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
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