I've gotten interested in beam-forming via a phased antenna array. I read a few articles/tutorials and they have very nice diagrams of antenna arrays and how the antennas interact to behind each antenna is the key piece: A phase shifter. And yet when searching for it it seems like more of a theoretical device rather than a physical one-yet really phased antenna arrays exist. What is it exactly? Is it something so simple it's just assumed I know what it is?
What, physically, is a phase shifter?