Measuring Common-Mode Chokes with the Y21 Method
Measuring Common-Mode Chokes with the Y21 Method
Y21 can extract the series branch of a choke fixture while separating local terminal shunts—provided the mode, calibration, full S-parameter data and π-network model are all correct.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The matrix calculation is the easy part. The difficult part is presenting the choke as the intended common-mode series element, placing the calibration planes correctly and proving that direct fixture coupling is not bypassing the DUT. This guide separates those jobs step by step.
Target result: ZCM(f) = RCM(f) + jXCM(f). Y21 does not directly produce “choke attenuation in dB,” and a small-signal VNA sweep does not establish the choke’s high-power rating.
Define the Mode Before Building the Fixture
A common-mode choke normally carries two or more conductors. In the wanted differential mode, their currents oppose. In common mode, their longitudinal current components flow in the same direction relative to the outside world, so the magnetic flux adds in the core.
For component-level common-mode impedance measurement, the conductors belonging to the bundle are commonly tied together at each end so they act as one common-mode terminal. The fixture then places that two-terminal common-mode DUT as the series branch between VNA ports.
- Coax choke: bond centre conductor and shield together at each DUT end for the common-mode component test, using short, repeatable connections.
- Two-wire choke: bond the two conductors together at each end.
- Multi-conductor choke: bond all conductors that are intended to move together in the tested common mode.
Do not confuse this with the operating connection. The bonds are part of the component-characterisation fixture. Differential insertion loss, return loss, balance and power stress must be measured separately with the conductors connected in their normal transmission-line configuration.
Understand the π-Network Extraction
Model the calibrated fixture near the DUT as three admittances:
- Y1: shunt admittance from port 1’s DUT terminal to the fixture reference;
- Y2: shunt admittance from port 2’s DUT terminal to the fixture reference; and
- Y3: series-branch admittance between the two DUT terminals.
The admittance matrix of that π network is:
Y = [ Y1 + Y3 −Y3 ; −Y3 Y2 + Y3 ]
Therefore:
ZDUT = −1/Y21
Zshunt1 = 1/(Y11 + Y21)
Zshunt2 = 1/(Y22 + Y12)
This is why Y21 is attractive: in the ideal π model, separate terminal-to-reference shunts appear in Y11 and Y22 while the middle branch appears in Y21.
Do Not Add Shunt Capacitors by Default
The π model does not require deliberate 47 pF, 100 pF or 220 pF capacitors from the port shields to a ground plane. Y1 and Y2 may represent unavoidable fixture capacitance. Y21 can estimate those shunts as well as the series branch.
Intentional capacitors may be useful in a specifically engineered experiment, but they also bring lead inductance, tolerance, self-resonance and a new RF return geometry. Large shunts can load the ports or hide other fixture defects. If used, their purpose and model must be documented and their behaviour verified over the sweep.
Better construction goal: make the fixture physically short, rigid, symmetrical and repeatable. Minimise uncontrolled direct coupling between the two DUT terminals instead of trying to “define” every shunt with a capacitor.
Build the Series Fixture
A practical fixture has two port connectors facing the two ends of the common-mode DUT, with a stable reference conductor or enclosure. Keep the DUT-terminal connections short and make the empty-fixture geometry repeatable.
- Use rigid connector mounting and mechanically stable DUT terminals.
- Keep the two terminal structures similar so reversal tests are meaningful.
- Minimise direct electric-field coupling across the DUT gap.
- Keep test cables away from the DUT and from each other after the calibration planes.
- Add ferrite to the external test cables if a lead-current check shows they participate in the transfer path.
- Use shielding or partitions when direct fixture coupling limits high-impedance measurement.
The fixture must support an open condition, a low-inductance short/through condition and connection of known impedance standards. Those states are essential for characterising leakage and validating the extraction.
Put the Calibration Planes in the Right Place
A full two-port calibration corrects systematic VNA and cable errors up to the chosen reference planes. Ideally, those planes are at the fixture connectors or DUT terminals. If calibration ends at the test-cable connectors, the remaining fixture must be compensated, port-extended or included explicitly in the model.
A female-to-female barrel is not automatically the correct THRU, and a short wire is not automatically wrong. The correct standard is the one whose electrical definition and physical reference planes match the calibration method. Connector sex, adapter delay, through length and fixture topology all matter.
Never substitute “normalize through” for full calibration without stating it. A response-through normalization does not correct the same reflection, source-match, load-match and tracking errors as a full two-port SOLT or suitable TRL calibration.
