The difference between a 1:1 current balun and a 1:1 voltage balun is
that the latter has a third (tertiary) winding. I'll assume for a moment
that the baluns are perfect, i.e., have an infinite common mode
impedance. If the load is balanced, that is, if the two load terminals
have equal impedances to the "cold" side of the balun input, the third
winding of the voltage balun carries no current, and there will be no
current on the outside of the coax. Because the third winding has no
current, it can be removed with no effect, so the voltage balun acts
exactly like a current balun when the load is balanced. However, if the
load isn't perfectly balanced relative to the input terminal of the
balun, a current balun will still prevent current from flowing on the
outside of the coax. The voltage balun, however, will force the voltages
at the two output terminals to be equal and opposite relative to the
cold side of the balun input. This will cause unequal currents from the
two terminals. The difference flows along the outside of the coax.

In summary, the best a voltage balun can do in preventing current flow
on the outside of coax is to be as good as a current balun, and this
happens only if the load is balanced. In all other cases, using a
voltage balun will cause current to flow on the outside of the coax
while a current balun will prevent this flow.

For more information, see the article posted at
http://www.eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

C.Aymon wrote:
A current balun (as described by W2DU in his book ''Reflexions''), put at
the center of a dipole, prevents HF current from flowing in the outer shield
of the coax. This is due to the high impedance caused by the ferrite rings.
Now, if instead, I put a voltage balun, what should prevent the current from
flowing in the outer shield of the coax? Am I missing something?

Thanks for helping.

Chris