This article documents a worked example of the matters discussed at Measurement of various loss quantities with a VNA.
Above is an air cored solenoid of about 20µH connected between Port 1 and Port 2 of a NanoVNA-H4 which has been calibrated. The whole lot is sitting on an inflated HDPE bag to isolate the DUT from the test bench.
Note that the calibration used a 250mm RG174 jumper from Port 1 to Port 2, and it is not in place for the measurement, so a correction will be made later.
Measurements will be made at 2MHz so that lead lengths are relatively insignificant.
Expectation
The 20µH inductor has a reactance of 250Ω at 2MHz, and some series resistance of the order of a couple of ohms.
We would expect:
- severe mismatch at Port 1, so relatively high |s11|;
- little conversion of RF energy to heat in the coil, so relatively low loss; and
- quite a reduction in power reaching Port 2, so reduced |s21|.
Measurement
Above is a screenshot showing |s11| and |s12| which are the inputs to the calculation process to find Loss, Insertion Loss, and Mismatch Loss.
The expected loss in the missing 250mm of RG174 mentioned earlier is 0.009dB at 2MHz, it is so small it will be ignored. There is no need to compensate for the phase difference of s21 as the phase is not used in the calculation.
Calculation
For convenience, this online calculator will be used.
Above, the adjusted measurement data is entered and results calculated. Not that the angles of |s11| and |s21| were not entered, they are not used in this calculation.
Reconciliation with expectation
As expected:
- severe mismatch at Port 1, |s11|=-0.6960dB and MismatchLoss=8.295dB;
- little conversion of RF energy to heat in the coil (and possibly some RF radiation), Loss=1.2dB;
- quite a reduction in power reaching Port 2, |s21|=-9.6dB giving an InsertionLoss=9.512dB.
The main contribution to InsertionLoss is MismatchLoss.