The nominal impedance of a 40 m dipole, at 30 ft above an average ground, is
4700 + j0. There may be some inductive or capacitive reactance present --
depending on the exact length of the antenna -- but it will not effect the
transmission line losses significantly.

100 ft of RG 58 exhibits a total line loss of about 13 dB when terminated
with the above impedance. i.e. 100 W in gives 5 W radiated.

For the above analysis I used NEC 2 for the antenna model, and ARRL's TLA
for the transmission line loss.

Regards,

Frank


"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