If, just for example I'm loading 20 meters into a 40 meter dipole
what's the formula for determining the resultant SWR ? And then what's
the formula for determining what my losses would be between say
feeding the 40 meter dipole with 100' of RG-58 vs 100' of 450 ohm
ladder line on 20 meters. Of course the 450 ohm twinlead would screw
up the (likely) close match between the dipole and the RG-58 on 40
meters. Maybe this requires some kind of antenna modeling program ?

Thanks in advance.

Gary