VLF Converter (G3XBM)

VLF Converter

A simple circuit allowing reception of SAQ on 17.2kHz

On Christmas Eve 2006 I wanted to try to copy the VLF morse code signals from SAQ at Grimeton in southern Sweden. These special commemorative CW transmissions happen every few months. This is a brief summary of this attempt. CW transmissions on VLF are rare today as most remaining VLF stations use data modes.

A breadboarded version

SAQ was built between 1922 and 1924. The 200kW transmitter was, and still is, unusual as it consists of an AC generator (alternator). The antenna

consists of six 127m high antenna towers placed at intervals of 380m with the 46m cross-arms carrying the eight copper antenna wires. On Dec 1st 1924 it first transmitted with the SAQ callsign on 16.1 kHz, but this was later changed to 17.2kHz on which frequency it operates occasionally today. Today the same station and antennas are preserved as a UNESCO World Heritage site some 80 years later.

A tidier version of the breadboard on the left is shown on the right after it had been boxed.

The converter below was built to use with my FT817, but it will work with most SW receivers. The circuit consists of a low pass filter (see yellow choke at bottom) feeding into an SBL1 double balanced mixer connected "back to front" with the IF port as the main RF input, a 12MHz crystal local oscillator, a 2N2904 IF pre-amp feeding the output at 12.0172MHz (12MHz + 17.2kHz) into the FT817 in CW mode. Of course the choice of crystal depends on what is in the junk box. The converter will work reasonably well from a few kHz up to around 100kHz. The antenna was my usual random wire down the garden tuned against the central heating ground. You will see that I am not into grand PCBs - rats nests are more my style! It shows that a circuit can be thrown together quickly and got to work with fully acceptable results.

Just before 8am SAQ was RST559 with me whereas MSF on 60kHz was RST599+. Unidentified data signals around 20kHz were RST589. The initial signals were just “VVV de SAQ” but then followed a short message in CW which I copied. I sent them an e-mail report with an MP3 recording of the signals as I heard them. I am not sure if they QSL, but I hope so as it would be good to have a VLF QSL on the wall.

My circuit, far from optimised as it was designed and built in just 20 minutes, is shown below. This is shown for ideas only and you are probably able to do something much better with more care. Reception would have been possible without the IF preamp too.

For use on 136kHz or in the LF NDB band, reduce the values of the low pass filter values. See this link for an on-line LC LPF value calculator by WA4DSY.