Coverage of earth by earth sensing satellites in visible spectrum is so high, that you can literally write different message into sand / earth every hour. No radio construction necessary XD
The biggest flaw of all prepper radio lists is that they do not contain knowledge about how easy it is to locate a radio transmitter. Every radio user should do at least one "radio fox hunt" to understand how to locate other people and / or how easy it is to be located.
You can do that even with radios on this list, no special equipment needed.