Which Q signal indicates that you are receiving interference from other stations?

The Q signal 'QRM' means that you are experiencing interference from other stations or signals. A helpful memory aid is to associate the 'M' in QRM with 'Man-made' interference, such as other operators talking over the frequency. It is highly important to distinguish QRM from other similar Q signals on the exam. 'QRN' refers to natural static or atmospheric noise (think 'N' for Nature). 'QTH' refers to a station's location, and 'QSB' indicates that the signal is fading in and out.