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Old September 22nd 08, 06:52 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?

Having received some very good advice about the Carolina Windom, I now
ask a question to which I THINK I already know the answer.

This particular version of the CW OCF dipole uses 300 ohm twin lead
feedline terminated after 33 feet to a 4:1 balun. Richard warned of
significant risk of RF on the outer (inner?) braid of the coax.

I've never read of using a 1:1 unun right after a 4:1 balun to minimize
RF on the outer coax.

My thinking is that the 4:1 balun acts as a voltage type and will do
nothing to ameliorate this. So, I will put some ferrite rings or snap
on ferrites just past the 4:1 balun on the coax.


Does this sound like a reasonable solution?

John
AB8O
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Old September 22nd 08, 07:16 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

john Wiener wrote:

...
I've never read of using a 1:1 unun right after a 4:1 balun to minimize
RF on the outer coax.

My thinking is that the 4:1 balun acts as a voltage type and will do
nothing to ameliorate this. So, I will put some ferrite rings or snap
on ferrites just past the 4:1 balun on the coax.


Does this sound like a reasonable solution?

John
AB8O


If you use a 4:1 Ruthroff, it would be a voltage balun ... if you use a
4:1 Guanella it would be a current balun ... the 1:1 current balun is
probably more useful behind a Ruthroff ... but hey, once you have tried
all these possible combinations, you can speak from experience! grin

If going from 300 ohm to 50 ohm, perhaps you would choose a 6:1. Or,
just go with the 4:1 now and when you need some diversion, later, try
the 6:1 to see what improvments can be had and if the loss in this
design is acceptable to you ... etc.

Anyway, in this document is a 6:1 (actually 6.25:1, resulting in 312ohm
to 50 ohm) made from two 4:1 baluns (I would think Guanella ununs ...
the 4:1 can be made from two 1:1, each wound on the opposite side of
toroid core, reversing coil directions on one side. This could also be
accomplished with 4 ferrite rods ... a 1:1 balun wound on each rod, two
rods combined to make a 4:1 balun, then these "two units" combined to
construct the 6.25:1 balun ...

Regards,
JS
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Old September 22nd 08, 07:18 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

John Smith wrote:
[stuff and forgot the URL, as usual :-( ]

The URL for the 6.25:1 ...

http://www.radioelectronicschool.net.../ocfdipole.pdf

Regards,
JS
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Old September 22nd 08, 07:36 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

john Wiener wrote:
Having received some very good advice about the Carolina Windom, I now
ask a question to which I THINK I already know the answer.

This particular version of the CW OCF dipole uses 300 ohm twin lead
feedline terminated after 33 feet to a 4:1 balun. Richard warned of
significant risk of RF on the outer (inner?) braid of the coax.

I've never read of using a 1:1 unun right after a 4:1 balun to minimize
RF on the outer coax.

My thinking is that the 4:1 balun acts as a voltage type and will do
nothing to ameliorate this. So, I will put some ferrite rings or snap
on ferrites just past the 4:1 balun on the coax.


Does this sound like a reasonable solution?


A while back I did some pretty careful measurements of an OCF dipole. I
found that ferrites were required at both the feedpoint and at one or
more places along the feedline. The ferrites at the feedpoint suppress
the conducted common mode current (which is actually forced to exist by
the voltage balun). But the asymmetry of the antenna results in common
mode current being induced onto the feedline by mutual coupling to the
antenna. This isn't a problem in a symmetrical dipole if the feedline is
positioned symmetrically relative to the antenna, since the currents
induced by the two equal halves cancel. But the OCF dipole can result in
quite a lot of induced common mode current.

Ideally, you'd put at least a second bunch of snap on cores about a
quarter wavelength from the feedpoint. But one of the main reasons
people use OCFs is for multi-band operation. So the thing to do is to
place the cores for maximum effectiveness on the band(s) where you have
the most trouble -- the common mode current also depends on the feedline
length and position, and will vary considerably from band to band even
if you do nothing.

My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, and the best you
can do is reduce it to a level you can tolerate.

Roy Lewallen, W7EL
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Old September 22nd 08, 08:04 PM posted to rec.radio.amateur.antenna
Tam Tam is offline
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Default Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?


"Roy Lewallen" wrote in message
treetonline...
john Wiener wrote:
Having received some very good advice about the Carolina Windom, I now
ask a question to which I THINK I already know the answer.

This particular version of the CW OCF dipole uses 300 ohm twin lead
feedline terminated after 33 feet to a 4:1 balun. Richard warned of
significant risk of RF on the outer (inner?) braid of the coax.

I've never read of using a 1:1 unun right after a 4:1 balun to minimize
RF on the outer coax.

My thinking is that the 4:1 balun acts as a voltage type and will do
nothing to ameliorate this. So, I will put some ferrite rings or snap on
ferrites just past the 4:1 balun on the coax.


Does this sound like a reasonable solution?


