History of LF Experimental Work on 73kHz/136kHz at Amateur Radio Station G3LDO

This page will describe experiments carried out, firstly on 73kHz and later on 136kHz and is currently being rewritten.

What follows is a description of the  last 136kHz installation before it was dismantled early 2005 to make way for further antenna experiments.

 The LF transmitter and antenna matching/tuning components are located in a shed at the bottom of the garden and remotely controlled from the comfort of the house.

The large unit was a modified ex-Decca transmitter.  It was driven by a Hands DDS unit and keyer, powered by a float charged battery. The box on top of the transmitter is the local/remote control unit. The output of the transmitter was fed to a variable matching transformer and the variometer tuned primary loading coil shown to the right. The output of this coil was fed to the main loading coil located away from the shack for safety reasons. The transmitter was  powered by batch of car batteries located under the bench, float charged from the PSU on the far bottom right.

The output from the primary coil in the transmitter shed was connected to the main loading coil (bottom wire in photo). The coil was a rewound Decca loading coil. This coil is multilayered (5 layers) with an inside diameter of 450mm and an outside diameter of 620mm. The coil was wound with 4mm diameter (including insulation) litz wire. It also an internal variometer with 16 turns.

The antenna was a sort of inverted L made up with two wires, which were spaced around 2m apart at the top and around 1m apart at the far end. The height of the top of the mast is 15m and the height of the antenna at the far end was 9m. The antenna was fed at the nearest point in the photo, where it connected to the top of the loading coil.

The antenna mast, which is still being used for other projects, is folded over when not in use or for antenna maintenance. The mast is mainly constructed from steel tube. Aluminium scaffolding tube is used for the top 3m of the fold over section and the lower section is of solid steel bar. This arrangement means that the mast can be folded over without the use of a winch and the antenna can be raised or lowered in less than a minute.

The main station radio equipment comprises a TS-850S, a FT-990 and a KW1000 linear. Most HF operation is confined to the five upper HF frequency bands.