Direction Finding

This article discusses the concept of radio direction finding, reviews the direction finding services that operated at Awarua Radio and records how these services supported New Zealand's national direction-finding requirements.

About Direction Finding

Radio direction finding (RDF or DF) is the ability to use a receiver to determine the bearing or direction of a transmitter signal. A bearing determined from one receiver site provides a long line, somewhere along which the transmitted signal has originated. By obtaining two simultaneous direction-finding bearings from well-separated receiving sites it is possible to identify a point where the bearings cross as the location of the transmitter.

In practice, there is normally a margin of error in the precise determination of a transmitter signal origin, so either taking a series of bearings or adding the support of additional DF stations helps in minimising errors and improving position identification.

Direction finding requires a directional antenna. The directional capability of an MF or HF antenna is obtained by using a rotatable loop antenna. When a loop is end-on to a signal it will receive maximum signal strength and when broadside-on to a signal it will receive minimum signal strength. A loop antenna has a 'figure-8' receiving pattern with two signal peaks from either end and two signal 'dips' or minimums received from either side. The minimums are more defined than the broad peaks, so are what is used to determine signal direction.

A key feature of direction finding was based on the work of Bellini and Tosi, two Italian engineers. In 1907 they experimented with a way to simulate rotating large, fixed loop aerials and they patented this system in 1909, subsequently selling the patent rights to the Marconi Company in 1912. The Bellini-Tosi concept was based on the discovery that a signal received from an external loop antenna could be connected to a small coil and the signal would be re-radiated. By connecting two physically large loop antennas to two small rotatable crossed coils it was possible to 'virtually' rotate the external fixed antennas. If a pointer was added to the knob used to rotate the crossed coil pair and a compass rose marked around the knob perimeter, it was possible to read off a signal bearing. This knob and coil unit was named a 'goniometer'.

Awarua Radio DF Sites

Three separate DF systems have been in use at Awarua Radio with an upgrade provided to the HFDF capability during WWII.

HFDF

A high-frequency direction finding (HFDF) system was in place at Awarua at the start of WWII. The site was located well down the rear of the Awarua station farm so that it was remote from other antennas and transmitting services. The receiving/direction finding equipment was a Marconi DFG12, probably a Mk2 version as used for fixed services operation. It came with four 30-foot tall masts mounted on insulated bases and which were placed in a square 30 feet apart at cardinal points around the building. A fifth 'sense' antenna wire was suspended vertically over the centre of the hut.

This arrangement of insulated masts was patented by British engineer Frank Adcock in 1909 to overcome issues of ionospheric signals interfering with stable ground-wave signals emanating from the same transmitter source.

MFDF

A medium frequency direction finding (MFDF) system was installed during WWII. It was housed in a hut closer to the main buildings and operated using the Bellini-Tosi system. This building had four tall masts equally spaced around it to form its two crossed loop antennas. While the MFDF hut still existed in 1967 its aerial structure had been removed, the hut was locked and the equipment was not maintained nor the site even visited.

Designed primarily for direction finding on the distress frequency 500 kc/s (kHz), it was mainly used to work the National Airways planes flying from Dunedin to Invercargill and to keep a watch on Captain Mercer's surveillance flights around the Fiordland coastline. James Mercer was an entrepreneurial pilot who had established air services from Christchurch to the West Coast when it was still a very isolated environment. The flights were commenced as a result of the German steamer Erlangen departing Dunedin in 1939 without bunkering and which was thought to be trying to refuel with wood. (See Coastwatcher Services).

HFDF2

About 1942/43 during WWII the HFDF system received an upgrade. The old Marconi DFG12 receiver/DF unit was removed and replaced with a more modern Marconi DFG24/2 unit. The antenna system remained the same. It is believed that the name of the HFDF facility was changed to HFDF2 in recognition of this upgrade.

The HFDF2 site was still operational in 1967 as an emergency capability. However, while technicians kept it maintained and calibrated, by that time its functions had mainly been overtaken by other advances in technology and operations.

HFDF2 Services

The services provided by HFDF2 were safety and military intelligence.

The safety service was a combined role supporting shipping and aviation needs. While both were important, the maritime safety aspect would have been to provide a bearing on any vessel sending a distress call, so a relatively uncommon need. The major role was supporting daily flying boat services by tracking flight progress across the Tasman Sea. Flying boats between New Zealand and Australia commenced in 1939 with Short Empire-class flying boats linking Auckland and Sydney. Later, flights also came out of Evans Bay, Wellington. Half-hourly position bearings were taken from three DF stations, these being Awarua, Musick Point (Auckland) and Australia's Lord Howe Island for these 7-8 hour-long daily flights.

Technical control of flights was at ZLF Musick Point, changing to VIS Sydney Radio at the halfway point of flights. The HFDF2 operator communicated with ZLF via a fixed-tuned receiver and transmitter located back in the main Awarua buildings.  (Reference: Maritime Radio Website, Frank Barlow memories 1939-1941)

The military intelligence role of HFDF2 was initially to search around the bands for Japanese radio-telegraph signals (Katakana code) in between supporting the flying boats.

However, in December 1941 following the Japanese attack on Pearl Harbour there came a distinct enhancement of the role. Communications within New Zealand were switched over to landline, a third HFDF station was installed at RNZAF Waipapakauri at the top of the North Island near Kaitaia and a connection was made to a Naval radio-fingerprinting detachment at Rapaura near Blenhiem staffed by WRENs. As a part of the US Pacific-wide DF network, the stations would respond to US Pacific Fleet Headquarters (Hawaii and later in Guam) requests for bearings of Japanese shipping. (Reference: Maritime Radio Website: Frank Barlow memories - Raparua).

Telefunken DF

About the 1950's era a 105 foot wooden pole (made from two poles spliced together) was erected about 50 meters behind the receiving office. On top of this pole was fitted a DF antenna unit which archive sources describe as being a Telefunken DF system.

The antenna unit looks designed to fit on top of a pole or ships mast and looks similar to a small cage of vertical bars with metal top, centre and bottom layers. There are nine bars consisting of:

  • a single centre bar (fixed at top, middle and bottom layers),
  • four dipoles equally spaced around the outside (fixed to the middle layer), and
  • four bars mounted halfway between the central bar and the outer bars- also equally spaced but offset from the outer bars by 45 degrees (fixed to top, middle and bottom layers).

We currently have no further information about this system.

Online research indicates this antenna unit is similar to the structures used for Telefunken's Telegon direction finding system The Telegon antenna used four equally spaced dipoles operating in an Adcock manner. Later adaptions use eight or 16 dipoles equally spaced.