Measurements, Testing, and Calibration Network Analysis Informational

How do I perform a multiport S-parameter measurement with a two-port VNA?

A multiport device (3+ ports) can be measured with a two-port VNA by measuring all possible port pairs sequentially, with unused ports terminated in 50 ohms, and then assembling the full S-parameter matrix from the individual measurements. The procedure for an N-port device: (1) The full S-parameter matrix has N×N entries. A two-port VNA measures four S-parameters per connection (S11, S21, S12, S22). Each connection measures one 2×2 submatrix. (2) Number of connections required: N×(N-1)/2 port pair combinations (for S21/S12), plus N connections for each port S11. In practice: C(N,2) = N×(N-1)/2 connections to capture all pairs. For a 4-port device: 6 connections. For an 8-port device: 28 connections. (3) For each connection: connect VNA Port 1 to DUT port i and VNA Port 2 to DUT port j. Terminate all other DUT ports in 50 ohms. Measure S11_ij, S21_ij, S12_ij, S22_ij. These correspond to Sii, Sji, Sij, Sjj of the full matrix. (4) Assemble the full matrix from all measured submatrices. Consistency check: each diagonal element Sii is measured N-1 times (once for each partner port). These values should be consistent (within measurement uncertainty). Average if small differences exist. (5) Important: the terminations on unused ports must be high-quality broadband loads. A poor termination (< 20 dB return loss) changes the impedance that the active ports see, altering the measured S-parameters. For precision measurements: use precision 50-ohm loads with RL > 40 dB. Alternative approach: a multiport test set (MTS) is a switching matrix between the VNA ports and the DUT ports. It automates the port-pair switching and reduces the measurement time from hours to minutes. The VNA software controls the MTS and assembles the N×N matrix automatically.

Category: Measurements, Testing, and Calibration

Updated: April 2026

Product Tie-In: VNAs, Calibration Kits, Cables

Multiport VNA Measurements

Multiport devices are increasingly common in modern RF systems: MIMO antenna arrays, switch matrices, power divider networks, coupled resonator filters, and balanced/differential amplifiers all require N-port S-parameter characterization.

Parameter	SOLT Cal	TRL Cal	eCal
Accuracy	Good	Excellent	Good-very good
Standards Needed	4 (S,O,L,T)	3 (T,R,L)	1 (module)
Bandwidth	Broadband	Band-limited	Broadband
Setup Time	5-10 min	10-20 min	1-2 min
Best For	Coaxial, general	On-wafer, waveguide	Production, speed

Calibration Procedure

For a 4-port device, the full S-matrix is: [S11 S12 S13 S14; S21 S22 S23 S24; S31 S32 S33 S34; S41 S42 S43 S44]. Connection 1 (VNA P1→DUT P1, VNA P2→DUT P2, P3 and P4 terminated): measures S11, S21, S12, S22 of the DUT. Connection 2 (P1→DUT P1, P2→DUT P3): measures S11, S31, S13, S33. Connection 3 (P1→DUT P1, P2→DUT P4): measures S11, S41, S14, S44. Connection 4 (P1→DUT P2, P2→DUT P3): measures S22, S32, S23, S33. Connection 5 (P1→DUT P2, P2→DUT P4): measures S22, S42, S24, S44. Connection 6 (P1→DUT P3, P2→DUT P4): measures S33, S43, S34, S44. Redundancy check: S11 is measured in connections 1, 2, and 3. S22 in connections 1, 4, 5. S33 in connections 2, 4, 6. S44 in connections 3, 5, 6. Averaging the redundant measurements improves accuracy.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades

Error Sources

A multiport test set automates the port-pair switching: (1) Architecture: the MTS contains SP4T or SP8T switches that route the VNA ports to any DUT port. Internal 50-ohm terminations are switched onto unused ports. All switching is controlled by the VNA software. (2) Calibration: the MTS must be calibrated as part of the measurement system. Two approaches: (a) calibrate each port pair individually (6 calibrations for 4-port), or (b) use a multiport calibration algorithm (unknown-thru: measure only reflection standards at each port + a thru connection between each pair). (3) Error considerations: the switches in the MTS add insertion loss (0.5-2 dB per switch), which reduces dynamic range. The switch isolation limits the minimum measurable S-parameter (typically 70-80 dB isolation for PIN switches, 90+ dB for mechanical switches). The switches must not perturb the unused-port termination when switching states. (4) Examples: Keysight U3042AE04 (4-port to 26.5 GHz), Keysight U3042AH08 (8-port), Rohde & Schwarz ZN-Z8x (8-port to 40 GHz). Cost: $10K-$50K on top of the VNA price.

Common Questions

Frequently Asked Questions

How much longer does multiport measurement take?

Time comparison for a 4-port device: 4-port VNA: one calibration + one measurement sweep = 2-5 minutes total. 2-port VNA with manual connections: 6 calibrations (or one cal + 6 measurement connections) = 30-60 minutes. 2-port VNA with multiport test set: one calibration + automated switching = 5-10 minutes. For an 8-port device: 4-port VNA with MTS: ~10-15 minutes. 2-port VNA manually: 28 connections = 2-4 hours. The multiport test set is essential for efficient multi-port characterization.

Do the terminations on unused ports affect the measurement?

Yes, significantly. When measuring Sij with all other ports terminated: the S-parameters assume the unused ports see exactly Z0 (50 ohms). If the termination has 20 dB return loss (|Gamma| = 0.1): the mismatch reflects energy back into the device, changing the measured S-parameters. For well-matched devices (high isolation between ports): the effect is small (< 0.1 dB). For devices with strong coupling between ports (e.g., a coupled-line filter or tightly coupled antenna array): the termination quality is critical. Use precision terminations with RL > 40 dB. For extreme accuracy: measure with different termination impedances and extrapolate to perfect termination using a mathematical correction.

Can I use a 4-port VNA to measure a 16-port device?

Yes, using the same sequential measurement principle. A 4-port VNA can measure any 4×4 submatrix in one connection. For a 16-port device: the full matrix has 256 entries (16×16). Number of 4-port submatrix connections: C(16,4) = 1820 if measuring all combinations, but many are redundant. A systematic approach: measure all port pairs (C(16,2) = 120 pairs) with the 4-port VNA measuring 4 ports at a time. With a 16-port MTS: the VNA switches through all combinations automatically. Alternatively: many VNA manufacturers offer 16-port and 32-port multiport solutions that handle the switching and matrix assembly automatically.

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