[Moon-net] Amplifier oscillation

[Koellner, Guenter (NSN - DE/Muenich)]

Hello,

I have a problem with oscillation between two amplifiers on 23cm since using 2 times MRF286 in one stage.

Basically I am using this set up:
(1) DD9DU-Transverter with 1W output
(2) LDMOS driver 13dB Gain, able to deliver about 20W
(3) 2 times MRF286 coupled with a Semi-Rigid made 90° hybrid, about 13dB gain, too.

Just connecting (2) and (3) while bias is enabled is enougth in order to let (3) oscillate. The oscillation is gone when inserting a 6dB or 10dB attenuator between them.
The oscillation also does not appear when using a BLF958 amplifier with just 10dB gain as (2)

How can that be? I always thougth that the hybrid with it's 50Ohm load at the input of a twin amplifier shows a perfect load to the stage before. Why do these stages then oscillate? All amplifiers are standard designs on PCBs, nothing really special.

Might an isolator between (2) and (3) solve the problem? (I do not have one for 23cm, so I cannot just try it)

Any help would be welcome! Thanks a lot in advance!

73, Günter (dl4mea)