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In the absence of VNA product documentation...... Now, -9 dB is equivalent to a ratio of 0.34 approximately, and -3.53 db is equivalent to a ratio of 0.66 approximately. I expected S11 to be nea...
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#1: Initial revision
by
Andy aka
·
2020-11-15T11:33:55Z (about 4 years ago)
In the absence of VNA product documentation......
> Now, -9 dB is equivalent to a ratio of 0.34 approximately, and -3.53 db is equivalent to a ratio of 0.66 approximately. I expected S11 to be near 0.
A [9 dB return loss](https://www.rfcafe.com/references/electrical/vswr.htm) is a reflection coefficient of \$\pm\$0.355. Accounting for slight discrepancies in the measurement, I suspect that the reflection coefficient (\$\Gamma\$) might in fact be -0.3333 (somewhere between 9 dB and 10 dB of return loss). And, a \$\Gamma\$ of -0.3333 would be obtained from a 25 Ω resistor relative to a measurement reference of 50 Ω: -
$$\Gamma = \dfrac{R_x- 50}{R_x + 50}$$
Plugging 25 Ω into \$R_x\$ means \$\Gamma\$ is -0.3333 and return loss is \$-20 log_{10}{|\Gamma|}\text{ dB}\$ = -9.54 dB.
So, why 25 Ω? It's the input impedance of the VNA in parallel with two 100 Ω resistors in parallel inside the soldered blob i.e. 25 Ω.
> Regarding s21, I didn't know exactly what to expect.
If you used a straight cable with no 100 Ω resistors fitted then I would expect 0 dB but, now that the power is shared between the two parallel 100 Ω resistors and the input of the VNA, the signal power level measured by the VNA is half or -3.01 dB. OK the cable and other errors makes this -3.53 dB.