Dave, your assumptions are off the mark.

On Fri, 10 Jun 2005 12:45:29 -0000, "Dave" wrote:

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.

NEC modeling of a 40-meter dipole at 66'

40-meters: Z= R69.5 J0 SWR (RG-58)= 1.4:1

20-meters Z= R4,198 +j1742 SWR(RG-58)= 98:1

Total Transmission line loss using RG-58

Total line loss (100 ft)= 1.278dB

Total line loss (100ft)= 11.64dB


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.


Same antenna using 100 feet of 450-ohm ladder line

40-meters: Z= R69.5 J0 SWR = 6.5:1

20-meters Z= R4,198 +j1742 SWR = 11:1

40-meters total line loss=0.19dB

20-meters total line loss=0.55dB

The antenna feed point impedance calculated using EZNEC

Transmission line loss calculated using TWL

Regards,

Danny, K6MHE