I’ve used the Langer probes in so many shops that I almost forgot there were other manufacturers of near field probes.

https://www.langer-emv.de/en/category/rf-passive-30-mhz-3-gh...

Generally these are connected to a high-end spectrum analyzer, but recently I’ve been working on a method to use a WiFi transceiver as sort of poor-man’s spectrum analyzer. This is useful only in applications where you want to snoop power levels in the range the chip works in. In my application we want to measure internal interference issues, within a device, where some non-WiFi subsystem creates noise in WiFi bands. Using the device’s own WiFi front-end to measure power becomes a very effective way to debug these issues.

The cool part is we did a correlation study between a $100k spectrum analyzer and the in-device WiFi chip, and across all noise sources and relevant frequency ranges the two agreed within 1-2dB. Not perfect, but good enough for our application.