Tour of ABC AM Transmitters

Club members at Transmitter site
Main antenna in distant background

During April 2012, Graeme VK3NE organised a tour of inspection of the site from which the Australian Broadcasting Commission’s Radio National 3RN (on 621 KHz) and the ABC Melbourne 774 (on 774 KHz!) programs are transmitted. It was interesting to note that the 774 transmitter is still labeled 3LO and is included as such on the ACMA Data base. So 3LO must still be its official call sign.

3LO commenced operation on 174 KHz (L/F) in 1924 as a commercial station, changing to 810 Khz (M/F) during 1925. The transmitting facilities (transmitter etc.) was located at Braybrook (West Footscray) and was owned by AWA.

The 25 KW Transmitter is made up of Multiple 2 KW units, combined.

Some frequency listings show that around 1930/4 it appears that it may have operated on 800 KHz. 3LO changed frequency again in 1935 to 770 KHz and relocated to Sydenham.

During 1975, to comply with the changeover to 9 KHz channeling in the AM broadcast band, it moved to 774 KHz.

Graeme explaining some of the monitoring and audio processing equipment

Currently each service (on 621 and 774 KHz), use two solid state 25 KW Nautel transmitters and each 25 KW transmitter is made up of 2 KW independent modules. The two 25KW units are fed into a combiner, providing a 50 KW output for each service. This is then connected via a rigid coaxial line to the Aerial Switches.

2KW Toroidal Combiners

A large, rigid, coaxial cable transfers the 50 KW to an Antenna Dual Coupling Unit located some 300 metres away at the base of the single vertical radiator. Each DCU contains both a matching circuit and a rejection circuit for each transmitter to ensure that the power from either transmitter is fed to the aerial and not back up the other transmitter’s transmission line!

The antenna, acting as a 5/8 wavelength vertical, was constructed in 1938. It is a guyed, triangular lattice, construction, 705 ft. (215 m) high, and sits on a single insulator with a buried earth mat 800ft (243m) diameter below it.

There are also two independent stand-by vertical antennas at Sydenham, each a nominal 1/4 wavelength long, with top loading. They also have an earth mat of about 400ft. diameter. A 50 KW water cooled Dummy Load is also on site.

The equipment is currently owned, operated and maintained by BAI.