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RFID antenna design using HFSS

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Simon_B

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Hi
I'm currently designing an RFID antenna for 2.45 GHz using HFSS. I choosed a bowtie-type antenna, got a decent s11 and bandwidth (around -35dB at 2.45GHz and a bandwidth at -10dB around 0.47GHz) nothing fancy.

However, I need to match the antenna to the chip, which has an impedance of 11-292j \[\Omega\]. I want to use a t-match structure, which is classic from what I've read. But when I try to match it to the antenna (complex conjugate matching, i.e. chip has 11-295j \[\Omega\] and antenna 11+295j \[\Omega\]), the antenna is not radiating (no E-field inside the antenna itself, power radiated and accepted about a hundred milliwatts for 1 watt or more incident power), s11 never goes below -3dB, etc.

I suspect a problem with the lumped port I'm using. Currently its parameters are impedance of 50 \[\Omega\] and renormalised to 50 \[\Omega\]. I also got an other curve for s11 with other settings: when I enter the conjugate of the chip (i.e. 11+295j \[\Omega\] and don't renormalise it, so both port and antenna have 11+295j \[\Omega\] ) I got a more standard plot for the s11 curve, but at some frequency it becomes positive, which don't make sense for me.

Does it already happened to someone? And how HFSS is computing s11? If it's not a problem with settings, does anyone knows how to control the radiation of the antenna?
Thank you in advance
 

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