Hi Karle-Arne, yes that's it. It should oscillate at about 353MHz. I thought you would spot this quickly so I posed the question for a bit of fun... For those who don't like maths etc the relevant part of the data file is here. 352.5885 1.06647 -178.94875 353.21312 1.06146 -179.54065 353.83775 1.05679 179.84584 354.46237 1.05193 179.24806 355.087 1.04718 178.6498 355.71162 1.04185 178.05712 The first column is frequency, the second is the magnitude of the reflection coefficient and the last column is the reflection angle. The key thing to be aware of is that when an EM wave (or any kind of wave) hits a short circuit all of the energy gets reflected but the phase changes 180degrees on the reflected wave. So to satisfy oscillation when there is a short at the output port you have to study the file and look for a frequency that has a reflection coefficient above 1.000 and it also has to have a 180degree phase angle. If you combine that 180degrees (from the data file at 353MHz) with another 180degrees caused by the short circuit you get zero degrees overall phase shift in the system when the short is connected to the output. This then sets up the conditions where you have a 1 port device that can top itself up with energy (because the reflection coefficient is above unity) and the top up happens 'with unity phase'. So you satisfy the requirements for an oscillator. The gain overcomes losses and the net phase of the top up is unity. So the JFET amplifier turns into a self fulfilling oscillator at about 353MHz where the phase of the reflection coefficient is closest to 180degrees. To demonstrate this I fitted a decent SMD cap (ATC porcelain) as a short circuit at the output of the real circuit . This cap has very low parasitic inductance and makes a very good short circuit. However, the spectrum analyser plot below shows an actual oscillation frequency of 360MHz. This is not at the expected 353MHz probably for two reasons. My clumsy soldering meant I probably disturbed the positioning of the output components slightly and also the 353MHz prediction is based on the small signal startup frequency. Once the JFET oscillates up to its amplitude limit the large signal oscillation frequency often shifts slightly as the device limits. But 360MHz is very close to 353MHz and this is a good demonstration I think!