radio wave / mobile phone signal / (slang) nonsense produced by a mentally unsound person, who is termed a 電波(でんぱ)系(けい) (denpa-kei, “radio-wave-type”) person

(slang) random and nonsensical

radio clock

(slang) denpa-kei person: one who acts strangely (disconnected from reality), often having wild fantasies and often saying strange things

denpa song, a Japanese genre of catchy electronic music featuring cute vocals and nonsensical or unusual lyrics.

(astronomy) radio astronomy

radar, radio detection and location, RADAL