Reiner and Arie thanks for your helpful comments.
I have learned something useful for future measurements. I should have mentioned that the preamp under test incorporates two SAW filters 1090 MHz +/- 15 MHz, so, in the plot I attached, the fundamental frequency likely did not have much effect, however it would explain some anomalous results with other amplifiers that I tested.
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Earl, 4Z4TJ
Earl