I previously stumbled on 11.175, but I have become aware of other related frequencies.
11.175 or 8.992 USB (Upper Side Band), Apparently transmitting from Offutt Air Force Base. Websites that mention this make an assumption that the messages are one time pad, thus are unbreakable codes assuming that the pad is only used once – https://en.wikipedia.org/wiki/One-time_pad.
Other frequencies I have seen listed are:
6.739, 4.724, 13.200, 15.016 – all USB
Technology does some amazing things, but its incredible delicate. You really can’t stop simple radio signals so maintaining this line of communication is a great idea. I imagine there was some resistance to keeping and maintaining it after modern Internet Protocol (IP) packet networks matured. Whatever it is used for, they want to ensure it survives modern communication network failures, which is likely in a high intensity conflict (cyberwar, space asset destruction, etc).
The messages themselves aren’t understandable without the key, but its interesting to listen to nonetheless.
I haven’t monitored these frequencies often in the last few months, but many claim that traffic increases when serious events occur around the globe, which makes these frequencies a sort of indicator of world tension.