Why i can't get oscillation at higher frequencies by cutting gate stub?

Here is oscillator scheme:

Varactor is removed. It works well at 5GHz.

I expected 10GHz oscillation when cut gate stub to half-length. I did it, but oscillation does not occur anymore.
Now if i put DRO from old LNA somewhere around transistor, 10GHz appears to be working, even if i put resonator in "wrong" places, somehow it produces good feedback.

Then I changed transistor to a better one (i have some stupid guess about transistor gain vs frequency, so i put one with typical F=15GHz), but stub or something still does not resonate. To be sure i restored stub to original length and get some around 5GHz with the new transistor.
I had a little access to spectrometer, so did not experiment more.

So why half-length stub did not give me double frequency, but DRO always does? What kind of problem is it?

I think oscillation conditions are not suffciently satisfied because of insufficient feedback capacitance of the transistor because as I saw there isn't any external feedback elements and your OSC works with
internal Ccb capacitance only.In additional to, since this circuit will present some negative input resistance when you look at through base,this is also depended on resonator circuit and its quality factor due to electrical and magnetical couplings..
It's just a guess..

insufficient feedback capacitance of the transistor because as I saw there isn't any external feedback elements and your OSC works with
internal Ccb capacitance only

Click to expand...

So it is not very good idea to modify this oscillator to get a higher freq?
Can i put lumped capacitor somewhere to make it work?

Terminator3 said:

So it is not very good idea to modify this oscillator to get a higher freq?
Can i put lumped capacitor somewhere to make it work?

Click to expand...

First of all, you should check the negative real part of the input impedance of your active part.
The best method using Oscillator port in ADS by changing frequency to able to see Loop Gain and Phase to define potential frequency of oscillation in a bandwidth.
For that, read user guide of ADS.
You should see at a frequency Loop Gain>1 and Loop Phase=0
To make it double the oscillation frequncy, active part of your oscillator must still satisfy this equation-at least- at that frequency.
You can not double the frequency by adding capacitor or something.Be precise and walk step by step..