Everyone from the The Legion Of Doom to V for Vendetta to Vandal Savage has done this: delivered a terse ultimatum to the public using radio and/or television. Not just a pirate signal, but one that actually temporarily takes over many existing broadcasts. Is this actually feasible, especially as a one-shot deal?
How exactly does one go about taking over television/radio broadcasts? What kind of quality are we talking, here? Do you and your army have to go to the television/radio studios or are there ways to do it using equipment in your underground bunker or mobile unit or broadcast antennae? How long can you be on air before the authorities triangulate your signal and find you?
If you wanted to broadcast to the American masses, you could use your evil super powers to hack into the Emergency Alert System (previously Emergency Broadcast System). That gets you pretty much full control over all AM, FM and TV signals (including cable).
If you’re going to operate your evil plans from the back of a truck, you’re going to run into problems. Broadcast TV and FM radio are only line of sight, so that probably wouldn’t work. AM radio will get you a lot further, but only a few hundred miles at best. Maybe your best bet would be to have a giant satellite dish on the truck and hijack all the satellite TV stations and XM radio.
We interrupt this thread to announce that the organisation representing the Beleaguered United Masses (BUM) has taken over this means of communication.