Query about gain of UWB MIMO antenna

I have designed an UWB MIMO antenna using low cost FR4 substrate. I am getting negative peak gain (-4 dBi) in lower and middle frequencies. But at higher frequencies, I am getting around 1 dBi. What would be the reason for this? The size of the UWB MIMO antenna is 50 mm x 50 mm. What is physical significance of negative peak gain?