How to Interpret S11 and S21 for Cable Assembly Evaluation

What S11 and S21 Show in a Cable Assembly Test Report

S11 is the reflection result seen from Port 1. In an RF cable assembly report, it helps check the matching condition at the input side. The result can reflect problems around the connector interface, termination area, adaptor transition, cable entry point, or the first section of the coaxial path.

S21 is the forward transmission result from Port 1 to Port 2. In cable assembly evaluation, it is commonly used to read insertion loss. For a finished cable assembly, S21 includes the cable itself, connector loss, termination process, adaptor influence, cable length, bend condition, and the test reference plane.

The two curves do not show the same issue. S11 looks at reflection and matching. S21 looks at signal transmission through the assembly.

Start with the Working Frequency Range

For GNSS antenna links, LTE modules, WiFi devices, 5G communication equipment, industrial antennas, TV antenna systems, and other RF links, the curve needs to be checked around the operating band. A wide sweep screenshot may look complete, yet the target frequency area may be too small to read clearly.

Frequency also changes how S21 is interpreted. A short RG316 cable assembly may be acceptable in one band. A longer small diameter coaxial cable may show higher loss in another band. The curve has to be read together with frequency, cable type, and length.

For a more complete setup view, you can also review our guide on the VNA test frequency range for RF cable assemblies.

How to Read S11 Without Confusing It with Return Loss

S11 is often displayed in dB. When it is shown as Log Mag, a lower value means less reflected signal at Port 1. A curve farther below 0 dB usually indicates better matching than a curve rising closer to 0 dB. The acceptance limit still comes from the project specification or the test requirement.

Return loss is related to reflection, yet the number direction is different. Higher return loss means less reflected power. This is a common point of confusion when reports from different instruments use different display names. One report may show S11 Log Mag, while another report may show return loss.

Understanding S21 in a Finished Cable Assembly

S21 shows how much signal is transmitted from one end of the assembly to the other. In RF cable assembly testing, it is commonly used as the insertion loss result. A curve closer to 0 dB means less transmission loss.

For a finished cable assembly, S21 is not the same as a cable loss chart. A cable loss chart estimates the coaxial cable itself. A finished assembly also includes connector interface, crimping or soldering quality, adaptor transition, cable length, connector orientation, and test fixture influence.

If you need to estimate the cable body loss before checking the finished assembly report, see our coaxial cable loss chart guide.

A smooth S21 curve gives more confidence than one acceptable data point. If the curve has a deep notch, visible ripple, or sudden jump, the assembly needs closer review. Possible causes include connector contact problems, loose adaptor connection, cable bending during test, shield termination inconsistency, or a test setup that does not match the approved sample condition.

S11 and S21 Need to Be Read Together

A cable assembly may show acceptable S11 at the input side, while S21 is still too high because the cable is long, the coaxial cable has higher attenuation, or the connector combination adds extra loss. In this case, the port matching may look acceptable, yet the transmitted signal level may still be too low for the system.

The opposite situation can also appear. S21 may look acceptable across the working band, while S11 shows a reflection peak near a sensitive frequency. The assembly may pass a simple loss review, yet the port connected to the module, antenna, receiver, or transmitter can still create matching concerns.

When you look at a report, S11 and S21 should clearly relate to the same sample. It should be easy to see which sample was tested, which connector was connected to Port 1, what impedance was used, and whether the frequency range actually matches the project requirement.

Match the Curve with Cable Length, Cable Type, and Connector Structure

S11 and S21 curves cannot be separated from the assembly structure. The same S21 value may be normal for a long small diameter coaxial cable and too high for a short low loss coaxial cable. The same reflection condition may be acceptable for an early prototype and unsuitable for a final production sample used in a narrow band link.

Cable type matters. RG174, RG316, LMR type low loss coaxial cable, flexible miniature coax, and 75 ohm TV coaxial cable do not show the same curve behavior. Connector structure also matters. Straight connectors, right angle connectors, bulkhead connectors, panel mount interfaces, FAKRA housings, F Type connectors, N Type connectors, and adaptor transitions can all affect the finished result.

If a short cable shows unexpectedly high S21 loss, check the connector termination, adaptor, test cable, and sample build. If a mixed connector assembly shows unstable S11, check the port direction, mating interface, center contact position, dielectric support, and reference impedance.

FAQ

Conclusion

S11 and S21 help evaluate different parts of an RF cable assembly. S11 shows reflection and matching at one port. S21 shows insertion loss through the finished assembly. A useful review connects both curves with the working frequency range, cable length, connector structure, impedance, port direction, and sample approval requirement. Clear and consistent interpretation of these parameters helps ensure the cable assembly meets system performance expectations.