Once in the early 90s, I was attached to a consulting firm whose primary goal was helping businesses get onto the Information Superhighway all the CEOs were buzzing about.
We were granted a guided tour one night of a facility at the top of a skyscraper in San Diego. It was shrouded in mystery and government TLA programs. My boss was explaining parts of it to me.
He said this is a TEMPEST secure facility. I'd never heard the term but when he described the principle to me it made a lot of sense. My father is a radio buff and raised me to learn all about electromagnetic stuff. The facility had conditioned power lines and shielded walls and partitions that could block EMR effectively to keep computing information safe from prying eyes.
There were other fascinating Top Secret features of the facility that were explained to me that night. Of course the facility was not in operation and unmanned at the time of our unclassified tour. I was gobsmacked at the depth of real bona fide security measures and countermeasures, even in those primitive times.
The RF encoder + cable acted as a transmitter.