EMC concepts explained
Inductor Impedance Evaluation from S-Parameter Measurements
Part 2: S21 Two-Port Shunt and Two-Port Series Methods
By Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame
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his is the second of two articles devoted to the topic of inductor impedance evaluation from the S parameter measurements using a network analyzer. The previous article [1] described the impedance measurements and calculations from the S11 parameters using the one-port shunt, two-port shunt, and two-port series methods. This article is devoted to the impedance measurements and calculations from the S21 parameters using the two-port shunt and two‑port series methods.

The overall conclusion of the previous article was that the inductor impedance evaluation from the S11 parameter measurements is not accurate. This article concludes that the two-port series method is the most accurate method for the inductor impedance evaluation from S21 parameters when using a network analyzer.

Two-Port Shunt Method
The two-port shunt configuration is shown in Figure 1.
diagram showing a two-port shunt configuration
Figure 1: Two-port shunt configuration
For this configuration, the inductor’s impedance in terms of the S21 parameter was derived in [2] as
equation 1
Two-Port Series Method
The two-port series configuration is shown in Figure 2.
diagram showing a two-port series configuration
Figure 2: Two-port series configuration
For this configuration, the inductor’s impedance in terms of the S21 parameter was derived in [3] as
equation 2
Impedance Measurement Setup and Results
The impedance measurement setup and the PCB boards are shown in Figure 3. The boards were populated with RF inductors [4] of the values 47 nH, 150 nH, and 270 nH.
measurement setup and PCBs
Figure 3: Measurement setup and PCBs
Figure 4 shows the impedance curves for a 47 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self‑resonant frequencies.
graph showing two-port shunt vs. two‑port series (L = 47 nH)
Figure 4: S21-based impedance curves – two-port shunt vs. two‑port series (L = 47 nH)
Figure 5 shows the inductor impedance curve obtained from support software [5].
support software impedance curve for 47 nH inductor
Figure 5: Support software impedance curve for 47 nH inductor
The two-port shunt, two-port series measurements, and the support software results are shown in Table 1.
table displaying impedances at 50 dB, 60 dB, and self-resonant frequencies
Table 1: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
It is apparent that the two-port series measurements are significantly closer to the support software results than the two-port shunt measurements.

Figure 6 shows the impedance curves for a 150 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self-resonant frequencies.

graph showing impedance curves - two-port shunt vs. two‑port series (L = 150 nH)
Figure 6: S21-based impedance curves – two-port shunt vs. two‑port series (L = 150 nH)
Figure 7 shows the inductor impedance curve obtained from the support software.
support software showing an impedance curve for 150 nH inductor
Figure 7: Support software impedance curve for 150 nH inductor
The two-port shunt, two-port series measurements, and the support software results are shown in Table 2.
table displaying impedances at 50 dB, 60 dB, and self-resonant frequencies
Table 2: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
Again, the two-port series measurements at 50 dB and self-resonant frequencies are significantly closer to the support software results than the two-port shunt measurements.

Figure 8 shows the impedance curves for a 270 nH inductor using a two-port shunt and two-port series methods. The shunt measurements were taken at 50 dB and self-resonant frequencies. The series measurements were taken at 60 dB and self-resonant frequencies.

graph showing two-port shunt vs. two‑port series (L = 270 nH)
Figure 8: S21-based impedance curves – two-port shunt vs. two‑port series (L = 270 nH)
Figure 9 shows the inductor impedance curve obtained from the support software.
support software showing an impedance curve for 270 nH inductor
Figure 9: Support software impedance curve for 270 nH inductor
The two-port shunt, two-port series measurements, and the support software results are shown in Table 3.
table showing impedances at 50 dB, 60 dB, and self-resonant frequencies
Table 3: Impedances at 50 dB, 60 dB, and self-resonant frequencies (S21 methods)
Once again, the two-port series measurements at 50 dB and self-resonant frequencies are significantly closer to the support software results than the two-port shunt measurements.

The overall conclusion is that the two-port series method is the most accurate method of the inductor’s impedance evaluation from the S21 parameter measurements.

References
  1. Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Inductor Impedance Evaluation from S Parameter Measurements – Part 1: S11 One-Port Shunt, Two-Port Shunt, and Two‑Port Series Methods,” In Compliance Magazine, April 2025.
  2. Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Capacitor Impedance Evaluation from S Parameter Measurements – Part 1: S11 One-Port Shunt, Two-Port Shunt, and Two-Port Series Methods,” In Compliance Magazine, February 2025.
  3. Bogdan Adamczyk, Patrick Cribbins, and Khalil Chame, “Capacitor Impedance Evaluation from S Parameter Measurements – Part 2: S21 Two‑Port Shunt and Two-Port Series Methods,” In Compliance Magazine, March 2025.
  4. Murata RF inductors LQG18HH47NJ00 (47 nH), LQC18HH15J00 (150 nH), and LQG18HH27J00 (270 nH).
  5. Murata Design Support Software, “SimSurfing.” https://ds.murata.co.jp/simsurfing/index.html?lcid=en-us
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Bogdan Adamczyk headshot
Dr. Bogdan Adamczyk is professor and director of the EMC Center at Grand Valley State University (http://www.gvsu.edu/emccenter) where he performs EMC educational research and regularly teaches EM/EMC courses and EMC certificate courses for industry. He is an iNARTE-certified EMC Master Design Engineer. He is the author of two textbooks, “Foundations of Electromagnetic Compatibility with Practical Applications” (Wiley, 2017) and “Principles of Electromagnetic Compatibility: Laboratory Exercises and Lectures” (Wiley, 2024). He has been writing “EMC Concepts Explained” monthly since January 2017. He can be reached at adamczyb@gvsu.edu.
Patrick Cribbins headshot
Patrick Cribbins is pursuing his Bachelor of Science in Electrical Engineering at Grand Valley State University. He currently works full time as an Electromagnetic Compatibility Engineering co‑op student at E3 Compliance, which specializes in EMC and high‑speed design, pre‑compliance testing and diagnostics. He can be reached at patrick.cribbins@e3compliance.com.
Khalil Chame headshot
Khalil Chame is pursuing his Bachelor of Science in Electrical Engineering at Grand Valley State University. He currently works full time as an Electromagnetic Compatibility Engineer co-op student at E3 Compliance, which specializes in EMC and high‑speed design, pre‑compliance and diagnostics. He can be reached at khalil.chame@e3compliance.com.
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