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