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Enquiry on Impedanze Zin and S11 relationship

Question asked by stanleymyc Employee on Dec 22, 2010
Latest reply on Dec 23, 2010 by stanleymyc
Hello everyone,

i would like to verify the relationship between this two formula using ENA/PNA:

r = (Zin - 50Ω)/(Zin + 50Ω)

S11 (dB) = 20*log(r)

where the 50ohm is the terminated 50ohm load.

First i connect the cable to port 1 with 50ohm terminated at the end of the cable,

the Zin should be the magnitude of the result shown in the screen. ( smith chart )

I tested using my unit, I saw -62dB S11 at around frequency 200MHz, and check in smith chart, I got around Zin = 49.9.

substitute the finding to r formula,

r = 0.1 / 99.9 = 0.001

Using S11 = 20log 0.001= I got around -60dB, which is almost same result as the one I saw in S11. so the relationship is verified.

Subsequently, i get DUT the TV cable , with impedance characteristic of around 100ohm, as ENA/PNA is using 50ohm system, i get a 100ohm to 50ohm impedance adpter, and connect in between.

i calibrate using 100ohm calibration kit ( self made open, short and load ) and perform as usual, this time the formula change to:

r = (Zin - 100Ω)/(Zin + 100Ω)

S11 (dB) = 20*log(r)

The result was obviously different this time, for S11 and r relationship.

my question would be
1) is the relationship between R and S11 still holds, even if i include the impedance adapter and using 100ohm cal kit?

2) if the relationship holds, obviously my cal kit definition file might have problem, then i will look into it further.

thanks if anyone can help on this.