Shortwave Radio:
http://www.worldofradio.com/index.html
https://wiki.radioreference.com/index.php/SWL_Broadcast
http://mt-shortwave.blogspot.com/
https://shortwaveschedule.com/?now=true
https://www.shortwaveradio.be/
https://shortwavedx.blogspot.com/
http://rx.linkfanel.net/ (worldwide SDR receivers online)
https://www.hard-core-dx.com/index.php?topic=Logs
https://shortwavedx.blogspot.com/
https://www.radioreference.com/
https://wiki.radioreference.com/index.php/SWL_Broadcast
https://www.swld.com.au/default.htm
https://www.bdxc.org.uk/links.html
https://wiki.radioreference.com/index.php/Utility_Monitoring
https://wiki.radioreference.com/index.php/RadioReference_UDXF_Portal
VLF/LF/NDB:
https://rxx.classaxe.com/en/rna/signals
https://www.dxinfocentre.com/ndb.htm
https://www.ndblist.info/datamodes.htm
https://www.ok1in.cz/downloads/ndb%20beacons.pdf
https://www.ka7oei.com/spec_ndb1.html
https://www.ndblist.info/ndbphotos/pics/andypics/ndbhunting.pdf