A signal reaches another station two ways:

• Line of sight: out to a range set by the band (VHF about 500 blocks), fading with distance.
• Skip: HF bounces off the sky and comes down far away, in a window of distances that differs by band and between day and night. In between is a silent zone.

Rules of thumb: 80 and 40 m are best at night, 20 m by day, 10 m only opens when the sun is lively.

/bandcalculate <from> <to> estimates every band between two points under the current conditions.

Being a few kHz off still lets you hear SSB, sounding higher or lower and thinner; 11 kHz off, nothing. FM is heard up to 2 kHz off.

Hills in the way weaken line of sight: badly on VHF, much less on 80 m, and HF also loses some at every cliff. Skip leaves at a low angle on 10 and 20 m, so a station deep in a valley loses it over the valley walls, while 80 m goes nearly straight up. A handheld indoors or underground is weaker. Build high!

HF radios hiss with static; a strong signal pushes it down, a weak one stays buried in it.

• Redstone switching near your coax or antenna crackles in the receiver: keep clocks away.
• Lightning crashes on every radio in the dimension.

It works both ways: a transmitter induces a signal in dust near its antenna and can flip levers, buttons and torches nearby, further with a bad SWR. QRP rigs reach less.