to determine the swr you have to figure the impedance, which can vary
depending on height, exact shape, etc... no single formula, you can model
with a program of course, but its likely easier to measure it.

as for losses... 100' of rg-58 on 20m has a loss of about 1.5db when
matched. raise that swr to 20:1 at the load and the loss goes up to 5db
(using graphs in old arrl antenna book so the exact numbers may be off a
bit, but who cares). so you might lose 1 s-unit if you try to use the 40m
dipole on 20m, likely something less.

now, if you use a common twin lead and drop that 1.5 db to .5db, then raise
the swr to 20:1 you still lose 3db. so for all the extra problems of
getting twin lead in and out of a house, and messing up the 40m match, you
save 2db.

"Gary" wrote in message
...
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