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EM and Circuit modeling of Slot Antenna

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Full Member level 3
Mar 13, 2015
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Hello all,

I am working to design a microstrip fed slot antenna. I have run into an unexpected result and I wasn’t sure how to interpret it correctly. Above is a picture of the antenna geometry. The left picture is a top down view: a 50 Ohm line feeding a slot in the ground plane, with the transmission line extending slightly past the slot. The right picture shows the line and slot at an oblique angle to show the separation, the substrate is hidden to make the ground plane visible. For reference, the substrate is 5 mil polyimide, this work was done in Microwave office, and the port setup was suggested by software makers.

From some literature I was able to find a good antenna structure. Above on the left, I am plotting the return loss of the antenna. The dashed red line is the EM simulated result of the antenna structure seen at the top of the post. The solid blue line plots the return loss of the circuit schematic seen next to the plot. The circuit schematic simply plots the s1p file of the EM simulated results, so its no surprise the two plots are identical.

Here is where I run into a result I didn’t understand. I started to extend the length of the 50 ohm feed in the EM simulation as shown above on the left. I also added the same length of transmission line to the circuit model, however, obviously as a microstrip circuit model and not an EM model.

Sorry to switch to smith charts, but it better illustrates the point. Above on the left is the result of the EM simulated antenna while increasing the feed length by increments of 2mm. Above on the right is the original EM simulated antenna but with the added microstrip model for the feed.
The results on the right, using the added microstrip model for the feed, makes sense to me. Because I am adding a 50 ohm line between the antenna structure and the 50 ohm port, I should simply rotate the results around the center of the smith chart, which it does. The results on the left, the fully simulated EM feed, confuse me a bit. Why is its center of rotation not the center of the smith cart?

One check I did was to make sure the microsctrip circuit model did indeed match well with the EM simulated line. Above I am showing an EM (red dotted line) and circuit simulated (solid blue) 6mm microstrip line and comparing their results. The results show two models expectedly match very well, and the lines truly model as 50 ohms.

I tend to believe the EM simulation’s results, but I can’t explain the data’s center of rotation not being the center of the smith chart, despite only adding a length of 50 ohm line to the geometry. Any thoughts?

Happy to provide more details and context, thanks in advance for any help,


In my opinion is practically impossible (or pure luck) to get a perfect match between a microstrip model and its EM counterpart.
In top of many factors that the EM simulation take in consideration and the linear model don't, I think the answer is mainly related to how the two situations relates to meshing.
Even for the same circuit when run two EM simulations having two different meshing choices, it may get two different results.

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