There were mentions of the repeater system last night, so my guess is they transmitted weakly somewhere and the repeater happily repeated it out to activate all the sirens.
You can do this pretty trivially with HAM gear, although some may require small modifications to allow transmitting on the necessary frequencies. You can also often buy the equipment on ebay ready to go.
You can do this pretty trivially with HAM gear, although some may require small modifications to allow transmitting on the necessary frequencies. You can also often buy the equipment on ebay ready to go.