I just happened to notice this along a road near where I live:
So these were in a pair, oppositely facing, "tip" to "tip" of the teardrop.
At first glance, this doesn't seem to be a bird diverter like here.
What is it for?
Note - I wrote "power lines" but I'm not actually sure what function this line has. Since its just one wire/cable, I didn't think it would be electrical transmission. Most other poles around here (rural) have a lot more on them than these do.
Minor addition - now that I'm looking for these, I'm seeing a lot of them in my area.