antenna-theory.com

[daneloctober]

Hello!

I was told that it is not very efficient to use S11 parameters to measure the performance of passive RFID tag antennas and to instead use Z11 parameters. Can anyone tell me why this is so? Thank you.

[bigSteve]

Hmmm....

I haven't heard that.

But supposing it is true, here is what I would guess may be the reason:

a) RFID is typically very low frequency, and use small antennas. As a result, the antennas aren't so much antennas as big inductors. Hence, the RFID antenna design is less about obtaining an efficient antenna and more about getting a loop with a large enough inductance. Inductors couple together when close together.

b) Since RFID antennas are so small, s11 will be very close to 1 (or 0 dB) because they are so inefficient. Hence, you can't really read on a network analyzer the difference between -0.1 dB and -0.13 dB and really see a difference.

Actually, after writing this I can see why you wouldn't use s11, but rather measure inductance.

[daneloctober]

Thanks for the reply!

Though I forgot to mention that we're into UHF RFID applications, which means operating frequencies of about 860-960MHz.

I've started using Z-parameters. Am I just to concern myself with the imaginary impedance component? Because I really can't see much difference in simulation results of the real impedance... I'm using HFSS by the way...

[bigSteve]

Your probably modeling with perfect conductors so there won't be much for ohmic losses (real). Or if you are using Cu it will be pretty low as well. The real part of the impedance will grow as the radiation efficiency improves. So keep an eye on it. But yeah, the imaginary part (which should be inductive) will be more critical.