[SOLVED] Microstrip Line design and Input impedance

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sujan2

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Hello,

I am designing a 50 ohm, lambda/2 microstrip line with two lumped ports each of 50 ohm in HFSS using FR4_epoxy (relative permittivity=4.4) substrate of 1.67mm thick at 1.5GHz. The width and length of the microstrip line is calculated using ADS linecalc method which is obtained to be width:3.193830 mm and length: 54.523000 mm. Since the load impedance and characteristic impedance of the microstrip line is both 50 ohm the input impedance z(1,1), I believe should be 50 ohm for at least 1.5GHz. However, I am not getting 50 ohms while extracting the report for Z(1,1) or Z(2,2). I am new to RF and microwave and it will be greatly appreciated for any help on this. I have attached the screenshot of the result and design here. There is radiation box too which is 0f 100mm cube.



Thank you.
 

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Z-parameters are "Open Circuited Parameters" so when one port is opened, the impedance that is seen from other ports. You have to check s-parameters to verify your line, not z-parameters.
 

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That's it.
By definition


Which means that you left open port 2 (current is 0), so you only sees an open line.
Checking the smith chart you will see that S(1,1) in centered at 50 Ohms.
 

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Just a comment.

Although FR4 is a fire retardant std. laminate using a certain epoxy/fiberglass ratio and dielectric constant. near and above 1GHz it also has a loss tangent that will affect performance, so there are improved options generally with lower Dk which raises Zo or increases the trace width/height requirement to meet 50 Ohms. Although the Dk tolerances are not precise. +/-10% typ.
 

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Thank you everyone. I found my mistake. I was connecting two port each of 50 ohms and looking for Z(1,1) which I realized later it is not input impedance for two port network. When I changed second port to Lumped RLC, it worked, I can see almost 50 ohms in all the frequency sweep.
 

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