This is a small list of open-source software to help develop antennas:
openEMS:
http://openems.de/start/index.php
NEC2++:
https://github.com/tmolteno/necpp
Angora:
Microstrip Patch Antenna Calculator:
http://www.emtalk.com/mpacalc.php
Update: Presentation about openEMS: https://archive.fosdem.org/2019/schedule/event/openems/attachments/slides/3367/export/events/attachments/openems/slides/3367/openEMS_FOSDEM19.pdf