2-18GHz cavity backed spiral antenna pattern distortion

hi everybody.

i am designing a cavity backed spiral antenna in 2-18GHz.
i used microstrip balun for transforming from 50ohm input impedance to 188ohm spiral impedance.
both balun and spiral have good behavior . and spiral have good patterns in all frequencies.
but when i attach this two part the resulting radiation patterns distorted in high frequencies (spetially above 14GHz).
please check attached pics.
in addition i wan to model an absorber(like AN75) in HFSS and i don't know what is the electrical properties(epsilon and mu and loss tangent) of absorbers .

i would be thankful if some experts help me.

have you checked current phase in balun
upload your file so that we can understand your problem.