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I am trying to design and understand a 30mmx45mm patch antenna. The setup is below, but it is a 50mm long, 50ohm T.L. feeding the patch antenna on Rogers5870 circuit board. I am trying to do impedance matching with an open circuit stub. My issue is two-fold: i) when I have a really good impedance match, where my S11 parameter is very low, my gain does not increase and ii) the 3dpolar plot is not symmetric. The details are below and I have attached the simulation file

Case 1: No tuning stub--just patch antenna and transmission feed line The electric field plot and the 3d polar gain plot showing asymmetry, here the gain is 5.8dB and the S11 parameter at 3.125GHz is -5dB. I deembeded the excitation by 50mm to the entrance of the patch antenna to estimate what the impedance was and Re(z)=89ohms and Im(z)=-75ohms

No stub-electric field No stub-ReZe of Impedance No stub-3d polar plot No stub-S11 plot

Case 2 included the tuning stub used for impedance matching As before, the electric field plot and the 3d polar gain plot showing asymmetry, here the gain is 5.8dB and the S11 parameter at 3.125GHz is now -37dB. I deembeded the excitation by 50mm to the entrance of the patch antenna to estimate what the impedance was and Re(z)=52ohms and Im(z)=-.6ohms which means the impedance matching is working quite well. My question is, why is my gain not improving?

W/ stub-electric field W/ stub-ReZe of Impedance W/ stub-3d polar plot W/ stub-S11 plot

Erik McKee
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1 Answers1

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A guess: Your simulation calculates the gain as radiation power per steradian divided by the radiation power per steradian caused by ideal omnidirectional antenna with same input power.

You may output more watts to the sky in case 2 but that doesn't affect the ratio. You should compare watts per steradian -patterns when the transmitter at the feeding end stays same.