Short note: Medium wave propagation

Medium wave (MW) propagation covers the MF band, 300 kHz – 3 MHz; AM broadcasting uses 535–1605 kHz (526.5–1606.5 kHz in Nepal/ITU Region 3).

Daytime

• Propagation is mainly by ground (surface) wave. Vertical polarisation is used (vertical mast radiators) because horizontal polarisation is quickly short-circuited by the earth.
• The D layer strongly absorbs MW sky waves, so there is almost no sky wave.
• Service range is about 100–300 km, depending on transmitter power and ground conductivity (larger over sea water, smaller over dry or rocky land).

Nighttime

• The D layer disappears, so the sky wave is reflected by the E layer with little loss and reaches 1000–2000 km.
• This gives long-distance reception but also interference between distant stations on the same channel and fading where ground wave and sky wave of similar strength meet (near-fading zone).

Other points

• Attenuation of ground wave rises with frequency, so the lower end of the band covers more area.
• Antennas are tall λ/4 Marconi masts with good radial ground systems; anti-fading antennas (≈ 0.53λ high) reduce high-angle radiation.
• Signal is stable in daytime, so MW is used for local and regional AM broadcast, maritime and aeronautical beacons.