The “Flame” radar site

One of the most secretive and technically advanced installations on Rottnest Island during WW2 was located at Cape Vlamingh, where a radar and searchlight site with the codename “Flame” was established.

 

The Flame site was part of Australia’s rapidly expanding radar network, developed in response to growing concerns about enemy naval and air attack. Instructions to proceed with radar installations around Australia were issued by Army Headquarters in August 1941 and Rottnest was quickly identified as a priority location, due to its strategic position guarding Fremantle.

 

Radar installation

The facilities at Cape Vlamingh were referred to at the time as SRDF or RDF, abbreviations for Shore Radio Direction Finding and Radio Direction Finding. These early radar systems were highly classified when they were installed and represented cutting-edge technology at the time.

 

The Cape Vlamingh installation, officially designated Station 0 with the codename Flame, was ready for occupation by 22 December 1942. A radar tower was constructed at the site to detect ships and aircraft approaching the coast, providing early warning to the island’s gun batteries and command posts.

 

Flame was one of three radar stations constructed on the island, each consisting of a standard concrete building with two rooms. One room housed the radar equipment, while the other served as a living and working space for personnel manning the station. The buildings were designed with defence in mind. Windows were fitted with external steel shutters, and the doors were also made from steel, making the structures resistant to shrapnel and blast fragments.

 

Because Rottnest Island did not have material suitable for precise radar calibration, special river-washed sand was transported from the mainland and used during installation to ensure the equipment functioned accurately.

 

Special river-washed sand was brought over from the mainland due to the way it behaved electrically. Early radar systems were extremely sensitive to the ground surface beneath and around them. Different soils reflect radio waves in different ways and uneven or unpredictable ground conditions could distort radar signals, create false echoes or interfere with accurate calibration.

 

Rottnest Island’s natural surface is dominated by limestone and shell-rich sands, with variable moisture content. These materials do not behave consistently when interacting with radar waves and could produce unwanted reflections or signal interference. For radar equipment that relied on precise measurements, this was a serious problem.

 

River-washed sand, by contrast, is more uniform in composition. Its rounded grains and relatively consistent structure provided a predictable surface that helped reduce ground interference. By importing this sand and using it around the radar installations, engineers were able to create controlled conditions that allowed the equipment to be calibrated accurately and function as intended.

 

Power and supporting infrastructure

A separate concrete engine room located around 20 yards from the main radar building, housed the power supply. This consisted of a 13 KVA petrol-driven generator, typically a three-phase alternator powered by a Ford V7 engine. These generators provided reliable electricity in the event that external power supplies failed or were unavailable.

 

Similar generator sets were installed at Flame and other shore defence stations on Rottnest by early 1943, alongside upgraded telephone equipment that allowed information to be transmitted quickly across the island’s defence network.

 

Radar towers and detection capability

The radar aerials were mounted on tall, self-supporting steel towers approximately 180 feet high, giving a total operating height of around 245 to 250 feet above ground. This elevation significantly increased detection range. The towers were engineered to withstand wind speeds of up to 120 miles per hour, an important consideration given Rottnest’s exposed coastal environment.

 

Each tower had a square base of around 30 square feet, tapering to about seven feet at the top. A circular platform near the top allowed access for maintenance, while a ladder fitted with a safety cage and a single-person hand-operated lift ran up the centre of the mast.

 

The Flame radar used a seven-foot parabolic aerial, capable of detecting targets at distances of up to 40 miles. Unlike the other two radar stations on Rottnest, which scanned only 200-degree arcs, the Flame aerial rotated through a full 360 degrees, providing comprehensive coverage of the surrounding sea approaches.

 

Plotting and coordination

At each RDF station, personnel used a Course and Speed Table to track the movement of ships. This was a physical plotting table, roughly three feet by two feet, marked with a map of the coastline around the station and the exact location of the radar site. Using observed range and azimuth (bearing), operators could plot a vessel’s course and speed over time.

 

Information gathered at the radar stations was fed through to a central plotting room, where it was displayed on RO Plotter Type 102 units. These were large, wall-mounted wooden plotting devices, roughly ten feet square, fixed in place with metal brackets. Each plotter corresponded to an individual radar unit, allowing commanders to build a coordinated picture of activity around the island.

 

Personnel and operation

By August 1944, two coastal defence radar sets were operating on Rottnest Island, manned by the 5th Australian Radar Detachment. During routine monitoring, each station required two operators. In periods of heightened activity or action, this number increased to four.

 

Radar units on the island were closely integrated with searchlight and gun operations. In March 1944, both a 90 cm Mk III searchlight and a 60-inch Sperry anti-aircraft searchlight were installed at Flame. Control pillars for directing these lights were added by June 1944 (NAA: MP742/1, 82/9/5), allowing radar detections to be visually confirmed and targets illuminated for coastal batteries.

 

Later changes and dismantling

As the threat of direct attack diminished after the war, the need for large radar towers declined. The tall towers at Cape Vlamingh were dismantled around 1947–1948. Generator sets were returned to the 5 Base Ordnance Depot in April 1950, while some plotting equipment remained in storage at Rottnest.

 

Personnel assignments also shifted during the war. In late 1943, the 2nd Australian Radar Detachment was posted to Garden Island to support coastal batteries there. When those batteries were disbanded at the end of 1944, the detachment was transferred to Rottnest to replace the 5th Australian Radar Detachment, which was then disbanded.

 

References

NAA: MP742/1, 82/9/5 - Fortress Installations, Fremantle and Albany WA

NAA 971273: Radar - Liaison - Army RDF (Radar Direction Finding) at Rottnest Island [1cm; box 1]