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Momentum RF S11 Measured vs. Simulated Discrepancies

Question asked by rjrodrig@yahoo.com on Feb 14, 2013
Latest reply on Mar 8, 2013 by rjrodrig@yahoo.com
Hello All,

I have designed a two turn antenna for RFID system.  The antenna is on FR-4 31 mils thick substrate, and using momentum RF I simulated S11 parameters sweeping from 100KHz to 300MHz.  I then compute the Z-parameters (Z_11=(50*(1+S(1,1)))/(1-S(1,1))), and respective inductance (L_11=(1e9*imag(Z_11))/(2*pi*freq)).  Using the real magnitude of Z_11, I locate the antenna resonance from the simulation plot which is incorrect according to the VNA.  I fabricated the antenna, and using a Vector Network analyzer I measure 48.20MHz resonance.  Momentum predicts 79.20MHz.  Why is there a discrepancy?

I suspect it is a setup issue in the substrate (see below), and/or the ports.  I am using ADS 2009 version.  I have used single port on one end of the antenna trace, and port 2 on the other end as ground reference to port 1.

Using CST EM simulation from a friend on the same design, his results are identical to the VNA, I am disappointed as to why I can't seem to match my results to measured results.  I don't understand why the resonant frequency in the VNA is not the same as the simulated data within some error margin.  I tried adding ground below to the substrate but that didn't work either.  I continues to predict around 79.20Mhz which is wrong since the VNA predicts lower resonance.  

Does anyone know why this is?  Is there an accuracy issue with momentum RF version 2009 or is this just a setup issue?  I want to believe my sims before I fabricate anything, so far I have no confidence in this simulation results.

Any help will be greatly appreciated.

Stackup

FreeSpace (open)
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Air (0.5 inches)
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Top_cond
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FR-4 (32mils)
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Bot_Cond
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Air (0.5 inches)
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Open freespace  

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