500 kHz receiving antenna at ZLD

by Ian Walker

The main antenna at the Musick Point receiving station was a twin wire Marconi Flat Top centre-fed antenna. Its height was 75 ft above the feed point.

Approaching the station this antenna stood out as the most impressive of its type in the paddock, mainly due to its size.

Because it was the main receiving antenna for maritime radio telegraph services on 500 kc/s it was decided to also use it for the maritime voice service on 2182 kc/s as an emergency backup should there be a disaster at the transmitting station 3 miles away.

A means of separation was required between the two frequencies. Hams use traps in the antenna, but in this case the traps were installed at the feed point at the base.

Looking at the circuit it can be seen there were large coils and capacitors to make up the traps.

When I inspected the base of the antenna in the 1960s I recall a wooden box containing a large coil of double cotton covered wire, about 4″ diameter with several taps near the bottom winding. This was attached to the coaxial feed. The inspection was necessitated by the amount of interference experienced by the operators at the station.

The copper feed point had become dry-jointed and verdigris was very obvious around the wire connection. A burn mark on the cotton insulation indicated that it may have been caused by lightning.

The Radio Depot having been advised of my find effected a repair the next day, after which the operators found some slight improvement in reception. The antenna was at this time mainly used for 500 kc/s receiving. (I think this was about the time they changed to the trap system of feeding the antenna.)

As a result of this fault a new system was designed by the engineers, which accommodated both emergency transmitters at the receiving station. New glass fibre boxes were built to house the 50Ω feed at the different antennas with the most suitable lead out insulators attached. Previously the feed was 600Ω open wire feeders.

The old wooden 500 kc/s box was replaced and the antenna traps installed into the new box. As can be seen, the circuit was printed in 3 May 1981.

The unit was very sturdily built in a weather tight wooden box, which was in turn mounted into the fibre glass box housing in the paddock. This fibre glass box had a multiplicity of self locking screws and a padlock to deter vandals. It also took longer for the technicians to open and check the internal connections.

Transmit power to the unit being 20 watts AM on 2182 kc/s and 20 watts CW on 500 kc/s, there was no real problem with very high power levels or special insulation requirements. Even so the greatest care was taken to maintain a very high standard of workmanship.