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