are a couple of other
short "junction only" wires in the model.

A look at the wires, transmission lines, transformers, and sources
tables should aid in understanding the model.

I by no means mean to imply I've modeled everything correctly. I just
wanted you to know what I was attempting when I built the model.

Thanks for the Carolina Windom model. Good reference for comparing to
what I have done.

Also, thanks for looking at this. Any advice much appreciated.

73

Dykes (AD5VS)

OCF Dipole and Window Line Choke
 

dykesc wrote in news:6c2bff9b-583d-4eb5-90fb-
:

You have two parts to your model, and they are disconnected (apart from
conductor coupling effects). You have complicated your model with your
counterpoise concept and it doesn't work.

Try a simple model with not more elements than necessary, then when you
have it working, current look rational etc, extend the model.

Hint: the TL element models the differential mode of the transmission line,
and NOT the common mode. To answer your original question, you do not need
to model differential mode TL, just put the source(s) up in the centre of
the dipole.

You do need to know the common mode impedance of your 4:1 transformer to
include it in the model. If it is a voltage balun, you can probably treat
it as zero ohms without introducing great error, but if it is a current
balun (as your mention of choke implies), you need to know its impedance at
the model frequency.

Owen