You can listen to all the radio frequencies at once, from low AM ones up to dozens of GHz using inexpensive radio components - it's how bug hunters you see in m

You can listen to all the radio frequencies at once, from low AM ones up to dozens of GHz using inexpensive radio components - it's how bug hunters you see in movies work, they just measure electromagnetic waves, regardless of their wave length. Granted, you'll not be able to differentiate between them, only that they exists. And at this point of searching for those signatures that's all you need though. Also Marconi was listening to lightning strikes generating radio waves using his radio, and that was over a century ago. Lightning strike generates radio waves all over the spectrum, so we had this capability right from beginning.