Radio is the traditional hobby application and is a good place to start. The ARRL publications vary in quality and level but are generally pretty good. You ca

Radio is the traditional hobby application and is a good place to start. The ARRL publications vary in quality and level but are generally pretty good. You can go as deep as you want into RF design at the hobby level as a ham. It's sort of a new golden age with good-enough test equipment so cheap these days and the sheer range of parts available on the internet.