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RFID Antenna complex impedance calculation

 
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rock1997
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Joined: 22 Sep 2017
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PostPosted: Fri Sep 22, 2017 9:23 am    Post subject: RFID Antenna complex impedance calculation Reply with quote

Hello,

I am new to antenna and RFID. I recently designed and made a RFID dipole antenna and would like to see the impedance change at certain frequencies.

I have obtained S matrix parameters s2p file using VNA and all four S parameters (including real and imag) can be seen. Since the complex impedance needs to be calculated using the following equation:

Zd=2*Z0*(1-S11^2+S21^2-2S12)/((1-S11)^2-S21^2), where Z0=50 Ohm.

I tried to plot Zd in EXCEL but it couldn`t handle four complex number operations, hence returning errors.

I then tried MATLAB but the results was very odd. I finally asked a friend of mine to plot using Keysight ADS which got the correct results.

From my side, I don`t have Keysight ADS. I would like to plot Zd using either EXCEL or MATLAB.

Anyone knows the correct way or even more convenient way to do the calculation?

Many thanks,

Rock
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admin
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Joined: 03 Jan 2007
Posts: 204

PostPosted: Fri Sep 22, 2017 5:32 pm    Post subject: Reply with quote

First question: Why are there 2 ports in your measurement? Antenna is a 1 port device.
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rock1997
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Joined: 22 Sep 2017
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PostPosted: Mon Sep 25, 2017 7:38 am    Post subject: Reply with quote

admin wrote:
First question: Why are there 2 ports in your measurement? Antenna is a 1 port device.


RFID is a balanced device so I used two ports measurement. What I did was that I made a bespoke fixture using RG405 cables and two SMA connectors. Both outer conductors of RG405 were soldered together to provide a common ground and both inner conductors were connected to port 1 and 2, respectively.

I did usual TOSM cal and then compensated the loss of the fixture. I then export S-matrix including all four S parameters. However, i am struggling on how to calculate the impedance using those four parameters..... complex number calculation....

Thanks.
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admin
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PostPosted: Mon Sep 25, 2017 5:07 pm    Post subject: Reply with quote

Makes no sense. Measure the antenna with a single port and you can get the impedance directly.
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