Re： Designing an aperture coupled antenna in ADS

I realise that a reflector adds on to the gain achieved by the antenna but when I tried out the same in ADS it is decreasing the radiation efficiency by some margin!
Is it a theoretical aspect that reflectors generally decrease radiation efficiency or is there any problem in my concept of designing it?

Any idea in why I am receiving these Warnings? and how to remove them?
An array simulation in momentum is taking almost 5 hours for me.. I was wondering if the time can be reduced by increasing the mesh size? Is it possible to control mesh size in ADS 2016?

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