I have a 5-element 10-meter yagi hung from a tall pine tree. It's not very
high, just about 20 feet. I used a piece of 1/4 inch wire rope as a support
cable. It goes up and over a limb and back down to support a 6-foot mast to
which the beam is attached at the bottom.
The mast helps to stabilize the yagi. It also has tie-lines of rope at both
ends of the boom to keep it steady. They are tied off to small trees to
keep it in the desired direction.
One of these days I'll get the support wire over a higher limb and raise it
a bit. It is fixed South, filling in the gap on my rotating side-mounted
10-meter yagi on one of the towers.
The wire rope allows for easy raising and lowering of the beam if
necessary. Using two wire ropes or steel guy line is even better; one to go
over the tree limb, with a pulley at the hanging end, and then another to
go through the pulley and raise the beam. That way if the tree grows around
the main cable that is over a limb you can still raise and lower the beam.
Another way is to string a guy wire between two trees; use it to hang a
rotor upside down and a beam hangs from the rotor.
I got these ideas from Richard, K3IPK, a fellow Frankford Radio Club
member, who had numerous beams hung from trees with cables.