Assuming you have at least a quarter wavelength of wire vertical on 40 mtrs.you
need to have separate radiators or at least stubs for each band to have a easy
to feed antenna on three bands as 40, 80, and 160. One quick way is to leave
the 160 L as is and put out another wire in the opposite direction to resonate
it as a 80 L. Then all you need to do is add a second vertical at your feed
point for a quarter on 40 with 4 in. or more spacing from 80/160 vertical of
the L.This gives you a low impedance element on band of interest and the other
two are at high impedance 1/8 or 1/2 lengths.
This type of antenna must be worked against a radial system for efficiency.
The higher you can get the radials the fewer you can get away with but less
than 4 is not a good idea unless a compromising layout necessitates.(I have
have used just a ground connection in portable operation but it was more of a
cloud burner than a distance communication antenna. Just 1 radial was a
improvement with the radial a foot or more off ground.) Also the more vertical
you can get for the L the lower the takeoff angle will be as it approaches the
angle of a true vertical 1/4 wave.
Radials for each band is better but you compromise to fit your situation. If
you get a very low SWR with 50 ohm coax direct feed be very suspicions that you
could use more radials or off ground ones!
This just a few ramblings on how to answer your question on 3 bands one L.
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