Two questions:

1. Why does the hole in shielding to block EMF need to be less (i've read approx 40% less) than the wavelength itself?

2. Regarding #1, why are really really long wavelengths (maybe ULF) not blockable in this way---given that just about any shielding you make is going to have holes smaller than a few km ?

 

Because holes are antennas.
https://en.wikipedia.org/wiki/Babinet's_principle

 

donno
? pg.8 https://docplayer.net/21741435-Advanced-materials-and-design-for-board-level-emi-shielding.html
? http://cdn.lairdtech.com/home/brand...ing Aperture Size Technical Note Download.pdf
https://www.google.com/search?q=shield+effectiveness+hole+mesh+emi

 

donno
? pg.8 https://docplayer.net/21741435-Advanced-materials-and-design-for-board-level-emi-shielding.html
? http://cdn.lairdtech.com/home/brandworld/files/EMI Rule-of-Thumb for Calculating Aperture Size Technical Note Download.pdf
https://www.google.com/search?q=shield+effectiveness+hole+mesh+emi

thanks, those are pretty helpful.