Offset/SNR: Some Ideas for Medium Wave DXing

Offset_5

Offset-DXing “on the fly” shows four different stations (spectrogram) on one nominal channel, namely 801 kHz. The window is baout 30 Hz wide and shows the carrier on HF level.

Although I use Simon Brown’s excellent software SDR Console V3 for years, I only now discovered a feature, being most valuable for medium wave DX.

Nearly each medium wave channel is populated by a couple of stations which mostly have a slight difference from each other, called offset. This often is specific to specific stations. It even reveals stations too weak to be heard. Software V3 will show these carriers of HF level during normal listening, being live or from an HF recording.

Read MW-Notes, to get some information on “how-to” on 6 pages, with 12 screenshots. There you will find also a hint for a method with even much more resolution (but: not “on the fly”) plus some information on how to measure signal strength and estimate/calculate the SNR of speech/music, rather than that of just the carrier.

You have to distinguish between absolute and relative frequency accuracy; the first is best achieved with a GPS-disciplined oscillator, the letter the normal case.

P.S. I started with these things back in 1997 with an evaluation board from Motorola, followed by sound card & software on audio level (“Soundtechnology zeigt Signale: Sieh’, wie es klingt!”, funk magazine 6/1998), to be continued on HF level from 2006, first with RFSpace’s groundbreaking SDR-14. Three years later, I published a survey of each and every 9- and 10-kHz-channel on medium wave by this method. After Apple closed their web service, these pages had gone astray, and the information is now not up-to-date anymore. State-of-the-Art now is the method described in the paper.

8 comments

  • Thanks for the nice article Nils ! I’ve been enjoying MW Carrier Offsets for about a decade … Console v3 is indeed a great tool for that purpose (speed is set to 1 here to get thin traces). For newcomers, you should have mentionned that the receiver needs to be properly calibrated so that the readings are accurate. Kind regards, Patrick (S.E France)

    • Thanks, and done. To maintain up an absolutely calibrated radio within the milli-Hertz range by amateur means, is quite an effort without using GPS as a reference. So I recommend the “relative method”, as the distances between the carriers don’t change at a drifting receiving. Yes, “Speed” might be reduced further to get an even better resolution – but you trade responsiveness by this. So one has to find his own compromise. 73 Nils, DK8OK

  • Indeed Nils. I usually check my SDR accuracy against CHU and re-check on MW tuned to RAI or BBC, both supposed to be dead on the nominal. However, accuracy with a Hz is enough, I don’t need more 😉

  • I meant <>

  • So far, I had not made use of the resolution settings. After trying it I can only say: Excellent tip. The visual presence of the signals gives an impression of what is possible. Signals are easier to track and easier to assign. So you can at least say, it’s not the antenna (or the SDR), but it’s the propagation conditions over which 5AN simply has not found the way. Thanks, Nils.

  • Great presentation, many thanks. Paul

  • An excellent paper, Nils, thank you! I’ll be trying your technique very soon, perhaps even in just a few hours. Can be a good way to find Trans-Pacific MW DX splits too.

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