AntennaPod does support the standard podcast subscription protocols:
itpc://www.example.com/feed.xml
pcast://www.example.com/feed.xml
feed://www.example.com/feed.xml
And a non-standard one:
antennapod-subscribe://www.example.com/feed.xml
I am a bit annoyed by the fact that everyone specifies their own links. If everyone just followed the standard, the Android system could open your podcast app automatically instead of having to display a huge list of clients on each website.