How to increase bandwidth of horn antenna

hi everyone!
I've been designing horn antenna for Ku band with realized gain >15db
But I can't rules out of learning the way to optimize bandwidth, S11 always about -3-->-7dB
thanx

I noticed you used lump ports.

But in actual implementation, how are you going to excite the horn?

I intend to excite by a short dipole, I tried to use waveport but I ruled out as soon as solve.