Bellini-Tosi Medium Frequency Direction Finder



The Bellini-Tosi Medium Frequency Direction Finder (commonly referred to as B-T DF) was Australia's first standardised aviation radio navigation system. Being a ground-based system, it had the advantage of not requiring any specialised airborne radio equipment, in those days both big and heavy, other than a standard radio transceiver.


De Forest and others had experimented with the directional properties of loop aerials in the early 1900s, and in 1906 two Italians, Dr Ettore Bellini and Captain Tosi, produced a direction-finding system which still bears their names and which in principle remained in use for over 70 years. Development at first was slow because the receivers used crystal detectors, and it was not until the introduction of vacuum tubes that the sensitivity of the receivers improved to a satisfactory level. The rights in the Bellini-Tosi system were bought by the Marconi Company in 1912 and from that point development went ahead rapidly. The first ship to be fitted was Mauritania, aboard which successful experiments were carried out, although the range limit was about fifteen miles.

The System in Australia

The first use of the system in Australia was for the 1934 London to Melbourne Air Race, when the RAAF sent a mobile communications and direction finding station to Charleville, Queensland, to communicate with aircraft in flight, to provide meteorological information, and to provide a direction finding service for the four competing aircraft fitted with radio equipment. At this time, the Civil Aviation Branch (CAB) was seeking advice both locally and overseas on the best wireless organisation to suit Australian conditions. The Lorenz Radio Range was selected, but it would be some years before it came into operation. Some form of interim radio navigation service was needed to meet the rapidly growing needs of civil aviation.

Top photo: The B-T DF operator's console from a 1930s AWA brochure. The operator is tuning the antenna using the goniometer. Click here to see the preserved console on display at the Airways Museum.

Photo below: Aeradio operator Phil Richardson, a former ship’s radio operator, at Sydney/Mascot c.1938. Behind him is the B-T console.


B-T MF DF operator'sconsoleHF radio recievers and transmitter controls - click here for a Virtual Tour of Cloncurry Aeradio station and more information

The success of the Charleville experience prompted the CAB to arrange with the RAAF and Amalgamated Wireless Australasia (AWA) for a temporary direction finding service to assist commercial aircraft flying across the Timor Sea on the Brisbane–Singapore route, and across Bass Strait for the Melbourne–Hobart service. The Charleville mobile installation was relocated to Launceston/Western Junction, and came into operation in March 1935.

AWA commenced operating a direction finding service from a hut inside the reservoir compound at Essendon Airport soon after. This, together with assistance from the Coastal Radio service, provided a continuous watch on all aircraft on the Melbourne to Hobart route. The RAAF installed a similar service at Darwin In January 1936 to guide aircraft to their destination, particularly during adverse weather conditions.

Early in 1937, AWA was contracted by the CAB to supply and operate temporary direction-finding radio stations at Sydney, Canberra, Albury, Adelaide, Forrest and Perth at a cost of £6,000 per year. The addition of these stations provided guidance for the whole of the Melbourne–Sydney, Adelaide–Melbourne routes and much of the Sydney–Brisbane route. Finally, in early 1938 a network of 17 permanent Aeradio Stations was installed in Australia and Papua New Guinea by AWA.

It had been proposed that Lorenz Radio Ranges would form part of each installation, but as they were not yet available, Bellini-Tosi DF receivers were provided as part of the network instead, as an interim measure. As part of the contract, AWA provided radio operators who were experienced in the operation of HF communications (voice and Morse code) and of the Bellini-Tosi DF equipment. The following year, the operators resigned from AWA and were immediately employed by the Department of Civil Aviation. Thus the Aeradio organization was now owned and operated by DCA.

Ultimately, some 37 DF receivers, manufactured by AWA, would be installed between early 1939 and 1945. Up until the early 1950s, DCA trained Aeradio Operators in the operation of the B-T DF, but in practice it was rarely used in later years except in an emergency. It was a cumbersome procedure, and difficult for the Operator to establish a good ‘fix’ due to atmospheric interference. The 1944/1945 DCA Annual Report stated that as practically all aircraft were now equipped with radio compasses and that use of the B-T DF was now rare, no additional DF equipment would be provided where homing beacons were installed.


The B-T DF system made use of the directional property of a loop aerial, capable of being rotated in the electro-magnetic field of a transmitter. If the loop was turned until a minimal signal is received (a 'null'), then the plane of the loop is at right angles to the direction of the transmitted signal. When rotated through 90° and the plane of loop was in line with the transmitter, a signal of maximum strength was received.

Originally used on board ships, the B-T system employed a rotating loop, but this was necessarily restricted in its dimensions. In order to improve sensitivity, the ground direction finding system used two large fixed loop aerials, placed at right angles. These aerials were fed to a radiogoniometer (commonly referred to as goniometer), a device not unlike an alternator in that it consisted of an armature rotating in the field produced by two stator windings. Upon request from an airborne pilot for a direction finding bearing, the operator would tune the radiogoniometer on his DF receiver to the aircraft’s transmission, and fine tune for a null, i.e. minimum, signal. The rotating pointer would indicate on a fixed ring the bearing of the aircraft.

This procedure could take minutes for the operator to obtain a clear null. Fixes were ideally obtained by two or three D-F stations, each determining the bearing of an incoming signal from the aircraft. The ground stations would then transmit the bearing of the aircraft from the station, to the aircraft. Triangulation would then give the position of the aircraft.

B-T MF DF consoleHF radio receivers


Left: Charleville Aeradio in the 1950s - note the B-T DF is still in use.

(Photos: CAHS collection)


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