Hi Frank and Topbanders,
For an open field use of aluminum fence wire will works just fine.
The only problem is one of mechanics...avoid kinking it when
installing...guaranteed to break then! If is does break, it can
be repaired first by polishing the wire at the break with fine
sandpaper, etc then make a tightly-wound "Western Union"
splice using about 6 inches of wire on each side of the splice.
I have had an open wire transmission line feeding my 160m
vertical for many years. One span is over 200 ft long. No
problems with stretching or breaking except once during
a severe ice storm.