Thanks! MRF3866 is spot on, the other is MRF5160. R&S apparently use the same sampling design for their hfh2-z2 loop antenna.
Too bad the transistors weren't the problem. Started measuring the YIG RF output directly by feeding it -5 to 0V to the PINYFO. I can't measure over 4GHz but from what i see 2-4GHz is the output power never exceeds 6 dBm and it jumps allot:
2.1 = 3 dBm
2.2 = 0.2 dBm
2.3= 2 dBm
2.4 = 6 dBm
2.5 = 5 dBm
2.6 = 2 dBm
2.7 = 0.7 dBm
2.8 = 3.3 dBm
2.9 = 5.9 dBm
etc...
Will see if the YIG driver board is at fault, maybe poor transistor or voltage regulator but the RF output power should be at least 15 dBm at any given time from what i understand.
Coil resistance measures 7.1 ohm on Main coil, 1.1 ohm on Tracking and 0.5 on FM coil (suppose to be 1.5 ohm?)
Meanwhile, some photos of the YFOM module opened.