Radiohound, I love your table and spectrum analyzer output! We should work this into the official HamShield wiki once we get that going so everyone can edit.
Yes, there is a bit of a non-linear jump and it is functioning as designed. What we are doing with power level is essentially controlling the AT1846S's internal PA. This effectively drives another PA and a final, and we have found that the 1846 PA doesn't exactly behave as expected. It is non-linear and its maximum output mode produces too much noise.
Also, the amplifier performs better at higher frequencies, which is why power at 2 meter will be different than 70 cm.
A little history of the HamShield prior to the Kickstarter. I have all of our schematic revisions, and I will post them somewhere so you can see our progression (and hopefully a simple mistake!).
HamShield was originally designed as a 5 watt transmitter. However, this design proved to be difficult for a number of reasons: The main fact that it was being placed exposed on top of an Arduino!
We originally started out with the RF6886 and controlled the VREG with a variable power supply controlled by an Arduino PWM line. The idea was to allow fine control of power output by modifying an arduino PWM. We had a variety of problems, which could be rooted in our lack of practical experience with power amplifiers. We had issues with self oscillation of the amplifier and a variety of other issues.
Ultimately, we moved on to several different amplifier designs and settled on one before we did the Kickstarter.
Then, we made the amateur crowdsourcing mistake of promising the moon for stretch goals, including spending the extra time to figure out how to boost the power if we hit the $100,000 mark. It all worked out though, but we went through heck and a lot of stress -- and ultimately a few months late on delivery.
With HamShield10 (1.0) on the drawing board, we are going to refactor the entire amplifier section. We will probably be around the same power output, but reducing the amplifier path redundancy (and using 1 final amp instead of 3!), will reduce costs of manufacturing. There are 3 amplifiers only because we ran out of time and wanted to get HamShield out the door.