VLF Converter

The converter was built to use with my FT817, but it will work with most SW receivers. The circuit consists of a low pass filter 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 and I have since received a nice QSL card from them which is now on the wall of my shack.

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.

The time lapse video is by OK1VEN and shows him building the converter.

Another version

Chris HB9DAO made some changes to the basic design with the aim of improving the large signal handling of the converter. His version is shown too.