We think that radio navigation and direction finding are quite modern things. However, you might be surprised that direction finding is almost as old as the radio itself. In 1888, Heinrich Hertz pointed out that the signal is strongest when the loop antenna is rotated in one direction and the weakest 90 degrees. By 1900, experimenters noticed that the dipole exhibited similar behavior, and soon the antenna was rotated to maximize the signal or locate the transmitter.
Of course, there is a problem. You can’t actually tell by loop or dipole which side of the antenna is pointing at the signal. Therefore, if the antenna points to the north, the signal may point to the north, but it may also point to the south. Nevertheless, in some cases, this is sufficient.
John Stone Obtained a patent There was such a system in 1901.The famous radio experimenter Lee De Forest also has A novel system In 1904. These systems have encountered various problems. At shortwave frequencies, multipath propagation can confuse the receiver, while longwave signals require very large antennas. Most antennas have moved, but some – such as Marconi’s antennas – use multiple components and a switch.
However, in some special circumstances, these restrictions are acceptable. For example, when Pan Am needed to navigate airplanes over the ocean in the 1930s, Hugo Leuteritz, who worked at RCA before Pan Am, used loop antennas to locate the aircraft’s transmitters at the airport. Since you know which side of the antenna the aircraft must be on, two-way detection is not a problem.
Radio navigation is largely due to ordinary celestial navigation and measurement. What you see is not a lighthouse, the sun, or the stars, but a radio transmitter.
Suppose you are in a field with a flagpole, and you know the exact location and height of the flagpole. If you are somewhere in the wild and want to know where you are, you can use a pole. You aim the pole and measure the angle to the pole. Since you know the height and angle, you can use geometric figures to draw a circle around the pole where you have to stand.
Of course, you can be anywhere on the circle-the position line called by the navigator. But what if you have two poles? You can draw two circles. If you are lucky, the circle will touch exactly one point, and this is where you are. However, it is more common to have two points, and—probably—one is far from where you should go and the other is close to where you should go.
Even with a simple pair of loops, if they are far enough apart, you can use the same technique. If the angle between station 1 and the transmitter is 30 degrees (or 210 degrees; this is not clear), and the angle displayed by station 2 is 300 degrees, you can triangulate by drawing two lines and paying attention to where they intersect Measurement.
Even so, there is still a need for better things. In 1909, Ettore Bellini and Alessandro Tosi introduced an innovation. The Bellini-Tosi system uses two right-angle antennas to feed the coils. The third loop moves inside the coil to find the direction. This allows the large antenna to remain stationary. These were very common in the 1920s and continued until the 1950s.
By 1919, British engineer Frank Adcock (Frank Adcock) proposed a system that used four vertical antennas, monopole antennas or dipole antennas. This arrangement connects the antennas, effectively forming a square loop, ignoring horizontally polarized signals, thereby reducing skywave reception. Adcock antennas are often used with Bellini-Tosi detectors.
In 1926, Englishman Robert Watson-Watt tried to detect lightning to help pilots and sailors avoid the storm. The lightning signal is very fast, but it takes about a minute for an experienced operator to line up the Bellini-Tosi detectors. By coupling an Adcock antenna and an oscilloscope, Watt can quickly lock onto a lightning or radio transmitter.
During the war, military high-frequency direction finder or huff-duff proved invaluable. German U-boats keep the transmission for a short period of time to avoid detection, but due to panting, it doesn’t matter. The Germans did not figure out the technological improvement, and it is estimated that 25% of U-boats sinking are caused by panting.
Modern systems use phase-locked loops and other technologies to be much more complicated. Although some of the early systems used by Pan Am used transmitters on aircraft and receivers on the ground, most systems did the opposite. Older ADF-automatic direction finding-set the electric antenna used to locate a known transmitter. Modern sets use a Marconi system with multiple antennas, although in this case the switch is electronic.
Amateur radio operators like fox hunting-which is part of an activity called “radio sports” in most parts of the world-essentially playing hide and seek with radio transmitters. You can see more in the video below.
You might think that GPS has made radio direction finding a thing of the past. However, if you think about it, GPS is a different form of radio direction finding. Instead of using the bearing of the antenna, you are measuring the time of arrival of the signal, but this is the same idea. The time delay gives you a circle from the known position of the satellite. Making multiple circles around multiple satellites can provide you with accurate locations.
Of course, this technology is far from the Hertz loop antenna. But the radio direction is still a key part of modern navigation systems.