As an antenna engineer, I've referred to this site hundreds of times. To be fair, it's almost an eli5 version of how antennas work, but it's been a good reference for basics.

http://www.antenna-theory.com/basics/whyantennasradiate.php is as deep as it gets.

For DEEP antenna theory, I can't recommend the Balanis book high enough. https://www.amazon.com/Antenna-Theory-Analysis-Constantine-B...