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Nmerna AntennaTheory.com Newbie
Joined: 11 Oct 2015 Posts: 1

Posted: Sun Oct 11, 2015 11:21 am Post subject: Microstrip antenna calculation vs simulation discrepancy 


For 2.45GHz, 1.5mm substrate thickness (epsilon_r = 4.3), Z_0=50ohm : I found these values using the formula given in Balanis' book
L=29.16mm
W=37.61mm
y_0=10.8mm
transmission line width: 3.04mm
But in the cst simulation (http://s000.tinyupload.com/index.php?file_id=50897352226886568710) these values give not so great results. A VSWR of 4.5 at 2.45GHz.
I played with the equations and the simulation for quite some time but could not achive good results with what those formulas/procedures tell me.
Am I doing something wrong, is this to be expected, how should I go about doing this. I used the optimizer to get something that gives VSWR=1 and Z_0 = 50ohm but it is definitely not what the formulas tell me (they are about 30% off from the formula length measurements). Can I trust those results? 

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helix Antenna Theory Regular
Joined: 29 Jan 2015 Posts: 64

Posted: Wed Oct 14, 2015 2:55 pm Post subject: 


The formulas should get you pretty close but some modest amount of tinkering in a code (and then in the lab) to get good performance is common. Is this an inset fed patch?
Are you using the time domain solver in CST? I would expect the patch you describe to be quite narrow band and the IFFT in CST may be missing the frequency domain feature you want to see.
If so, check your simulation time settings. Due to the sampling theorem (I think that's the right name), what you do in the time domain affects what you see in the frequency domain. Increasing max run time decreases the frequency step, decreasing the time step increases the max frequency. 

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