On Sep 3, 8:04*am, Cecil Moore wrote:
wrote:
Many US amateurs do not understand how a balun works.

Quoting "The ARRL Antenna Book" re a w2du balun:
"Maxwell made a test balun by slipping 300 #73 beads
over a piece of RG-303 coaxial cable. The impedance
of the outer conductor of the cable measured
4500+j3800 at 4.0 MHz."

The differential current emerging from the inside
of the coax braid encounters, e.g. 30 ohms, looking
toward the antenna. It encounters 4500+j3800 = 5890
ohms looking back down the outside of the coax.
Ohm's law does the rest.

Without the beads, that differential current might
see an impedance lower than 30 ohms looking back
down the outside of the coax for certain unfortunate
lengths of coax. The common-mode choking impedance
forces ~equal currents in each dipole leg at the
BALanced antenna to UNbalanced coax junction. That
common-mode choking impedance causes the balun
function to occur.
--
73, Cecil *http://www.w5dxp.com

Because the input and the out windings of the balun must be isolated
at least at RF of interest, OF COURSE there should be a high impedance
between the ballanced current flow and the unbalanced current flow.
However, the impedance of the source looking into the impedance from
the souurce should match source impedance and the impedance of the
load looking in the output winding of the balun should match the load
impedance (say 70 ohms, or 600 ohms or whatever your antenna or
trnasmission line impedance is. The impedence at frequency of interest
between input winding and output winding should be infinity for
isolation purposes (obviously you do not want to mix the unbalanced
input and balanced output currents.