Hi Bret:

If you're talking about guying an antenna, rope is fine.  However, due to
the torquing forces experienced by a tower, you should consider heavy-duty
stranded guy wire and anchors.

The stuff I use isn't that easy to work with, you need to cut it with a
hatchet or chisel  or something similar.  I got some guy wrap to hold the
wire in place on the tower legs.  This stuff won't deteriorate in th4e sun,
although I inspect the turnbuckles periodically to assure proper tensioning.