Yep, the combination of alias products and harmonics makes for interesting results. And of course the MMIC is downstream from the anti-alias filter.
I have several follow-up experiments in mind - along the lines of Steve's suggestions.
1. repeat the scan with a Type-2 9850 DDS on an Experimenter Board that has only the tapped transformer and a 50 ohm load - no attenuators or anti alias filter yet.
2. add the attenuators and anti-alias filter to the Experimenter Board and repeat.
3. return to the Type-2 PHSNA and vary the input voltage down to the drop out of the 5 V regulator - this will vary the bias current to the ERA-3+.
The darn little 1 minute movies with my digi camera are over 100 MB, and still take 12 MB after washing through Windows Movie Maker as a "windows media video" (wmv) file. I'm going to hunt for a better way. I'll try using my tablet's camera at lower resolution and mp4 format. At 320x240 it's about 6 MB per minute