Amateur radio is an amazing sandbox for radio science. It has worldwide frequency allocations from 2200m (136kHz) all the way up to visible light, including an

Amateur radio is an amazing sandbox for radio science. It has worldwide frequency allocations from 2200m (136kHz) all the way up to visible light, including an allocation on the 160m (1.8MHz) band that behaves almost identically to the AM broadcast band. Beyond ~30-50 MHz (depending on solar activity (solar flux and/or sunspot number)), the rules of propagation change from mostly ionospheric to line-of-sight, but can still have some interesting long-range propagation phenomena like tropospheric inversion ducting, rain/hail/airplane/building/mountain/water tower scatter, earth-moon-earth, Sporadic-E, etc. Ham radio has kind of a bad rap for being an expensive hobby full of crotchety geriatric men, but it really has a lot more going for it. It's like the national park of radio spectrum. Some people litter and cause problems, but the sheer beauty and utility of the spectrum outshine them.