The effect of supply voltage on transmitter output power.
My friend Monk related an interesting experience where his battery charger has an intermittent fault. Meaning that the actual voltage being supplied to his QRP transmitter was varying. This was resulting in inexplicable power output variations in his transmitter.
I calculated that in his case a variation in Vcc from 13.7volts down to 9.68volts (4.02volts) will in fact result in a 3db power output change from 6watts to 3watts!
The moral of the story. Measure all voltages all the time.