FEKO is an EM simulation tool that is typically used for large body EM simulations such as cars, aircraft, boat/ships, ... etc using the Method of Moments (MOM) approach to simulate the environment that the antenna will be subjected to, rather than the antenna itself.
https://en.wikipedia.org/wiki/FEKO
Although FEKO can be used for antennas, it is mostly for used to help find the best location placement of the antenna, not the antenna itself as HFSS and CST are the go-to EM simulators for antenna structures :
https://www.antenna-theory.com/tutorial/feko/feko.php
Typically it can be used to find the 'sweet spot' where an antenna can exist in or on a body :
https://www.designtechproducts.com/product-details/altair-feko
This video showcases how FEKO is used for the EM of an antenna on a ship :
https://www.altair.com/resource/altair-feko-for-marine-emc-applications
Again FEKO is not used to design the antenna, but to find the perfect placement 'sweet-spot' for the antenna.
