I'm trying to simulate microstrip-antennas with ADS in the range up to 1 GHz. I defined a 1.6 mm FR4-Substrate with two conductor layers. On the upper layer i built my antenna and on the lower layer i built the ground. There is one differential port with at the antenna and at the ground. After simulating with FEM and Momentum, i could find huge differences between the results. While the Momentum results showing an antenna efficiency of about 1 %, the FEM results showing an efficiency of about 60 %. Even the farfields look quite different. Both simulations were done with default settings. Which result is the correct one or what should i change to get more similar results?