Topband: New TX antenna for 80 and 160?

[Rick Kiessig]

Until recently, I was planning to put up an inverted-L as a TX antenna for
80 and 160. However, things have changed, and it no longer looks like I'll
be able to run the radials I would need to have an effective vertical. Now
I'm leaning toward a low dipole.

My site is highly constrained: it's near the top of a ridge, on a slope,
facing the ocean (100m above sea level, 300m from the water). The highest
spot above sea level is the top of my tower: it's only 8.5m above immediate
ground level (next to the house), although the ground 10m away is 7m lower.
I can't run more than a wire or two and a coax feed on (not above) the
ground - an FCP, for example, would be much too large. Due to limitations
imposed by the city, I can't go higher than 10.5m above ground level.

If I run a wire around three sides of my property in a U shape, hung from
the tower near (but not at) the feedpoint, I can just hit 80m total length,
with a 46m long center section and two 17m long end sections. The wire would
attach to 10m high fiberglass poles near each of the four corners of the
property.

I recently built a dipole for 40/20/15 using 300 ohm twinlead, which worked
out very well, so I'm thinking of using a similar approach for this antenna,
using the full length for 160, and trimming one wire to be resonant on 80.
I'd like to do whatever I can to maximize bandwidth. If I could cover both
80 and 75, for example, that would be great.

Questions:

-- Is a low dipole for 80 and 160 on a sloping site like mine worth the
effort? I'm interested in DX, not NVIS.
-- Given my constraints, are there other types of TX antennas I should
consider?
-- What's the best wire to use to minimize stretching and to maximize
bandwidth and efficiency, and that can handle full legal power? I will need
at least two (maybe three?) wires to cover both bands. Copperweld is strong,
but I've heard it can be lossy, too. Twinlead has two wires, but it's
stranded and doesn't feel very strong.
-- I'm thinking about using Spiderbeam black fiberglass telescopic poles at
the corners. However, I'm concerned about durability in high winds and
having enough strength to be able to tension the wires so they don't droop
terribly. Is there a better choice?
-- I'm planning to put a common mode choke at the feedpoint and run coax
from there, as I've done with the other dipole. Any reason to do it
differently?

73, Rick ZL2HAM