SDR- Software Defined Radio
Radio Propagation- The Behavior of radio waves as they travel
Antenna- A metallic device (such as a rod or wire) for radiating or receiving radio waves
This source has information on skywave radio propagation and its application to amateur radio in the High Frequency band. This site can be very useful for people interested in learning how radio signals can travel very far. This source provides detailed tutorials about the topic and tells you how to read the charts used in this kind of amateur radio. Other websites also show live charts, but this source is more focused on educating users. Its navigation, however, can be a bit cluttered, so it can be difficult to try and return to previous material viewed sometimes.
This website is an excellent way to tune in and experience live amateur radio. People and volunteer groups link radios to computers and allow people online to listen to different frequencies and locations. It's a good way for people interested in amateur radio to experience the real thing before investing in a radio. The website, however, does not give much instruction on operating the radio itself. I still would recommend interacting with the application and trying to find a conversation. I recommend the Northern Utah WebSDR on the front page. It tends to have the most consistent activity.
This website has simple summaries and information on many electrical engineering topics. I recommend this site for amateur radio because it has summaries on radio and antenna topics that are relevant to the hobby. I recommend using this source to fill gaps in other research as you explore and learn more about amateur radio. Its navigation can make it hard to find things, but the search bar works well. Remember, it's not a website focused on amateur radio, so it's not always going to explicitly say how the topics connect to amateur radio.