Why Network Analyzers Must Be Calibrated Using Cal Kits

At Testwall we regularly see excellent RF and microwave designs let down by poor measurements rather than poor engineering. Vector Network Analyzers (VNAs) are extremely precise instruments, but they are only as accurate as their last calibration and the quality of the calibration kit used.

What Does a Network Analyzer Actually Measure?

A VNA measures the S-parameters of a device under test (DUT) – typically S11 for input reflection, S21 for forward transmission, and so on. Engineers rely on these measurements to judge return loss, insertion loss, phase and group delay across frequency. Small, systematic errors in magnitude or phase can completely change whether a filter, amplifier or antenna appears to meet its specification.

Why Calibration Is Essential

Any VNA, no matter how high-end, contains systematic error sources such as limited directivity, imperfect source and load match, frequency response tracking errors and cable/connector losses. These errors add to the DUT behaviour and, if left uncorrected, the VNA is effectively measuring “DUT + test setup + instrument error” instead of the DUT alone. A proper calibration using a suitable cal kit characterises these errors and mathematically removes them so that the reference plane is moved to the end of the test cables or fixture.

What a Cal Kit Really Does

A coaxial calibration kit provides a set of precisely characterised standards – typically an open, short, load and through connection – matched to a specific connector family and frequency range. When the VNA measures these standards, it solves for a set of error coefficients that describe the behaviour of the test system. Once those coefficients are applied, subsequent measurements are corrected so that the DUT is seen at a well-defined reference plane with known uncertainty. High quality cal kits are themselves traceable standards, so they transfer metrological traceability from national laboratories down into day-to-day production and service measurements.

What Happens If You Don’t Calibrate?

Skipping calibration, or using a worn or inappropriate cal kit, has very real consequences in RF work. Instead of a clean response from the DUT, the display will include errors from the test leads and instrument itself. This can lead to:

Misleading return loss: Reflections from cables and adaptors can look like genuine mismatch at the DUT, giving the impression of poor antenna or filter performance.
Passband ripple that is not really there: Uncorrected mismatch and tracking errors can introduce artificial ripple in S21, which may be mistaken for a design problem.
Reduced dynamic range: Systematic errors consume part of the available dynamic range, making low-level signals and subtle distortions harder to see.
Costly rework and returns: Decisions based on bad data can drive unnecessary design changes, failed compliance tests and avoidable field returns.

Good Practice When Using Cal Kits

To get the best from your network analyzer and protect your investment in RF design and test, it is worth adopting disciplined calibration practices. We typically recommend:

Calibrate whenever you change the test setup significantly – new cables, different fixtures, different frequency range or after a large temperature change.
Use the correct cal kit for the connector type, gender and frequency range you are working with; avoid improvised stacks of cheap adaptors in the calibration path.
Treat cal kits as precision standards: keep connectors clean, use correct torque wrenches and have the kit periodically verified or recalibrated.

How Testwall Can Help

At Testwall we supply refurbished VNAs from leading manufacturers together with appropriate calibration kits, test cables and accessories. All instruments are fully tested in our in-house calibration laboratory, and we can arrange traceable calibration to support your quality system. If you are unsure which cal kit or calibration strategy is appropriate for your frequency range and connector system, our engineers are happy to advise on a cost-effective setup for your application.

Visit our Contact Us page to discuss calibration options for your network analyzers, or to source a suitable VNA and cal kit combination.