The greatly increased use of the radio-frequency spectrum above 300 megacycles since World War II made necessary a re-examination of the interference problem. Not only may interference occur between equipments and services operating at these high frequencies, but also interference may be caused by equipments operating at lower frequencies with appreciable harmonic radiation above 300 megacycles. In particular, electromagnetic radiation from radio-frequency industrial, scientific and medical equipments may cause interference to communication services above 300 megacycles.