Longwave

Propagation

Because of their long wavelength, radio waves in this frequency range can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ground wave, is the main mode in the longwave band. The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies, and falls with frequency. Low frequency ground waves can be received up to 2,000 kilometres (1,200 mi) from the transmitting antenna.

Low frequency waves can also occasionally travel long distances by reflecting from the ionosphere (the actual mechanism is one of refraction), although this method, called skywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric E layer or F layers. Skywave signals can be detected at distances exceeding 300 kilometres (190 mi) from the transmitting antenna.

Non-directional beacons

Non-directional beacons transmit continuously for the benefit of radio direction finders in marine and aeronautical navigation. They identify themselves by a callsign in Morse code. They can occupy any frequency in the range 190–1750 kHz. In North America, they occupy 190–535 kHz. In ITU Region 1 the lower limit is 280 kHz.

Time signals

There are stations in the range 40–80 kHz that transmit time signals to radio clocks. For example:

Long-waves travel by groundwaves that hug the surface of the earth, rather than mediumwaves or shortwaves, whose signals can travel as skywaves, ‘bouncing’ off different layers of the ionosphere at different times of day, which makes the time lag different for every signal received. The delay between when the long-wave signal was sent from the transmitter (and the coded time was correct), and when the signal is received by the clock (when the coded time is slightly late), depends on the overland distance between the clock and the transmitter and the speed of light through the air, which is also very nearly constant. Since the time lag is essentially the same, a single constant shift forward from the time coded in the signal can compensate for all long-wave signals received at any one location from the same time signal station.

Military communication

The military of the United Kingdom, Russian Federation, United States, Germany, and Sweden use frequencies below 50 kHz to communicate with submerged submarines.

LowFER

In North America during the 1970s, the frequencies 167, 179 and 191 kHz were assigned to the short-lived Public Emergency Radio of the United States. Nowadays, in the United States, Part 15 of FCC regulations allow unlicensed use of 136 kHz and the 160–190 kHz band at output power up to 1 watt with up to a 15-meter antenna. This is called Low Frequency Experimental Radio (LowFER). The 190–435 kHz band is used for navigational beacons.

Historic

Swedish station SAQ, located at the Varberg Radio Station facility in Grimeton, is the last remaining operational Alexanderson alternator long-wave transmitter. Although the station ended regular service in 1996, it has been maintained as a World Heritage Site, and makes at least two demonstration transmissions yearly, on 17.2 kHz.

Broadcasting

Long-wave is used for broadcasting only within ITU Region 1. The long-wave broadcasters are located in west, north, central and south-east Europe, the former Soviet Union, Mongolia, Algeria and Morocco.

Typically, a larger geographic area can be covered by a long-wave broadcast transmitter compared to a medium-wave one. This is because ground-wave propagation suffers less attenuation due to limited ground conductivity at lower frequencies.

Carrier frequencies

Long-wave carrier frequencies are exact multiples of 9 kHz; ranging from 153 to 279 kHz, except for a French language station Europe #1 in Germany. This station did keep to correctly spaced channels spacing for 4 months—only 7 years ago, and all Mongolian transmitters are spaced at 10 kHz.

Until the 1970s, some long-wave stations in northern and eastern Europe, and the Soviet Union, operated on frequencies as high as 433 kHz.

Some stations, for instance Droitwich transmitting station in the UK, derive their carrier frequencies from an atomic clock. They can be therefore used as frequency standards. Droitwich also broadcasts a low bit-rate data channel, using narrow-shift phase-shift keying of the carrier, for Radio Teleswitch Services.

In January 2014, Russia closed all of its LW broadcast transmitters, except for one in Caucasus, which was subsequently shut down in 2015.

Long distance reception

Because long wave signals can travel very long distances, some radio amateurs and shortwave listeners engage in an activity called DXing. DXers attempt to listen in to far away transmissions, and they will often send a reception report to the sending station to let them know where they were heard. After receiving a report, the sending station may mail the listener a QSL card to acknowledge this reception.

The longest distance over which a long wave signal has been received is 18,451 kilometres (11,465 mi). It occurred on 27 July 2015, when Mike Thayne of England, received Radio NL on 358 kHz from New Zealand.