page_title: Low-frequency radio range
section_title: Ground
hierarchical_section_title: Low-frequency radio range / Technology / Ground
caption_attribution_description: Photo w:Low frequency radio range installation
context_page_description: The low-frequency radio range, also known as the four-course radio range, LF/MF four-course radio range, A-N radio range, Adcock radio range, or commonly "the range", was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s, until the advent of the VHF omnidirectional range, beginning in the late 1940s. It was used for en route navigation as well as instrument approaches and holds.
  Based on a network of radio towers which transmitted directional radio signals, the radio range defined specific airways in the sky. Pilots navigated using low-frequency radio by listening to a stream of automated "A" and "N" Morse codes. For example, they would turn the aircraft to the right when hearing an "N" stream, to the left when hearing an "A" stream, and fly straight ahead while hearing a steady tone.
  As the VOR system was phased in around the world, low-frequency radio range was gradually phased out, mostly disappearing by the 1970s. There are no remaining operational facilities today. At its maximum deployment, there were nearly 400 stations exclusively using low-frequency radio range in the U.S. alone.
context_section_description: The low-frequency radio ground component consisted of a network of radio transmission stations which were strategically located around the country, often near larger airports, approximately 200 miles apart. Early low-frequency stations used crossed loop antennas, but later designs were all based on the Adcock vertical antenna array for improved performance, especially at night.
  Each Adcock range station had four 134-foot-tall (41 m) antenna towers erected on the corners of a 425 × 425 ft square, with an optional extra tower in the center for voice transmission and homing. The stations emitted directional electromagnetic radiation at 190 to 535 kHz and 1,500 watts, into four quadrants. The radiation of one opposing quadrant pair was modulated (at an audio frequency of 1,020 Hz) with a Morse code for the letter A (· —), and the other pair with the letter N (— ·). The intersections between the quadrants defined four course lines emanating from the transmitting station, along four compass directions, where the A and N signals were of equal intensity, with their combined Morse codes merging into a steady 1,020 Hz audio tone. These course lines (also called "legs"), where only a tone could be heard, defined the airways.
  In addition to the repeating A or N modulation signal, each transmitting station would also transmit its two-letter Morse code identifier once every thirty seconds for positive identification. The station identification would be sent twice: first on the N pair of transmitters, then on the A, to ensure coverage in all quadrants. Also, in some installations local weather conditions were periodically broadcast in voice over the range frequency, preempting the navigational signals, but eventually this was done on the central fifth tower.
  The low frequency radio range was originally accompanied by airway beacons, which were used as a visual backup, especially for night flights. Additional "marker beacons" (low power VHF radio transmitters) were sometimes included as supplementary orientation points.
caption: Adcock range station. The central fifth tower was typically used for voice transmissions.
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