s11 device
Hello,
You said you designed means simulated or fabricated a prototype!!!
The S11 can't be positive their may be some problem with the device model
parameter or file...
Yes, it is unstable (oscillations), plot the K or B1 stability factor plots or stability circle measurements on smith chart....
See more here **broken link removed**
Or verify the matching network & the device models...!
By the way what is your operating frequency range...!!!
---manju---