I'm looking into using aluminum fence wire for a horizontal loop too, and I was curious if icing or even rain might have any effects on the uninsulated wire. Any odd behavior I should look out for?
If you are expecting serious ice build-up, then you may need the strength of a steel core
rather than pure aluminum.
You might consider Teflon insulated wire to reduce ice build-up, as sleet tends to run off the
surface faster. Otherwise I've heard of hams using car wax or other coatings (also to reduce
the changes in wet twinlead) though I haven't tried it myself.
Insulated wire might
show slightly less detuning when covered with ice and snow, but
I don't know if it would be enough to make a difference.
At least with a loop you can run some current though it to help melt the ice: a low voltage,
high current transformer winding may do the job.