[moved] Resonance frequency mismatches bias tee

I put a 2~4 GHz bias tee in the circuit, however the resonance frequency dip showed on the VNA S11 trace is around 11.8 GHz. And I'm sure my resonator is long enough for 2-4 GHz. When fixing at this frequency, 11.8 GHz, it's very oscillating (~2 dB) under the ambience. Does it mean most of the signal I got is reflected from the bias tee instead of real intended one from the sample? How can I avoid this from happening?

Thank you!

It looks like your circuit is oscillating when the choke coil in the bias tee is connected to it.

Why do you worry about 11.8 GHz S11 of a "2-4 GHz" bias tee? How are S11 numbers in the specified range?