Klopfenstein taper design microstrip problem

Hello,

I am to design a klopfenstein taper with Z1=90Ω and Z2=180Ω with S11 less than -15dB. I first find the impedances required to design.

Then I used these impedances on ADS using Tline. I got the appropriate S11 results.

However when I tried to implement this on Mline on ADS using TMM3 and also TMM4(at different occasions) using the height of 12.7mm the results I got were extremely bad. The S11 parameter goes to -6 to nearly 0dB instead of what I designed it for.

Any suggestions as to what I should to try maybe bigger dielectric constant or smaller height. However 1 thing I know for sure is that using bigger dielectric will decrease the width of the microstrip to less than 1mm and I am told that it is not good.

Any help or suggestions welcome.

Thanks in advance.

What frequency band are you trying to design your taper at? I was able to achieve a simulated 17dB RL from 1 to 6GHz using TMM10i, 25mil thick. I never got to fabricating the circuit, so I'm not certain of the measured S11, S22.

Do you discretize your taper or how do you model it in ADS? Tline is very ideal and when you convert to Mline, you can get a lot of differences. Try tuning your elements to see if you can get it to "fall back" to good performance?

the frequency i am designing it for is 1.5GHz and above. I am using a 21 step taper. I designed one for 15 to 50 Ohms which worked fine in both Mline and Tline... however this high impedance seems to be the problem... what do you mean by 'tuning my elements to get it to fall back'?

My designs were for 50ohm to 6ohm, 50ohm to 9ohm, and 50ohm to 12ohm, so I have never tried a non-50ohm transition like you are designing. I don't think there were any limitations in the impedance from Klopfenstein's paper anyways.

I asked to "tune the elements" by changing the Zo and length of your discrete steps to see if any one segment is causing problems. You said the S11 goes from -6dB to 0dB which indicates to me that you may have some resonance conditions in the taper transition at those points.

I assume your simulation setup is correct since you have demonstrated to yourself that a 15/50 transition works correctly.

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