UWB Vivaldi/Tapered Slot Design at low frequencies

I want to design a 0.5-3 GHz Vivaldi or Horn UWB Antenna, The problem with horn is that its dimensions get very large- and vivaldi is a good option but most of the designs have good performance above 3 GHz and Designs are not available for lower frequency wideband operation,Could anyone help in achieving such a design?

Or suggest LPDA would do the required function,We need antenna Gain to be atleast 7-8 db and High directivity.