Brian,
After you double checked with the cal output I can only say that your tinySA should be within the +/- 2dB error and it is not messed up.
I can not explain the difference with your wattmeter.
Maybe one more test you can do is to use the 40dB attenuator as the dummy load for your wattmeter so you can measure with the tinySA and the wattmeter at the same time using exactly the same load to your HT
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For more info on the tinySA go to https://tinysa.org/wiki/
