Listening at 18kHz with WebSDR

[Circlotron]

Was listening to this site http://websdr.ewi.utwente.nl:8901/m.html with WebSDR iPad app and there are signals as low as 18kHz. Are they likely to be real transmitted signals or just spurious receiver artifacts?

[uncle_bob]

Was listening to this site http://websdr.ewi.utwente.nl:8901/m.html with WebSDR iPad app and there are signals as low as 18kHz. Are they likely to be real transmitted signals or just spurious receiver artifacts?

Hi

There are indeed real signals down there. Most of what you pick up is EMI from switching regulators and similar stuff.

Bob

[mark03]

VLF comms are really interesting.  One of the US Navy's big transmitters is not far from me here in Seattle:

https://en.wikipedia.org/wiki/Jim_Creek_Naval_Radio_Station

When you're only dealing with the groundwave you can really push the Shannon limit by going low/slow, because the phase coherence of the channel is much greater than at shortwave frequencies.  Here's a fun write-up of an amateur transatlantic communications experiment at 8971 Hz:

http://www.w4dex.com/vlf/8971Hz/index.htm

For a test of the web SDR receiver, see if you can pick up the nearest VLF time-standards stations in the UK (MSF, 60 kHz) or Germany (DCF77, 77.5 kHz).