Acquire Genuine Complex Two-Port Data
For a general two-port conversion, obtain the four complex terms:
S = [ S11 S12 ; S21 S22 ]
Many low-cost T/R instruments measure S11 and forward S21 in one orientation; an exported Touchstone file may contain zeros, copies or unmeasured placeholders for S12 and S22. Inspect the data rather than trusting the file extension.
For a stable reciprocal passive fixture, a second calibrated measurement with the DUT/fixture ports reversed can supply the reverse terms, provided the reference planes and orientation are handled consistently. A true full two-port VNA is simpler and allows direct reciprocity checks.
- Use complex magnitude-and-phase data, not only LOGMAG traces.
- Choose an IF bandwidth and averaging that provide adequate transmission dynamic range.
- Check that S21 and S12 agree within the expected fixture uncertainty.
- Record sweep power; ferrite measurement on a VNA is normally small-signal.
Convert S to Y Correctly
For equal, real port reference impedances Z0, the matrix conversion is:
Y = (1/Z0)(I − S)(I + S)−1
For unequal or complex port impedances, use the general normalised conversion implemented by a trusted RF network library or instrument. Do not silently apply the scalar formula.
The explicit equal-Z0 transfer term is:
Y21 = (−2S21/Z0) / [(1 + S11)(1 + S22) − S12S21]
Then calculate ZDUT = −1/Y21 at every frequency point and plot R, X and |Z|.
Check the instrument menu carefully. Some VNAs distinguish a full matrix Y-parameter from a “converted admittance” calculated from one displayed S-parameter. For the π-network method you need the full two-port S-to-Y conversion, not a single-trace equivalent-admittance shortcut.
Prove That You Measured the DUT
Y21 separates the branches of the assumed model; it does not identify the physical source of every branch. Direct port-to-port capacitance, magnetic coupling, radiation and test-cable common mode all contribute to transfer admittance and therefore remain in Y21.
- Measure the open fixture. This reveals direct leakage and the high-impedance measurement floor.
- Measure a low-inductance short. This exposes series residual resistance, inductance and reference-plane error.
- Measure known standards. Use resistors and simple R-L/R-C networks spanning the intended impedance range.
- Reverse the DUT. A reciprocal choke should give materially the same extracted impedance.
- Change fixture spacing. A large result change identifies mutual coupling or radiation.
- Move and choke the test cables. A stable component result should not follow cable routing around the bench.
- Compare methods in their overlap. Properly calibrated S11, series-through S21 and Y21 should agree where their error mechanisms are small.
Interpret the Result Correctly
The extracted complex impedance describes the series branch of the measured small-signal model. It does not by itself say how many decibels common-mode current will fall in a station. For a simplified installed path Zpath:
AI = 20 log10|(Zpath + ZCM)/Zpath|
Zpath is complex, frequency-dependent and separate from the antenna’s differential feedpoint impedance. If it is unknown, installed attenuation is unknown. Verify the station with a clamp-current probe at several positions along the feedline.
Also treat VNA data as small-signal. High common-mode voltage or current may heat the ferrite, change permeability, stress insulation or move the impedance curve. Power capability needs separate thermal and voltage testing under documented frequency, SWR and duty-cycle conditions.
Practical Workflow
- Define the tested common mode and bond the DUT conductors accordingly.
- Build a short, symmetrical series fixture without unnecessary deliberate shunts.
- Calibrate or de-embed to documented reference planes.
- Measure open, short and known standards.
- Acquire all four genuine complex S-parameters.
- Convert the full S matrix to the full Y matrix.
- Calculate ZDUT = −1/Y21.
- Plot R, X and |Z|, plus fixture shunts if useful.
- Run reversal, spacing, cable-routing and method-comparison checks.
- Measure differential performance and high-power behaviour separately.
Bottom line: Y21 is powerful because it separates local shunt branches in a valid π model. Accuracy still comes from correct modal wiring, genuine full-matrix data, calibration, fixture control and validation—not from the equation alone.
Mini-FAQ
- Must I add 100 pF capacitors to the fixture? No. Deliberate shunts are not required by the Y21 method and can introduce new errors.
- Can a NanoVNA-class instrument be used? Yes, if genuine complex forward and reverse data are measured or carefully reconstructed and the dynamic range is adequate.
- Is a barrel always the correct THRU? No. The standard must match the connector topology, calibration definition and intended reference planes.
- Can the VNA display Y21 directly? Sometimes, but confirm that it performs a full matrix conversion rather than a single-trace equivalent-admittance conversion.
- Does −1/Y21 remove every fixture error? No. Direct transfer coupling, radiation, cable common mode and calibration residuals remain possible.
- Does the result prove power handling? No. A normal VNA sweep is a small-signal characterisation.