They are basically junk. At best, they are no better than a piece of wire. At worst, they can be dangerous or lethal if poorly made. They work by loosely coupling the antenna to the house wiring. They do pick up signals but not very well. They also pick up large amounts of interference and multipath from the house wiring. If you want to do something similar, just loosely wrap some insulated wire around the outside of the power cable and connect it to the antenna input. I've seen AM/FM radios that did something similar but the results were mixed. A properly designed antenna always works better.