Receiving SAQ transmissions

Reception of SAQ 17.2 kHz CW transmission on 24 December 2009.

Radiostation Grimeton SAQ went into service on 1 December 1924 on 16.1 kHz. The frequency was changed to 17.2 kHz soon after this. The station was designed to carry trans-Atlantic traffic. The station is now maintained and operated by a dedicated band of volunteers and test transmissions are sometimes made. This report describes what happened when GM3YEH decided to try and receive the transmission from SAQ scheduled for 0800UTC on 24 December 2009. More information about SAQ can be found at http://www.alexander.n.se/

The transmissions were monitored at GM3YEH using the VLF soundcard software by SM6LKM (click here to download). The LINE IN input of the soundcard was used as the antenna input. The antenna was a 5m length of wire.

The online WebSDR system by PA3WEG (http://websdr.pa3weg.nl/) was also monitored during the transmission for comparison with local reception.

Below is a series of screenshots taken before and during the SAQ transmission. Audio recordings taken during the transmission from GM3YEH and PA3WEG are provided. Audacity (http://audacity.sourceforge.net/download/) was used for the audio recording.

Before the start of the SAQ transmission.

On the screenshots below, note the group of signals shown by both receivers between 19.5 kHz and 22 kHz. The general signal strengths shown at PA3WEG are higher than at GM3YEH. This is likely to be due to the poor antenna system at GM3YEH. The signal at 18.5 kHz is strong at PA3WEG and weak at GM3YEH. The signal at 19.5 kHz is stronger relative to the 18.5 kHz signal at GM3YEH but is the opposite at PA3WEG. This probably indicates propagation path differences at the two receiving sites.

GM3YEH - 0745UTC 24 December 2009

PA3WEG - 0747UTC 24 December 2009

During the SAQ transmission.

SAQ began transmission before the scheduled time of 0800UTC and was heard at both sites. Although the SAQ signal was audible at GM3YEH, it is not very clearly visible on the GM3YEH spectrum. On the PA3WEG spectrum screenshots below, the presence of SAQ is clearly visible on 17.2 kHz. The lower signal strength of SAQ at GM3YEH is likely to be due to better antenna performance at PA3WEG.

GM3YEH - 0756UTC 24 December 2009

Click here to listen to the recorded audio from GM3YEH

PA3WEG - 0758UTC 24 December 2009

Click here to listen to the recorded audio from PA3WEG

Conclusions

The audio clips above are from the same time segment to allow a direct comparison of audio at the two receiving locations. Although it is easier to decode the PA3WEG signal, the signal at GM3YEH was perfectly readable. The decoded characters in both audio clips are as follows:

VVV DE VQSAQ SAQ VT A DE SQ AQ SAQ VA

The VVV is a common pre-transmission indicator. DE means ‘from’. SAQ is obviously the station callsign. VA is a common end of message indicator. It is not know what the rest of the decoded characters mean but there may have been a few keying errors.

This was the first attempt to receive VLF signals at GM3YEH and it is considered to have been a success. SAQ was received successfully. Use of the SDR soundcard software receiver by SM6LKM was fairly straightforward, but it is recommended that the LINE IN input to the soundcard is used as the antenna input instead of the LEFT mic input as suggested in the software help file. Using a very long antenna resulted in an ‘OVERLOAD’ indication from the software and so a fairly short wire (about 5m) was used. For future VLF work it would be useful to try to improve the antenna system. There are a number of online resources where ideas are given for VLF antennas and pre-amplifier systems.

73 de Barry GM3YEH