Editing Radio navigation (section)

===Radio direction finding===
[[File:Earhart-electra 10.jpg|thumb|right|[[Amelia Earhart]]'s [[Lockheed Model 10 Electra|Lockheed Electra]] had a prominent RDF loop on the cockpit roof.]]
{{Main|Radio direction finder}}

The first system of radio navigation was the ''Radio Direction Finder'', or RDF.<ref name="KF116">Kayton, Fried 1977, p.116</ref>  By tuning in a [[radio station]] and then using a [[directional antenna]], one could determine the direction to the broadcasting antenna. A second measurement using another station was then taken. Using [[triangulation]], the two directions can be plotted on a map where their [[Line-line intersection|intersection]] reveals the location of the navigator.<ref name="Ecologist 2021 f124">{{cite web | title=An Introduction to Radio Direction Finding | website=Cognitive Ecology | date=October 22, 2021 | url=https://cogecog.com/radio-direction-finding/ | access-date=April 4, 2024}}</ref><ref name="Murphy_1983">{{cite report |last= Murphy |first= Charles J. |date=1983-06-01 |title=Evaluation of VHF-FM Shore-Based Direction Finding Triangulation System in Massachusetts Bay Area |url=https://rosap.ntl.bts.gov/view/dot/10712 |publisher=United States Coast Guard |access-date=2024-04-04}}</ref> Commercial [[AM radio]] stations can be used for this task due to their long range and high power, but strings of low-power [[Nondirectional beacon|radio beacon]]s were also set up specifically for this task, especially near [[airport]]s and harbours.{{fact|date=July 2022}}

Early RDF systems normally used a [[loop antenna]], a small loop of metal wire that is mounted so it can be rotated around a vertical axis.<ref name="KF116"/> At most angles the loop has a fairly flat reception pattern, but when it is aligned perpendicular to the station the signal received on one side of the loop cancels the signal in the other, producing a sharp drop in reception known as the "null". By rotating the loop and looking for the angle of the null, the relative bearing of the station can be determined. Loop antennas can be seen on most pre-1950s aircraft and ships.{{fact|date=July 2022}}