That is not correct. You cannot legally build something that interferes with FCC-licensed devices. Various devices fall under different FCC licensing categori

That is not correct. You cannot legally build something that interferes with FCC-licensed devices. Various devices fall under different FCC licensing categories, which basically define a hierarchy of who-gets-to-interfere-with-whom. "Part 15" devices, basically home and office equipment, is basically at the bottom of the stack (must not interfere, must accept interference received). Moving up the pecking order, Amateur Radio, under "Part 97", can under some circumstances interfere with Part 15. There are other sections for microwave ovens (Part 1030?), commercial broadcasting, medical equipment, military / government use, radionavigation... anything that uses the RF spectrum basically. The FCC regs apply everywhere in the US, all the time. With certain exceptions (very low power applications, amateur radio equipment operated according to regs), you can't build and operate random RF-emitting gear legally. (Much of it you're not even allowed to legally modify. If you run a microwave with the cover off, that's a violation.) Constructing a cellphone jammer is much like constructing a machinegun: it's technically not that hard, and you can probably get away with it too if you're careful, but it was illegal from the moment you brought it into existence without the proper procedure. (Less prison time associated with it, though, and the rules governing simple possession aren't quite as strict, admittedly.)