On Sep 14, 5:51*pm, John Smith wrote:
OK, I'll ask, "I am correct in thinking that most are using a 'voltage
balun' (ruthroff 1:4) as the 'Low Loss Matching Transformer' and a
'current balun' (guanella 1:1) as the 'Special-High Isolation Line
Isolator?'"

That's the theory behind the Carolina Windom. Whether it actually
performs up to expectations is questionable. The marketing hype
acknowledges that the 4:1 voltage balun will cause common-mode current
on the coax which is expected for off-center-fed dipoles. They seem to
assume that the "Isolator" located 20 feet from the dipole feedpoint
is an ideal device with an infinite choking impedance. If it were
that, the antenna would work exactly as advertised. Unfortunately,
there's no such thing as an ideal isolator so we are zeroing in on
what is possible in reality and what must remain as marketing hype.
From my modeling, it appears that the isolator works well for any
frequency where the length of the coax from the feedpoint to the
isolator is approximately an odd multiple of 1/4WL. That occurs on 20m
and 10m. Unfortunately, my modeling indicates that RF-in-the-shack may
be a problem on 40m and I would be tempted to add an ugly balun.
--
73, Cecil, w5dxp.com