Bellini–Tosi direction finder

BTDF was invented by a pair of Italian officers in the early 1900s, and is sometimes known as a Marconi–Bellini–Tosi after they joined forces with the Marconi Company in 1912.

BTDF was the most prevalent form of naval direction finding from the 1920s to well into the 1980s, and was used as a major part of early long-distance air navigation systems from the 1930s until after World War II.

During the war, new techniques like huff-duff began to replace radiogoniometers in the intelligence gathering role, reducing the time needed to take an accurate fix from minutes to seconds.

The ability to inexpensively process radio signals using microcontrollers allowed pseudo-doppler direction finders to take over most of the radiogoniometer's remaining roles from the 1980s.

The earliest experiments in RDF were carried out in 1888 when Heinrich Hertz discovered the directionality of an open loop of wire used as an antenna.

[7] During experiments in 1907,[8][b] Ettore Bellini and Alessandro Tosi noticed that they could cause the received signal to be re-radiated by forming a loop with multiple winds of wire.

For longwave use, the two crossed antennae could be easily built by running four wires from a single mast to the ground to form triangular shapes.

[4][10] When used with shorter wavelengths, the system of two crossed loop antennae proved to be more mechanically robust than a single rotating one.

Early experiments carried out aboard Eskimo and Royal George, as well as the RMS Mauretania were successful, but the range was limited to about 15 miles (24 km).

This allowed the signal to hop over very long distances by reflecting multiple times off the ground and ionosphere.

By 1923 a number of amateur radio operators (hams) demonstrated excellent performance at 100 m and started routine trans-Atlantic communications the next year.

This led to a number of new frequency bands being defined in this shortwave region, as short as 10 m (which is very long by today's standards).

In 1917 Frank Adcock was trying to solve the problem of making large antennae suitable for use with the radiogoniometer at even the longest wavelengths.

However, it was later found that the underground connections between the antennae shielded them from skywaves, allowing only the direct-line groundwave to reach the goniometer.

The limitations of these frequencies to line-of-sight communications during the day was not a serious issue for air-to-ground use, where the local horizon might be hundreds of miles away for an aircraft flying at even moderate altitudes.

A good example of the advantages of shorter wavelengths can be seen on the Supermarine Spitfire, which started WWII with an HF radio that broadcast from a cable antenna stretched from the cockpit to the top of the vertical stabilizer.

A good example of such a system was first installed in Australia in 1934 as part of the 11,300 miles (18,200 km) MacRobertson Air Race.

The ITT team fled France in front of the German invasion and destroyed their equipment before leaving.

In the UK, the high-frequency direction finding (HFDF or "huff-duff") system largely had displaced BTDF by about 1943.

The second advance was the introduction of the automatic direction finder (ADF), which completely automated the RDF procedure.

Improvements continued to be made to both systems throughout this period, especially the introduction of solenoids in place of conventional loops in some roles.

However, the introduction of the doppler direction finder, and especially the low-cost electronics to implement it, led to the disappearance of the traditional loop systems by the mid-1990s.

Doppler systems use fixed antennae, like BTDF, but handle the direction finding via signal processing alone.

For vertically polarized signals, reception on the top and bottom of the loop is very low,[c] so it has little contribution or effect on the output.

The total energy radiated by the coil is less than what is received on the antenna, but it broadcasts this into a much smaller physical area, so the flux may be much higher than the original signal.

As the B–T system relies on the comparison of signal volumes, this results in a non-uniform output, rising and falling every 45 degrees, eight times around a full circuit.

[25] Even minor changes in the weather, physical layout or even bumping the chassis containing the tunable capacitors can cause the tuning to vary.

The operator timed from the end of the start signal to the maximum in the continuous tone, and then divided by the rotation rate to determine the angle.

[10] The advantage of the B–T system in terms of mechanical simplicity was generally difficult to use in this role due to the normally small amount of energy it could tune.