Changing the modeled line impedance to 300 ohms is correct if you really
have a 6:1 transformer. But you probably don't.

The impedance transformation of a typical 6:1 balun is around 6:1 when
the antenna impedance is near 50 ohms resistive. However, when it's not,
the transformation can be vastly different. And the balun is likely to
add substantial series and/or shunt reactance to boot. So there's no way
to know what impedance you'll really see looking into the transformer
unless you measure one when loaded with various impedances.

You should also know that a "voltage balun", which a 6:1 balun typically
is, produces equal currents in the dipole halves, and no current on the
feedline, only if the antenna is perfectly symmetrical. If the antenna
isn't symmetrical (e.g. if it's fed off center), the "balun" will force
common mode current to flow along the outside of the coax. This will
affect the feedpoint impedance as well. See
http://eznec.com/Amateur/Articles/Baluns.pdf for more information about
this.

Roy Lewallen, W7EL

wrote:
I am using Nec Win Plus (NEC2) to view the SWR curve of my modeled
antenna. The actual antenna uses a 6:1 ratio balun. Is it incorrect
to just change the characteristic impedance of the line from 50 ohms to
300 ohms to see what effect a 6:1 balun would have on the SWR values? I
was just assuming that the line impedance was only really needed to
calcuate SWR - hopefully I am right.

Thanks,
Scott, WU2X