Direction finding
"Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication. By combining the direction information from two or more suitably spaced receivers, the source of a transmission may be located in space via triangulation. This is called a cross-cut or fix."[1]
On the flight from Lae to Howland, Earhart and Noonan planned to use the simplest form of radio direction finding: find where the transmission was coming from and fly toward it. On the flight from Oakland to Honolulu, Manning and Noonan took seven radio bearings.[2]
From the Chater Report:
"Miss Earhart and Captain Noonan spent a considerable time in the radio office and as previously mentioned it was learned that neither of them could read morse at any speed but could only distinguish letters made individually slowly and repeated often; in that case their direction finding apparatus would be useless or misleading unless they were taking a bearing on a station using radiophone which could give the station position on voice. We understand the 'Itasca' was to do this but if the plane was unable to pick up the 'Itasca' it is doubtful if the direction finder would be any use to her."
Finding a minimum
Synonymous phrase: "finding a null."
If one has a directional receiver antenna, it is possible to find the direction from which a transmission is coming by rotating the antenna (as with Eahart's loop antenna or, if the antenna is fixed, fly the aircraft in a circle. In either case, the goal is to find the point in the rotation or the circle where the signal is weakest. At that point, the transmitter must be at a 90 degree angle to the axis in which the antenna is most sensitive. If no other information is available other than that derived by the rotation of the antenna, one would not know whether the signal was to the 90 degrees to the left or 90 degrees to the right of the antenna's bearing at the minimum.
There are many ways to resolve the ambiguity in the information provided by the directional antenna. One way is to move to a new position and take a new reading. The second reading should provide a triangulation pointing to the radio source.
A moment's thought will show that it is only necessary to rotate the antenna 180 degrees. A full 360-degree sweep should produce two nulls.
High frequency direction finding (HFDF)
Pronounced "huff-duff."
It is important to understand the relationship between frequency and wavelength and perhaps something about radio propagation in order to see why low-frequency direction finding worked better than HFDF in 1937. Because low-frequency transmissions have a longer wavelength, they interact differently with the ionosphere than high frequency "short waves" do. Because of the longer wavelength and different kind of propagation, it is easier to find the bearing of low-frequency wavelengths.
"Modern broadband loop antenna depends on a carefully engineered amplifier to keep the sensitivity constant over a wide range of frequencies" (Hue Miller, 30 June 2000 Forum.)