Medium wave (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. During the daytime, reception is usually limited to more local stations, though this is dependent on the signal conditions and quality of radio receiver used. Improved signal propagation at night allows the reception of much longer distance signals (within a range of about 2,000 km or 1,200 miles). This can cause increased interference because on most channels multiple transmitters operate simultaneously worldwide. In addition, amplitude modulation (AM) is often more prone to interference by various electronic devices, especially power supplies and computers. Strong transmitters cover larger areas than on the FM broadcast band but require more energy and longer antennas. Digital modes are possible but have not reached momentum yet.
MW was the main radio band for broadcasting from the beginnings in the 1920s into the 1950s until FM with a better sound quality took over. In Europe, digital radio is gaining popularity and offers AM stations the chance to switch over if no frequency in the FM band is available, (however digital radio still has coverage issues in many parts of Europe). Many countries in Europe have switched off or limited their MW transmitters since the 2010s.
The term is a historic one, dating from the early 20th century, when the radio spectrum was divided on the basis of the wavelength of the waves into long wave (LW), medium wave, and short wave (SW) radio bands.
Spectrum and channel allocation
{ | class="wikitable" |- ! area ! kHz (centre) ! spacing ! channels |- | Europe, Asia, Africa | 531–1,602 | 9 kHz | 120 |- | Australia/New Zealand | 531–1,701 | 9 kHz | 131 |- | North and South America | 530–1,700 | 10 kHz | 118 |} For Europe, Africa and Asia the MW band consists of 120 channels with carrier frequencies from 531–1602 kHz spaced every 9 kHz. Frequency coordination avoids the use of adjacent channels in one area. The total allocated spectrum including the modulated audio ranges from 526.5–1606.5 kHz. Australia uses an expanded band up to 1701 kHz.
North America uses 118 channels from 530–1700 kHz using 10 kHz spaced channels. The range above 1610 kHz is primarily only used by low-power stations; it is the preferred range for services with automated traffic, weather, and tourist information.
Sound quality
The 9/10 kHz channel stepping on MW requires limiting the audio bandwidth to 4.5/5 kHz because the audio spectrum is transmitted twice on each side band. This is adequate for talk and news but not for high-fidelity music. However, many stations use audio bandwidths up 10 kHz, which is not Hi-Fi but sufficient for casual listening. In the UK, most stations use a bandwidth of 6.3 kHz. With AM, it largely depends on the frequency filters of each receiver how the audio is reproduced. This is a major disadvantage compared to FM and digital modes where the demodulated audio is more objective. Extended audio bandwidths cause interference on adjacent channels.
Propagation characteristics
Wavelengths in this band are long enough that radio waves are not blocked by buildings and hills and can propagate beyond the horizon following the curvature of the Earth; this is called the groundwave. Practical groundwave reception of strong transmitters typically extends to , with greater distances over terrain with higher ground conductivity, and greatest distances over salt water. The groundwave reaches further on lower medium wave frequencies.