How to match the impedance to excite a Plastic wave guide to get its attenuation for a particular frequency? Is there any other simple way to get the attenuation of a plastic waveguide? I tried with waveguide port excitation but i am facing issues like getting very poor S11( High refection on incident power). I tried with Vaccum creation on the both sides of the wave guide its also not working for me! 

Boundaries were electric and background were PEC.

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