problem with source edit in HFSS

hi there
i am working with two PIFA antenna on a same ground plane. i really get stuck when i found that radiation pattern for gain changes drastically on changing the source of ports like - keeping one open circuited ,
- terninating one port ,
strange thing is that on terminating an open port(feeding other) i got certain pattern but on termination a port with feed initially (and feeding other one) i got entirely different values .....pattern was somewhat same.

its all very confusing anybody please help ........