"Dan Richardson arrl net" k6mhatdot wrote in message
...
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


The nominal impedance of a 40 m dipole on 20 m, 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.

The results are very similar to K6MHE, but should show zero reactance on
some frequency. Anyway the losses are close.

Regards,

Frank