External Antenna for Raspberry Pi 4

That’s slightly different, that’s unwanted emissions (but also relevant for products for sure). Every country has a regulatory body for wireless, e.g. in the USA it is FCC, and in the UK it is Ofcom (those are the only ones I know), so their websites will have the information, but it can be hard to find.

For instance on the ofcom site, there’s a PDF doc which mentions the amount of radiated power allowed (and there will be other restrictions too). So for instance, with 5 GHz WLAN, there would be the particular restriction below (but it would need an expert at navigating through their sites to be sure; not me!). 

In that doc, they list it as EIRP, meaning that they expect the antenna to be taken into account. There are online calculators to experiment with (e.g. m0ukd.com/calculators/erp-eirp-calculator/ ); for instance, if you had a transmitter of 100 mW, and no other loss and (say) connected to an antenna labeled 6 dBi, then the radiated power (there are several ways to measure it; they are using EIRP in the PDF document, so that is the box to look at in the screenshot below) would be almost 400 mW, so that would exceed the allowed limit in that example. If the user wished to stay within the 200 mW allowed limit in that case, they would need to either reduce the transmitter power to 50 mW, or instead, change the antenna to one that is 3 dBi max.