Editing Direction finding (section)

===Early mechanical systems===
[[File:Wade performing RDF.jpg|thumb|W.G. Wade of the National Bureau of Standards uses a large multi-loop antenna to perform RDF in this 1919 photo. This is a fairly small unit for the era.]]

The earliest experiments in RDF were carried out in 1888 when [[Heinrich Hertz]] discovered the directionality of an [[loop antenna|open loop of wire]] used as an antenna. When the antenna was aligned so it pointed at the signal it produced maximum gain, and produced zero signal when face on. This meant there was always an ambiguity in the location of the signal: it would produce the same output if the signal was in front or back of the antenna. Later experimenters also used [[dipole antenna]]s, which worked in the opposite sense, reaching maximum gain at right angles and zero when aligned. RDF systems using mechanically swung loop or dipole antennas were common by the turn of the 20th century. Prominent examples were patented by [[John Stone Stone]] in 1902 (U.S. Patent 716,134) and [[Lee de Forest]] in 1904 (U.S. Patent 771,819), among many other examples.

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 [[medium wave]] and [[longwave]] 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.{{sfn|Yeang|2013|p=187}}

Antennas are generally sensitive to signals only when they have a length that is a significant portion of the wavelength, or larger. Most antennas are at least {{frac|1|4}} of the wavelength, more commonly {{frac|1|2}} – the [[half-wave dipole]] is a very common design. For longwave use, this resulted in loop antennas tens of feet on a side, often with more than one loop connected together to improve the signal. Another solution to this problem was developed by the [[Marconi]] company in 1905. This consisted of a number of 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.{{sfn|Baker|2013|p=150}} The [[US Navy]] overcame this problem, to a point, by mounting antennas on ships and sailing in circles.{{sfn|Howeth|1963|p=261}} Such systems were unwieldily and impractical for many uses.{{sfn|Yeang|2013|p=188}}