The Low Frequency (LF) and Very Low Frequency (VLF) bands, spanning 15 to 120 kHz, play a vital role in global communication. These frequencies are known for their long-range propagation and ability to penetrate water and soil, making them indispensable for time signal dissemination, navigation, and military applications.
DCF77: Germany’s Time Signal Transmitter
One of the most well-known transmitters in this range is DCF77, located in Mainflingen, Germany. Operating at 77.5 kHz with a transmission power of 50 kW, DCF77 has been in operation since January 1, 1959. Initially designed as a standard-frequency station, it began transmitting date and time information in June 1973. Controlled by the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, DCF77 serves as the official time reference for Germany and much of Europe. Its signal can be reliably received up to 2,000 km away.
How DCF77 Works
DCF77 transmits time information using amplitude modulation. At the start of each second, the carrier power is reduced to 15% for either 0.1 or 0.2 seconds, representing binary ‘0’ and ‘1’ respectively. This sequence encodes the current time and date over the course of a minute. The carrier frequency is synchronized with atomic clocks at the PTB, ensuring deviations do not exceed 5.5 ± 0.3 microseconds.
Grimeton SAQ: A Historic VLF Station
Another significant transmitter is Grimeton SAQ, a unique Alexanderson alternator-driven VLF station in Sweden. Operating at 17.2 kHz, SAQ is one of the oldest functional radio transmitters in the world. Built in the 1920s, it was part of a global network for transatlantic communication before the advent of modern technologies. While no longer in regular operation, SAQ still transmits on special occasions, such as UNESCO World Radio Day, to demonstrate early radio technology.
Notable Transmitters in the 15–120 kHz Range
Several other transmitters operate within this frequency range:
- MSF (Anthorn, UK) – 60 kHz, 17 kW: The UK’s national time reference.
- WWVB (Fort Collins, USA) – 60 kHz, 70 kW: Provides time signals across North America.
- JJY (Japan) – 40 kHz & 60 kHz, 50 kW each: Transmits time signals across Japan.
- BPC (Lintong, China) – 68.5 kHz, 90 kW: China’s primary time signal transmitter.
- RBU (Moscow, Russia) – 66.6 kHz, 10 kW: Transmits time signals across Russia.
- NAA (Cutler, USA) – 24.0 kHz, 1.8 MW: A US Navy transmitter primarily for submarine communications.
- HWU (Rosnay, France) – 15.1, 18.3 & 21.75 kHz: Used for naval communications.
- VTX (Vijayanarayanam, India) – 17.0 kHz: Military communication station with a 471-meter tower.
Historical Context
The use of LF and VLF frequencies dates back to the early 20th century. These frequencies were essential for early transatlantic radio communication, including naval and military transmissions during World War I and II. Their ability to diffract over obstacles and follow the Earth’s curvature made them ideal for secure, long-distance communication. Even today, despite the rise of satellite technology, VLF remains crucial for submarine communications and backup time signals.
The 15 to 120 kHz frequency range hosts a variety of critical transmitters, from DCF77 for European time synchronization to Grimeton SAQ as a historic monument of early radio technology. These transmitters continue to play a vital role in global infrastructure, ensuring reliable communication, navigation, and timekeeping services.