WiFi Explorer MacSparky
macsparky.com/blog/2014/3/wifi-explorer
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
WiFi Explorer provides the mother lode of WiFi data. The app sports a top pane that lists every available wireless network. The second pane presents a graphical representation of the selected networks. There are four potential displays: Network Details, Signal Strength, 2.4 Ghz Channels, and 5Ghz Channels. You can also filter the list between open, secure, ad hoc, and preferred networks.