[TowerTalk] OCF dipole

[Tom Rauch]

> In reality, what is needed is an "un-un",  and Sevick's
work may prove helpful, as well.

We better be careful here.

A current balun is always useful as an "un-un", but an
"un-un" is not always a useful current balun. Some un-uns
have no common mode isolation, and would be totally useless.

In the OCF application the antenna requires a decoupling
device that provides a *very high* common mode impedance.
The only device guaranteed to do that is an exceptionally
good current balun. Certainly a string of beads would never
be adequate. The more we move the feedpoint towards an end,
the more complex or critical decoupling becomes.

> As both K4SAV and W8JI observed, an unbalanced antenna
will tend to have current on the outside of the feedline.
The CW is no exception.   My solution is use of  ferrite
chokes in the feed system,  on a ground point directly below
the antenna.   The coax shield is grounded through a
bulkhead connector fastened to the ground rod.  Clip on
ferrites both upstream and downstream of this point tamed
the common mode problem.

If that is true, the Carolina Windom lacks adequate feedline
decoupling.  That isn't any sudden surprise. Measuring a RW
line isolator, you'll see it doesn't even have 10% of the
claimed impedance of 50,000 ohms over most of HF. It takes a
lot more than a string of beads with a few hundred ohms
common mode impedance to decouple an OCF antenna.

A voltage balun or any other single core non-unity ratio
balun won't work either. It has to be a current balun or
common mode choke, and a good one.

73 Tom