Calculate Resistance and Voltage Drop on the iPhone
For the last few weeks, there’s been a new source of QRM on the most popular shortwave pirate radio frequency in North America, 6925 kHz. Last night, it was particularly bad. Here’s a recording of what it sounds like, when tuned to 6923 khz USB.
Putin on the QRM
The offending signal is a Russian Military 12 Tone PSK AT-3004D modem, often referred to as the “Russian 12 Tone Modem” in utility DXer circles, and also the MS5.
There is a very good writeup about it at this site: http://www.signals.taunus.de/FFT/CIS12CH.HTML
Here is an SDR waterfall image of the modem:
You can see the stronger pilot carrier on the extreme right (highest frequency) as well as the 12 PSK channels.
The signal faded in at around 2300 UTC, was quite strong around 0030 to 0200 UTC, and faded out around 0400 UTC. The local fade in time is due to the Sun starting to set here, and the path being mostly dark, to support propagation on this frequency. The fade out time is due to sunrise at the transmitter site. This suggests a location somewhere in Europe.
If this modem continues to transmit around 6925 kHz, operators may wish to work around it, by avoiding 6925 kHz during the time the 2300-0400 UTC time the signal is present. Unfortunately this is also the most popular time for pirate transmissions. Due to the strong signal strength, and the wideband nature of the signal, it can to obliterate any weaker stations on the frequency.