A transistor/ fet can have its bias adjusted to a
level so it responds to a very low threshold signal from an antenna. Then amplifies it. This is a step better than the diode detector idea.
The link below has a gallery of wideband rf detector circuits. An internet search turned up myriad more. I received a simple field strength detector with a kit I purchased, which broadcast mic audio to a nearby fm radio.
https://www.hobbyprojects.com/F/Field_Strength_Meters.html