Medium Wave: Offset Atlas – all 9 kHz channels Plus VLF & Longwave, 24 hours

The “Atlas” shows screenshots of all 9kHz channels on Medium Wave within a 50Hz window, sometimes better. It also shows some odd channels plus Time Signal Stations on VLF and all Broadcasting Longwave Channels. You can download it for free to determine accurate and stable offset readings over 24 hours (zoom in by e.g. 400%)

With the new Elad FDM-S3 and its OCXO/GNSS-stabilized clock, I did a 24h recording of the whole medium wave band on January 19, 2021 in Northern Germany; plus longwave on Januar 21, 2021. Free software SDRC V3 enabled me to make up a spectrogram of each channel within a window of 50Hz width, and at a frequency raster of 9kHz on medium wave. You can easily see:

  • sign-on/sign-off
  • fade-in/fade-out
  • accurate and stable frequency offset over full 24h down to a millihertz
  • frequency control of the transmitter’s oscillator (stable, drift, sinus, sawtooth …)
  • propagational effects (doppler, scatter …)

The format is PDF, DIN-A4, landscape, resolution 300dpi – see screenshot at the bottom. This allows you to zoom to a factor of about 400% to search for details and better read out of the time/frequency scale. It weighs 865MB. You can download it here, and open it with your PDF reader (you can also point your mouse cursor onto the link, click right mouse key, and choose “Save under …”). Leafing from one page to another gives an interesting overview.

A similar Atlas showing a raster of 10kHz is also available for free – just scroll to the previous post of this blog. It is also planned to publish a general article about the background, about what to do with such a tool, and how to do this by yourself.

I am sure that it will open some new horizons on Medium Wave DXing, including accurate offsets over up to 24h.


  • Nice work! I’ve got a similar dataset from the Boston area, but on an SDRplay receiver. Elad is sold out so I can’t get one yet, and I’m struggling to get my Flex6700 (which does have a GPSDO) to work with Carrier Sleuth.

    Did you script the production of the panels or did you have to do each manually? I’d also be interested in hearing if you have any scheme to identify the individual carriers. My methods are tedious at best and not really very effective.


    • … thanks, Mike, and you got it: “tedious at best” … But the most recent version of SDRC V3 software features a Data File Editor. Here you can take a whopping file (e.g. 1536kHz over 24 hours) and automatically (!) cut it into pieces. In our case: 1kHz wide channels with a distance of either 10kHz or 9kHz. This leaves you with a bunch of files which you can easily handle to make up spectrograms. An SSD will further speed this process. To be honest: It was just this feature which let me realize the atlases … 73 Nils, DK8OK

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