About a 50 ohm stripline: freq range 0 to 18 GHz
I assumed that your material was FR4, and the height between the top and ground layer 1mm , TXLINE gives me 11 ohms, pretty flat, but the width is greater than a quarter wavelenght at 18Ghz, so there will be modes. Also 11 ohms. 5dB return loss is almost exactly what I read out from my smith chart. The correct width for FR4 for 50 ohm should be around 2mm, not 4.3cm. 

Also, the bottom (ground layer) should be covering the entire bottom side of the PCB, or fringing effects will destroy the model.