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.

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

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

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 -

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

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)

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)

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)

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)