An alternative to screw-in anchors is earth anchors that aren't screwed in.
Most people probably aren't aware that W3LPL, for instance, has no concrete in
any anchors, with multiple 200' towers and other shorter ones. He hired a
backhoe to dig trenches in a T shape and put "big disk" anchors in the slot,
then backfilled. A key to the strength of all anchors is that the anchor,
whether it's an anchor rod in concrete or an anchor rod without concrete, is
that it pulls against "undisturbed earth." W3LPL uses a separate anchor for
each individual guy wire. Just another "data point." 73 - Rich, KE3Q