I am designing Low noise amplifiers/power amplifier/filters in Ku-Band using microstrip technology in Agilent Advanced design system (ADS-2008) Substrate material is mostly RO4003C or RO5880. At frequencies below X-Band, there is close coherence between simulation and measured results. But at X-Band and above, I always observe a frequency shift between simulation and measured results and this wastes lot of precious resources. the most interesting thing is that shape of performance curves (S11, S21, S22) remains almost same in both simulation and measured results. this shift is sometimes positive and sometimes negative. I always simulate the circuits with maximum possible mesh density and minimum possible arc resolution in Microwave mode. Substrate is defined as follows FreeSpace Substrate ////GND////// Metallization layers and vias are also correctly mapped. I suspect that for certain geometries, effective dielectric constant of substrate varies and this causes the shift but I also think that ADS2008 Momentum simulator should also calculate the effective Dielectric constant depending on the geometry and frequency during simulation. Thus, this shift should not be observed. Please help in this regard, How I can improve simulation to avoid this shift in frequency.