HF propagation and antennas:
https://www.rohde-schwarz.com/us/knowledge-center/videos/understanding-hf-propagation-video-detailpage_251220-909312.html
https://www.rohde-schwarz.com/us/knowledge-center/videos/understanding-near-vertical-incidence-skywave-nvis-video-detailpage_251220-908736.html
https://www.arrl.org/files/file/Technology/tis/info/pdf/8312011.pdf
https://www.arrl.org/files/file/Technology/pdf/119962.pdf
https://www.arrl.org/files/file/Technology/tis/info/pdf/9106023.pdf
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General radio theory and practice:
https://www.cisa.gov/sites/default/files/publications/COMT%20Pre-Course%20Study%20Guide_v5_0.pdf
