How to simulate straight line inductor in HFSS

Hi there:
I am sorry if this is a repeat of a common question, but I can find a way to simulate a simple straight line inductor that the results generated can be verified by empirical formula. the model is shown below and also attached to this topic.

to be more specific, I use a PEC line with length=8mm & radius=0.5mm, the well known formula for straight line inductor is
which in this case gives inductance of 4.34nH;
One the other hand, the simulation of HFSS model gives inductance of 2.57nH, which i think must be wrong.So the question is How to set HFSS model appropriately to get the right inductance?

Thanks

attachment is the HFSS model file.

Oops, something wrong in the formula given above,
the line inductance is

You need to understand the effect of a conducting wall or ground plane on the value of inductance of a straight line. The formula you gave may only valid for free space. With a ground plane as close as you depicted from HFSS, I am not surprised the value is considerably lower.

Thanks Phytech,
I fact I have already thought of this effect, and simulated accordingly, but the inductance got enormous large as the figure below shown:

There I increases the lower solve-space from 0.2mm to 15mm and you can see, things get messy. I think maybe the boundary conditions or lumped ports should be modified somehow, but I have no clue how to do to generate the acclaimed value the formula guarantees. BTW the I use Y(1,1) to get the inductance


Best Regards
Julian

I cannot make an accurate comment without knowing exactly how you did the simulation. If you used the same configuration as picture above, then when you increase the lower space, you also increase the overall length of the line thus the value of inductance. Can you do the simulation with input and output ports from two ends and lift the line from the ground plane?