HF bands explained:
http://www.cvarc.org/personalities-of-hf-bands/
https://www.eham.net/article/23337
https://www.qsl.net/ad4dx/pdf/radio02.pdf
https://www.hamuniverse.com/hfbands.html
https://www.electronics-notes.com/articles/ham_radio/ham-bands/hf-amateur-frequencies.php