A while back I did some pretty careful measurements of an OCF dipole. I
found that ferrites were required at both the feedpoint and at one or more
places along the feedline. The ferrites at the feedpoint suppress the
conducted common mode current (which is actually forced to exist by the
voltage balun). But the asymmetry of the antenna results in common mode
current being induced onto the feedline by mutual coupling to the antenna.
This isn't a problem in a symmetrical dipole if the feedline is positioned
symmetrically relative to the antenna, since the currents induced by the
two equal halves cancel. But the OCF dipole can result in quite a lot of
induced common mode current.

Ideally, you'd put at least a second bunch of snap on cores about a
quarter wavelength from the feedpoint. But one of the main reasons people
use OCFs is for multi-band operation. So the thing to do is to place the
cores for maximum effectiveness on the band(s) where you have the most
trouble -- the common mode current also depends on the feedline length and
position, and will vary considerably from band to band even if you do
nothing.

My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, and the best you can
do is reduce it to a level you can tolerate.

Roy Lewallen, W7EL


Roy,
What's your opinion on the 4: or 6:1 balun between the 300 Ohm line and the
coax? I see no reason whatever to think that the impedance coming off the
300 Ohm line is anywhere near 300 Ohms. Also, where is it written that a 50
Ohm balun will work at, say, 2000 Ohms. The ferrites as you suggest will
clearly work if you use enough of them.

The reason for asking this is that a friend is in the process of putting up
a 75 m dipole, which he only plans to use on 75 m. Everybody is telling him
to feed it with ladder line going to coax through a balun. Why in the world
would you do that?

Tam/WB2TT



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Old September 22nd 08, 08:05 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

john Wiener wrote:
Having received some very good advice about the Carolina Windom, I now
ask a question to which I THINK I already know the answer.

This particular version of the CW OCF dipole uses 300 ohm twin lead
feedline terminated after 33 feet to a 4:1 balun. Richard warned of
significant risk of RF on the outer (inner?) braid of the coax.

I've never read of using a 1:1 unun right after a 4:1 balun to minimize
RF on the outer coax.

My thinking is that the 4:1 balun acts as a voltage type and will do
nothing to ameliorate this. So, I will put some ferrite rings or snap
on ferrites just past the 4:1 balun on the coax.


Does this sound like a reasonable solution?

John
AB8O

Ouch! Sorry about multiple posts! Not sure what happened there.
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Old September 22nd 08, 08:45 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

Roy Lewallen wrote:
My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, ...


Isn't the section between the voltage balun and the choke
designed to radiate?
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein
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Old September 22nd 08, 09:03 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

Cecil Moore wrote:
Roy Lewallen wrote:
My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, ...


Isn't the section between the voltage balun and the choke
designed to radiate?

Cecil
In the version I am putting up, the 4:1 balun is at the bottom of the
twin lead feedline, so the feedline can radiate.

John
AB8O
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Old September 23rd 08, 05:11 AM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?

Is this OCF dipole a single band antenna or a multiband antenna?
- 'Doc
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Old September 23rd 08, 01:16 PM posted to rec.radio.amateur.antenna
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Default Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?

wrote:
Is this OCF dipole a single band antenna or a multiband antenna?
- 'Doc


It is alleged to be a multi-band antenna. Assuming that
the standing-wave current is 90 degrees out of phase
with the standing wave voltage, one can actually calculate
an approximation for the feedpoint impedance by dividing
the standing-wave voltage by the standing-wave current at
the feedpoint. With an OCF, the feedpoint impedance will
always be lower than Vmax/Imin and always higher than
Vmin/Imax and thus provide a reasonable SWR for 450 ohm
ladder-line.

The free demo version of EZNEC available from
http://www.eznec.com
will provide a reasonable guess at the feedpoint impedance.

Here are what EZNEC 4.0 says about the
feedpoint impedance of the following:

------------83'----------FP-----47'--------

3.8 MHz, 85.5+j98.5 ohms, SWR(300)= 3.9, SWR(450)= 5.5
7.2 MHz, 177-j181 ohms, SWR(300)= 2.5, SWR(450)= 3
10.125 MHz, 3569-j102 ohms, SWR(300)= 12, SWR(450)= 7.9
14.2 MHz, 96-j357 ohms, SWR(300)= 7.7, SWR(450)= 7.7
18.14 MHz, 797-j911 ohms, SWR(300)= 6.3, SWR(450)= 4.4
21.3 MHz, 933-j1239 ohms, SWR(300)= 8.8, SWR(450)= 6
24.95 MHz, 218-j652 ohms, SWR(300)= 8.5, SWR(450)= 6.7
28.4 MHz, 757+j1019 ohms, SWR(300)= 7.4, SWR(450)= 5.1

As you can see, the myth that this antenna has a 300
ohm feedpoint impedance is just an old wives' tale but
it does have reasonable SWRs when fed with 450 ohm
ladder-line.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein
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