Wouldn't people with backpacks use something a little higher frequency to
communicate?

Tokyo Hi-Power makesa handheld 40 meter (7 MHz) handheld SSB unit, that is just
a little bigger than an older Radio Shack 5 watt CB set (about 4x2x12 inches,
give or take). In fact some pictures of Osama Bin Laden show him with such a
unit in his hand. It uses a "loaded whip" antenna, and is probably the only
thing that would work from valley to valley comms...by beaming sigs upward to
be bounced downward off the F2 or other layer.