Thought I'd chime in on this one...and say G'day to Rich, as well.
We humans have this problem of seeing earth in the visible spectrum.
Doesn't mean that an antenna at 14MHz will see the same thing. So,
your horizontal distance from the sea is less important than your
vertical distance to the water table. I would assume that the mobile
ions somewhere around that region would define 'ground', for purposes
of first reflection. Or maybe it's water trapped in the clay layer.
Dunno. Doesn't matter, really. Ground at 14MHz ain't the same as
300 TeraHertz. And radials on Carribean beach sand are effectively
elevated above the water...which may explain the effect Rich mentioned.
(An aside, before I launch into antenna interaction... I'm located along
the Chesepeake Bay. About 250 yards inland from a harbor, and 22' above
sea level. My 80m halfwave is strung at 75' in trees. It is an absolute
killer into EU and Asia. Definitely better in the direction of water...
but good in any case. I seriously doubt that its effective height is
under 90-100'. No productive way to determine this, of course...but it
So...stacking a 20m yagi near an LPDA 18-30? 10' separation should get you
far enough apart that vswr won't be hosed too badly. But pattern
due to coupling and re-radiation? That will be a problem which may be hard
model. You could always try it and see what happens.
If it were me, I'd seriously consider extending the lpda boom, and adding a
of elements for 20m. Or, contemplate what happens if you put a 2 el 20 with
feed on one end, and a director on the other end of the lpda boom (passively
the director). Will it affect the lpda? Sure. But it'll be predictable.
lucky, manageable and acceptable as well.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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