ACWWong
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The port can be trusted in the mismatch case, you just have to be careful what you are trying to measure. If you are using spectreRF, your small signal analyses (pac, pxf etc.) may incur the 6dB discrepency due to the mismatch voltage gain. You can always do transient analysis to compare.
Usually I work completely in the voltage domain, and specify my LNA & mixers in terms of voltage gains (& input referred noise, V1dB etc.), this is because I generally work on integrated RFIC, where signals are volatges or currents and not power. In your case I think this would make things clearer for you.
But yes you are correct, you can always do the LNA mixer design together to clarify the matter (assuming you have a conjugated matched input LNA, else you will have to again accommodate mismatch gain/loss).
anyway good luck with your design...
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