Sunday 3rd was Alexanderson Day, an annual commemoration of the Alexanderson Alternator, an early form of transmitter that utilized very high revolution rate motor-generators to produce a high power RF signal. Only one of these beasts remains in operation, at SAQ Grimeton, in Sweden. Each year, this VLF transmitter is run up and a message transmitted in CW on 17.2kHz.
The frequency this station operates on is so low that none of my receivers go that low! My FRG-100 will go down to 30kHz. So to receive these signals, a clever trick is used - a fast sample rate sound-card! In this case an extremely cheap Chinese USB device. A long random wire antenna is fed directly to the mic input channel, and then spectrum analysis software used.
This year SAQ transmitted its message at 09:00 and 12:00 UTC. For the first message, I quickly run out one of my Clansman HF antenna wire spools, just draped over everything, little more than 6ft off the ground. This was pretty much ineffective for SAQ, picking up just two of the stronger VLF encrypted naval teletype transmissions.
I realised I needed a vertical component! With a couple of hours until the second transmission, I changed the antenna config to a hastily rigged inverted L. With the connector of the Clansman wire held precariously in the screw thread on my VLF adaptor (actually just some static and overload protection) and wrapped around the door handles and a hook in the eaves, and the far end run up the flagpole, I was now receiving several of the worlds encrypted naval broadcasts.
And then, at about a quarter to, the first V's were clearly heard! On the spectrogram above, eight teletype broadcasts can be seen, as well as some interference lines. But, over to the left, and below the red marker on the scale, it should be possible to make out a broken trace. This is the spectrogram of SAQs CW transmission.
The message itself, whilst at a speed that I could actually copy, was rather rough sounding and the QRN meant that my rusty Morse was insufficient to copy it. I did manage to pick out bits, including the Tx power of 200kW!