Den senaste sändningarna från SAQ i Grimeton har mottagits med goda signaler i SM2. D.v.s. av de som har mottagare som klarar av den mycket låga 17.2 kHz sändningen från Alexandersonalternatorn. Med en enkel VLF-konverter som Roger G3XBM konstruerat kan man lyssna i sin vanliga mottagare eller transciever. Konvertern kan även lätt anpassas för 137 kHz. Passa på att bygga den innan nästa SAQ sändning. OK1VEN har en liten munter video när han bygger convertern. https://www.youtube.com/user/G3XBM#p/a/f/0/x-Azk9wmIvg
G3XBM nya hemsida (blog) https://g3xbm-qrp.blogspot.com/
G3XBM har även andra konstruktioner som finns att se på hans egen YouTube kanal
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.