"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