Home Radio navigation Where is this radio? A brief history of direction finding

Where is this radio? A brief history of direction finding

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We consider radio navigation and direction-finding something quite modern. However, you might be surprised that DF is almost as old as the radio itself. In 1888, Heinrich Hertz noted that the signals were strongest when in a loop antenna orientation and the weakest rotated 90 degrees. In 1900, the experimenters noticed that the dipoles exhibited a similar behavior and that it did not take long for the antennas to rotate to maximize the signal or locate the emitter.

British direction finding truck from 1927; public domain

Of course there is a problem. You can’t actually tell which side of the antenna is pointing to the signal with a loop or dipole. So if the antenna is pointing north, the signal can be north but it can also be south. Still, in some cases, that’s enough information.

John Stone patented a system like this in 1901. The famous radio experimenter Lee De Forest also had a new system in 1904. These systems all suffered from various problems. At shortwave frequencies, multipath propagation can disturb the receiver and longwave signals require very large antennas. Most of the antennas moved, but some, like Marconi’s, used multiple elements and a switch.

However, there are special cases where these limitations are acceptable. For example, when Pan Am needed to fly planes over the ocean in the 1930s, Hugo Leuteritz, who had worked at RCA before Pan Am, used a loop antenna at the airport to locate a transmitter. in the plane. Since you knew which side of the antenna the plane should be on, two-way sensing was no problem.

Basic navigation

Radio navigation owes much to ordinary celestial navigation and surveying. Instead of seeing a lighthouse, the sun or a star, you see a radio transmitter.

Using the sun and moon gives two circles (lines of positions) and you can assume that your ship is not above the mainland around Argentina or Paraguay.  Public domain.
Using the sun and moon gives two circles (lines of positions) and you can assume that your ship is not above the mainland around Argentina or Paraguay. Public domain.

Assume that you are in a field that has a mast and you know the exact location and height of the mast. If you are somewhere in the field and want to know where you are, you can use the pole. You aim at the post and measure the angle to the post. Since you know the height and angle, you can use the geometry to draw a circle around the column you need to be on.

Of course, you could be anywhere on the circle – what browsers call a position line. But what if you had two poles? You can draw two circles. If you’re lucky, the circles will touch at exactly one point and that’s where you are. However, it is more common to have two points and – likely – one will be very far from where you should be and the other will be close to where you should be.

Even with a simple pair of loops, you can do the same trick if they are far enough apart. If station 1 shows a 30 degree angle (or 210 degrees; it’s ambiguous) to the transmitter and station 2 shows a 300 degree angle, you can triangulate by drawing two lines and noting where they are. cross.

Improvements

A 2 MHz Adcock installation;  public domain
A 2 MHz Adcock installation; public domain

Even so, there was a demand for something better. In 1909 Ettore Bellini and Alessandro Tosi introduced an innovation. The Bellini-Tosi system used two right-angled antennas that fed coils. A third loop has moved inside the coils to find the direction. This allowed the large antennas to remain stationary. In the 1920s, these were quite common and remained so until the 1950s.

In 1919, the British engineer Frank Adcock developed a system using four vertical antennas, either monopoles or dipoles. This arrangement wired the antennas to effectively form a square loop that ignores horizontally polarized signals, thereby reducing reception of celestial waves. Adcock antennas were often used with Bellini-Tosi detectors.

Lightning

Huff Duff equipment;  Photo by Rémi Kaupp CC-BY-SA-3.0
Huff Duff equipment; Photo by Rémi Kaupp CC-BY-SA-3.0

In 1926, Briton Robert Watson-Watt attempted to detect lightning to help aviators and sailors avoid storms. Lightning signals are very fast, but it took about a minute for an experienced operator to line up a Bellini-Tosi detector. By coupling an Adcock antenna and an oscilloscope, Watt was able to quickly lock onto a lightning bolt or radio transmitter.

The high frequency or huff-duff military direction finder proved invaluable during the war. The German U-boats kept the transmissions short to avoid detection, but with the huff-duff it didn’t matter. The Germans did not understand the improvement in technology, and estimates are that 25% of U boat sinks were due to blast.

Modern times

Modern systems are much more sophisticated using phase locked loops and other techniques. Although some of the early systems like the one used by Pan Am used transmitters on the plane and receivers on the ground, most systems do the opposite. The older ADF – automatic direction finding – defines motorized antennas used to locate known transmitters. Modern sets use the Marconi system with multiple antennas, although the switch is electronic in this case.

Radio amateurs enjoy fox hunting – part of the event known as “radiosport” in most countries of the world – which is basically playing hide and seek with a radio transmitter. You can see more of it in the video below.

You might think GPS has made direction finding a thing of the past. However, if you think about it, GPS is sort of a different form of direction finding. Instead of using the bearing of an antenna, you measure the arrival time of the signal, but it’s the same idea. The time delay gives you a circle from the known satellite position. Making several circles around several satellites gives you an exact position.

Of course, the technology is a far cry from the Hertz loop antenna. But radio steering is still a key part of modern navigation systems.


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