If ETs sent a radio signal, you would never even know about it. It would be covered up and black projects would take control of the situation. They would print their messages on land for the simple reason of it being an extremely public method (everyone can see the message). If someone wanted to send a message to the people of a planet they would most likely do it openly instead of sending their message to a government owned radio facility where it could be easily silenced.

Disregard what I've just said. It was obviously a couple of people who knew of the original transmission working with boards and ropes in the blackness of night outside of an extremely secure and survailed (infrared cameras, sonic devices, motion censors) government facility for a few hours. What's on TV tonight?