1-4GHz horn antenna design, lower frequencies s11 is bad

hi!
i am designing a horn for 1-4 Ghz and i have been successful to some extent i-e for 2.5 to 4 performance is good but for lower frequencies s11 is not much low.

can any one guide me on it how can i get lower s11 on lower frequencies?

Only by increasing the antennas size.

That is a big % bandwidth. Are you using a horn with eplane ridges?