I am designing my own Test Fixture to measure a packaged transistor, the TF is composed of 2 launches (with biasing path) and a line to make the TRL calibration.
The TF center frequency is 2185MHz and it's 50 ohms reference impedance. I would like to be able to perform measurement from 0.5GHZ to 8GHz, but It seems that the launch has S21 resonnant frequency about 4.5 GHz, it is a problem.
This is due to bias circuit. i don't know how to get a flat S21 full bandwidth.
My project is available here.
I don't have ADS, so I cannot open the project, but from the image, it seems that the biasing path is acting like a stub. Try to remove it and simulate.
You need to design a wideband biasing or use an external bias tee.
I hope it can help.
Mazz
I havent designed any large bandwidth tee bias, but...
Those inductors are for very large bandwidth (>10 GHz, if I well remember.)
Some time ago I opened up and repaired a bias tee: they used a circuit made by different sized inductors (there was also a transformer) to prevent self resonant freq (see https://www.qsl.net/n9zia/wireless/pics/bias_tee.png to have an idea).
Or you can try using large bandwith stubs + some discrete.