Tom,
I modeled your antenna sketches on NEC-4 using a height of 40' above
average ground. For near 50 ohm use, the wire lengths are way short,
quite possibly because, like most antenna formulas, they do not take into
account the proximity of the antenna to the ground on these bands. 40' is
a small fraction of a waveength.
In any event, for use with the feedline length you have, lengths closer
to 180' and 90' respectively might be needed--or you may have to
experiment with the parallel feedline length to find a length that yields
something close to 50 ohms.
The source impedances at the ends of the parallel line all had high
capacitive reactance values, more than 100 ohms.
Using a combination of parallel line and coax makes these wires
essentially 1-band each, since the match on other bands is likely to be
poor. The benefit of using parallel line all the way to the shack and a
good ATU is that each antenna can be used on all bands. Heving two wires
at right angles is not a bad way to work, since the lobes and nulls on the
various bands may be different enough where switching antennas can make a
big difference in signal strength. Too boot, wire length becomes far less
critical, since shifts in reactance can usually be handling by most good
ATU designs.
On the other hand, should you prefer to keep these as 80 only and 40 only
antennas, then either a change in wire length of a change in 450-ohm
feedline length is indicated for your situation. Unfortunately, antenna
formulas are never very precise, and a low horizontal antenna height in
terms of a fraction of a wavelength (270' on 80 and 140' on 40, roughly)
tends to put them far off of the reality that antenna builders face.
Hence, trimming, extending, and experimenting are always the order of the
day.
-73-
LB, W4RNL
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