# Microstrip length problem

I'm simulating a single microstrip half-wave resonator (1.8 GHz) in Sonnet and using ADS LineCalc tool for calculating the microstrip length but I'm getting a wrong number.

Substrate parameters (LineCalc):

• Er = 10
• Mur = 1
• H = 0.4 mm
• Hu = 3.9e+34 mm
• T = 0.001 mm
• Cond = 5.8e7
• TanD = 0.002

With Z0 = 50 and E_Eff = 180º I get: W = 0.379400 mm , L = 32.239500 mm.

Substrate parameters (Sonnet)

• Bottom dielectric layer

• Thickness = 0.4 mm
• Erel = 10
• Dielectric Loss Tan = 0.002
• Diel Cond = 0 S/m
• Mrel = 1
• Mag Loss Tan = 0
• Conductor(copper)

• Thickness = 0.001 mm
• Conductivity 58000000.0 S/m
• Current Ratio = 0.0

But I've found empirically that the length should be approximately 30.4 mm instead:

Why is this happening?

• When you say you "found empirically" that the length should be 30.4 mm, does that mean you built a physical sample and tested it, or that you simulated it with a different tool? Commented Mar 20, 2015 at 0:52
• And if you built it physically, what was the actual substrate material? Commented Mar 20, 2015 at 0:53
• I mean that I changed the length of the microstrip until I saw the expected result in the simulation without any calculation. Commented Mar 20, 2015 at 1:58

I don't know the details of how Sonnet works, but some typical things that could make the EM simulation differ from the idealized transmission line model are

• Fringing fields at the end of the stub. A microstrip stub doesn't act like a lumped model of a perfect open circuit. There is some equivalent capacitive or inductive termination. This termination will change the phase of the reflected wave, and so change the line length that gives a resonant response. The 3D solver will model this more accurately than LineCalc.

• Launch conditions. Depending how Sonnet models the input port, which is essentially a transition from a lumped-element model to the 3D geometry, there could be a discontinuity at the input to the line. This would again produce a reflection which would change the length of the stub needed to see resonance.

Similarly, if you were to build a real sample of this circuit, any reflection due to a discontinuity at the input would change the stub length needed to get an equivalent to a half-wave resonance.

• Meshing. If you have chosen too sparse a mesh for the 3D simulation, its results will be inaccurate.

• Boundary conditions. The choice of how the borders of the simulation region are modeled can affect the simulated result.

Similarly, if you built a real circuit other conductive features like a lid on a package or an unrelated trace on the same substrate could slightly perturb the behavior of the stub.

As I've mentioned, there's several effects that will also make a real stub behave differently from your model, beyond which there's also the simple inaccuracy in manufacturing that means your W, H, and T parameters will never be exactly what you designed them to be. With this in mind, it's really best practice to make a design that is insensitive to small errors in any parameter; or, if necessary, to provide a tuning mechanism to allow the circuit to be adjusted individually to optimize the performance.