GCPW model simulation 70GHz in EMPRO

GCPW simulation @ 70GHz.

hello all, I'm currently doing simulation of a GCPW line at 70GHz. But I recently a testboard and the results is far from the simulations. AFter investigation I suspect some atmospheric attenuation of top ground. About simulation, I tried FEM simulation in ADS and EMpro, and for both there's no issue and return loss is below -20dB. Does anyone already experienced such issue? And how was it solved?

thank you

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