Back in the 1980's, these were called SLHFBs (Single Letter High
Frequency Beacons). There was quite a bit of speculation as to what they
were for. The best assessment was that they were some sort of a "last
chance" alert system for the soviet ballistic missile submarines. These
beacons were probably located near high priority targets. In the event a
missile sub was unable to raise higher authority, they would check for
the beacons. If ALL of them were off the air, the assmption was that an
attack had taken out the beacons as well as National Command, and the
subs were thus authorized to fire off their missiles.

"Peace is that brief moment in history when everybody stands around
reloading."
Rev. Skip Heitzig