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

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

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

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 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

Tam wrote:

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?

As you suspect, the impedances encountered by the transformer on some
bands are wildly different than its nominal design impedances. In the
one which I carefully measured, the result was no surprise. When the
antenna impedance was substantially different from 300 + j0, the
transformation ratio wasn't 6:1, and the transformer added series and/or
shunt reactance, sometimes a pretty large amount. And this was the case
on most bands.

This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands. But when it does, it's not working at all like predicted by a
simplified analysis which ignores the strong feedline coupling and very
non-ideal transformer effects.

Roy Lewallen, W7EL

Roy Lewallen wrote:

...
This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands. But when it does, it's not working at all like predicted by a
simplified analysis which ignores the strong feedline coupling and very
non-ideal transformer effects.

Roy Lewallen, W7EL

I have switched my balun designs to the one in this URL, Figure 4 -
Improved 4:1 Current Balun, page 3:

http://home.earthlink.net/~christras...k4to1Balun.pdf

Can be used balanced, or forced to unun fashion ... bandwidth becomes
increased (and, it "just works better for me!")

But, what works for you is the most important, always ...

Regards,
JS

Roy Lewallen wrote:
This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands.

I bought the 300 ohm feedpoint myth when I was at Texas A&M
in 1958 when I didn't know any better. I had an OCF fed with
300 ohm twinlead fed through a 6:1 air-core Heathkit balun
driven by a DX-40. The results were amazing to me at the
time. Now I know the pi-net output of the DX-40 would achieve
a match to almost anything and I should have been using a 1:1
balun. The Heathkit balun didn't have much loss and the 300
ohm twinlead didn't have much loss. I have no idea what the
actual impedances were, but losses were minimized and the
system successfully worked the world at the height of the
most active sunspot cycle in recorded history.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

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

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