page_title: Bellini–Tosi direction finder
section_title: Early RDF
hierarchical_section_title: Bellini–Tosi direction finder / History / Early RDF
caption_attribution_description: English: W.G. Wade received signals from a coil (or loop) antenna - mainly used for direction finding - at a Kennsington Maryland field station in 1919 -from NIST Photographic Collection, Radio Direction Finder
context_page_description: A Bellini–Tosi direction finder is a type of radio direction finder, which determines the direction to, or bearing of, a radio transmitter. Earlier RDF systems used very large rotating loop antennae, which the B–T system replaced with two fixed antennae and a small rotating loop, known as a radiogoniometer. This made RDF much more practical, especially on large vehicles like ships or when using very long wavelengths that demand large antennae.
  BTDF was invented by a pair of Italian officers in the early 1900s, and is sometimes known as a Marconi–Bellini–Tosi after they joined forces with the Marconi Company in 1912. BTDF was the most prevalent form of naval direction finding from the 1920s to well into the 1980s, and was used as a major part of early long-distance air navigation systems from the 1930s until after World War II. BTDF systems were also widely used for military signals intelligence gathering.
  During the war, new techniques like huff-duff began to replace radiogoniometers in the intelligence gathering role, reducing the time needed to take an accurate fix from minutes to seconds.
context_section_description: The earliest experiments in RDF were carried out in 1888 when Heinrich Hertz discovered the directionality of an open loop of wire used as an antenna. He noticed that the spark generated at the open gap between the ends of the loop was much stronger when the loop was end-on to the transmitter, and disappeared entirely when it was aligned face-on to the transmitter.
  By the early 1900s, many experimenters were looking for ways to use this concept for locating the position of a transmitter. Early radio systems generally used longwave or medium wave signals. Longwave, in particular, had good long-distance transmission characteristics due to their limited interaction with the ground, and thereby provided excellent great circle route ground wave propagation that pointed directly to the transmitter. Methods of performing RDF on longwave signals was a major area of research during the 1900s and 1910s.
  Antennae are generally sensitive to signals only when they have a length that is a significant portion of the wavelength, or larger. A common example is the half-wave dipole. For longwave use, this resulted in loop antennae tens of feet on a side, often with more than one loop connected together to improve the signal. This presented a significant problem in arranging for the antenna to be rotated. The US Navy overcame this problem, to a point, by mounting long antennae on ships and sailing in circles.
  One solution to this problem was developed by the Marconi company in 1905. This consisted of a number of long horizontal wires or rods arranged to point outward from a common center point. A movable switch could connect opposite pairs of these wires to form a dipole, and by rotating the switch the operator could hunt for the strongest signal. All of these systems were unwieldily and impractical for many uses.
caption: Early RDF systems used large rotating loop antennae built on wooden frames. This 1919 example, from the National Bureau of Standards, is relatively small for the era.
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