Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. In most of the world, mediumwave serves as the most common band for broadcasting. The standard AM broadcast band is 525kHz to 1715kHz in North America, but remains only up to 1615kHz elsewhere.
Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably. Many stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range.
In the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of 5kHz. In the rest of the world, the separation is 9 kHz, with sidebands of 4.5kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands.
Stereo transmission is possible and offered by some stations in the US, Australia, South Africa, France. However, there are multiple standards for AM stereo, and receivers that actually implement the technologies are relatively rare.
In September 2002, the United States Federal Communications Commission approved the iBiquity in-band on-channel (IBOC) system of digital audio broadcasting, which is meant to improve the audio quality of signals. The Digital Radio Mondiale (DRM) IBOC system has been approved by the ITU for use outside the Americas.
In US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts.
As aerials mostly mast antennas are used.
For most of the 20th century, the radio frequency 500 KHz was reserved world wide as the Morse code international calling and distress frequency for ships on the high seas. The frequency 2182 kHz is still used for this purpose, but employing voice transmission.