Input impedance of a shorted-circuit quarter-Wavelength Transmission Line

[An_RF_Newbie]

Hello,
Here is my circuit:

Regarding the transmission line theory , the input impedance of a shorted-circuit is Zin=Rin+JXin=J*Z0*tan(theta) and Rin=0. For a shorted-circuit quarter-Wavelength that theta=90, if should show the Xin=Infinity and again Rin=0. During my simulation I see the Rin became -50 ohm at resonance frequency and the Xin is a high number(we can assume infinity). I need to know why the Rin became -50 instead of zero.

 

[BigBoss]

Numerical Tolerances/Rounding/Limited Number Computation Space...
Haven't you ever heard one of those ??
Enter tangent(90) in your calculator, see what'll happen ?

 

[An_RF_Newbie]

You are right, I was concerned about the ADS result. As I add a series resistor I see the result become fine.