Electromagnetic interference (EMI)
EMI is unwanted electromagnetic energy that leaks out of one circuit and degrades another — the electrical equivalent of one radio station bleeding into the channel next door. You've heard it: a buzzing speaker when your phone is about to ring, snow on an old TV when a drill runs nearby, or a wifi router that chokes near a microwave oven. Every switching wire, spinning motor and ticking clock is a tiny, unintentional transmitter.
EMI travels by two routes. Conducted EMI rides along physical wires — power cords, ground lines, data cables — and dominates below ~30 MHz. Radiated EMI launches off any conductor acting as an accidental antenna and flies through the air, dominating at higher frequencies. The cure always attacks one of three things in the chain — the source (slow the edges, add filters), the coupling path (shield, twist, separate), or the victim (improve its immunity) — and a seasoned engineer asks 'which of the three is cheapest to fix?'
Every EMI problem is a chain of three; break any one link and the interference stops.
EMI that lands in the radio band is often called RFI (radio-frequency interference). The notorious culprits are fast digital edges and switch-mode power supplies, whose sharp transitions are rich in high-frequency harmonics.