Radio horizon

Radio horizon is like when you're standing on a hilltop and you can only see a certain distance in front of you, kind of like how when you're underwater you can only see so far because of all the bubbles and the water.

Radio waves work kind of like how sound works, where if you shout really loudly, the people closest to you can hear you, but if you shout really loudly from far away, even the people closest to you can't hear you anymore. Radio waves work in the same way, where they start to weaken and become more fuzzy the farther away they travel.

So, the radio horizon is the limit of how far a radio signal can travel before it becomes too weak to pick up on the other side. It depends on a lot of things, like how strong the signal is in the first place, how high up the antennas are, and how much stuff is in the way (like trees, buildings, or hills).

Basically, if you imagine a big bubble around the antenna sending the radio signal, the radio horizon is how far out that bubble can expand before it starts to shrink back down and get weaker.