RadiMation Application Note 166

How to perform an IEC 61000-4-20, Radiated emission test in a GTEM

Introduction

This application note describes how a radiated emission test can be performed in a GTEM (Gigahertz Transverse Electromagnetic) cell using the RadiMation^® software. It explains the practical configuration steps in RadiMation^®, and also the basic theory behind the GTEM concept and the OATS (Open Area Test Site) correlation.

Radiated emission testing is normally performed on an OATS or in a semi-anechoic chamber. However, a GTEM cell provides a compact, shielded alternative that allows emissions from an Equipment Under Test (EUT) to be measured quickly and reproducibly. The GTEM cell is designed to approximate free-space electromagnetic field conditions while maintaining good shielding and predictable field behaviour.

Because the GTEM is a closed environment and does not reproduce the exact geometry of an OATS, the results obtained inside the GTEM must be **correlated** to what would be measured on an OATS. RadiMation^® includes a dedicated function that applies the OATS correlation algorithm, allowing the GTEM results to be expressed in equivalent OATS terms and compared against the same radiated emission limits.

Test Concept

During a radiated emission GTEM test, the EUT is placed inside the GTEM cell and operated in three orthogonal orientations (commonly referred to as the X, Y, and Z axes). For each orientation, the emission spectrum is measured using a spectrum analyser or receiver connected to the GTEM’s output port.

These three measurements are then combined to determine the maximum field strength that would be radiated in free space. RadiMation^® uses this information to simulate what the emission from the EUT would look like if it were measured on an OATS at a defined antenna distance and height.

The correlation is based on a physical model that considers:

• The geometry of the EUT inside the GTEM.
• The equivalent distance to the receiving antenna as defined in the OATS standard (typically 3 m or 10 m).
• The antenna height variation and table height used in a standard OATS test.
• The direct and reflected wave components that occur on an OATS due to ground reflection.

Although no physical antenna tower or ground plane exists in the GTEM setup, these parameters are still applied in the correlation calculation to ensure that the resulting graph corresponds directly to the OATS test limits.

Purpose of the OATS Correlation

The goal of the OATS correlation is to make GTEM measurements directly comparable to the limits defined for OATS measurements in EMC standards such as CISPR 16 or EN 55032.

On a real OATS, the receiving antenna moves vertically (typically between 1 m and 4 m height) to capture the maximum signal caused by the combination of the direct and ground-reflected waves. This results in a characteristic **standing wave pattern** — the measured field strength varies with antenna height and forms an envelope of peaks.

The RadiMation^® OATS correlation algorithm uses the calculations as specified in the IEC 61000-4-20 to reproduce this effect by mathematically modelling the variation of field strength as a function of antenna height and phase difference between direct and reflected components. As a result, the correlated curve produced by RadiMation^® shows a similar “rising and falling” envelope as would be obtained from an actual OATS measurement.

This allows users to:

• Evaluate emissions of the EUT measured in GTEM to be using the same limit lines as for OATS testing.
• Understand whether the EUT complies with the radiated emission limits without performing a full OATS test.
• Compare the results of GTEM tests with final compliance tests on an OATS.

Test Setup

The GTEM cell is connected to the measurement receiver via a coaxial cable. The EUT is positioned at the centre of the GTEM test volume, where the electromagnetic field is most uniform. Each of the three orientations (X, Y, and Z) represents a rotation of the EUT so that its main emission axes are aligned with the electric field inside the GTEM.

A simplified setup includes:

• The GTEM cell connected to the measurement receiver or spectrum analyser.
• The EUT powered and operating in its normal mode.
• Control lines and support equipment passing through the GTEM feedthroughs.

The test setup is controlled entirely from within RadiMation^®. The user can define the test parameters, receiver settings, and OATS correlation configuration.

Configuration in RadiMation

To perform the GTEM radiated emission measurement in RadiMation^®, follow these steps:

1. Open RadiMation and select **Radiated Emission Test** from the main test list.
2. In the **Test Type** dialog, select **GTEM Measurement**.
3. Define the measurement range and frequency steps according to the standard or internal test plan.
4. In the **EUT Orientation** section, select the three orientations: **X**, **Y**, and **Z**.
5. Configure the receiver settings: resolution bandwidth, detector type (Quasi-Peak, Peak, or Average), and dwell time.
6. In the **OATS Correlation** section, enable the option *“Calculate correlated OATS result”*.

 Here you can define:
 * OATS antenna distance (e.g., 3 m or 10 m)
 * Antenna height range and step (e.g., 1 m to 4 m, 0.25 m step)
 * Table height (e.g., 0.8 m)

1. Store the configuration and start the measurement.

RadiMation^® will automatically perform the measurements for each EUT orientation and calculate the OATS-correlated result.

Performing the Test

Once the configuration is complete:

• Start the measurement sequence in RadiMation^®.
• The software will communicate with the receiver, perform the frequency sweep, and store the data for each EUT orientation.
• After all orientations are completed, RadiMation combines the results to determine the maximum field strength per frequency.
• The OATS correlation algorithm is applied, producing a result equivalent to an OATS measurement.

Results and Interpretation

The resulting graph shows:

• The measured field strength (after OATS correlation) as a function of frequency.
• The applicable OATS limit line (for 3 m or 10 m, depending on configuration).
• The EUT orientations (X, Y, Z) and the combined correlated result.

The curve may exhibit an **increasing pattern** across the frequency range, reflecting the influence of the simulated antenna height movement and the ground reflection effect that is typical for OATS results.

Conclusion

Using the RadiMation GTEM measurement function allows users to perform radiated emission tests in a compact, shielded environment while maintaining full comparability to standard OATS limits.

By understanding the basic principles of GTEM measurements and OATS correlation, users can confidently interpret their results and make reliable compliance assessments before proceeding to formal certification testing.

Institute of Electrical and Electronics Engineers

Gigahertz Transverse Electromagnetic

Open Area Test Site

Equipment Under Test

ElectroMagnetic Compatibility

Comité International Spécial des Perturbations Radioélectriques3