I got so curious that I had to test a 10dB attenuator as David Eckhardt suggested even it is late here. The gain went down 10dB which is good I think, nothing changed much. I noticed that S11 is almost normal but noisy below 200-300Mhz, so I quess this is some kind of feedback. BF199 is not made for 200MHz or more. And the attenuator is next to VNA and there is some 30cm of coaxial between that and the amplifier, so it is not so effective taming the circuit.
To
Dana Whitlow. I build this some time ago and now I noticed that there are two capacitors 22pF and 68pF from collector to ground and output is between them. I wish I could remember what coil inductance was.
