Understanding SAE J1113-23: Radiated Immunity Testing with the Strip Line Method

This article provides an overview of SAE J1113-23, a recommended practice for determining the electric field immunity of automotive electronic devices using a strip line test setup. Although cancelled in 2002 and superseded by the tri-plate line method (SAE J1113-25), understanding this standard is valuable for legacy testing and historical context. The method covers the frequency range from 10 kHz to 200 MHz and is designed to expose wiring harnesses and components to controlled radiated electromagnetic fields.

Overview of the Strip Line Method

The strip line method creates a known electromagnetic field between a driven conductor (the strip) and a ground plane. The device under test (DUT) and its harness are placed within this field to evaluate immunity. The test setup includes a signal source, power amplifier, directional coupler, RF voltmeter, and field strength meter. A key engineering insight is that the strip line simulates radiated electromagnetic conditions by generating a field that couples into the DUT primarily through the wiring harness, which acts as an antenna.

Test Configurations and Setup

Three test configurations are feasible: exposure of the wiring harness alone, exposure of the DUT alone, or exposure of both. The following table summarizes their characteristics:

Configuration	Description	Typical Use
Harness Alone	The wiring harness is placed in the strip line field, using the harness as an antenna to couple RF energy into the DUT.	Most common; allows testing across a wider frequency range, especially above the frequency where wavelength is less than twice the strip line length.
DUT Alone	The DUT is placed between the driven conductor and ground plane with the harness exiting at 90 degrees to minimize coupling.	When direct immunity of the DUT is needed; frequency is limited to below the wavelength condition to avoid field non-uniformity.
Both	Simultaneous exposure of the DUT and harness.	Requires special agreement between test parties.

The DUT must be positioned 200 mm (+20 -0 mm) from the driven conductor, and its height cannot exceed one-third of the strip line height to avoid field distortion. Non-conductive fixtures with low dielectric constant (εR ≤ 1.4), such as polystyrene, are required to minimize field perturbation. Peripheral devices with metal housings must be bonded to the ground plane if they are electrically connected to vehicle mass.

Frequently Asked Questions

What is the maximum frequency for a uniform field?

For most strip line designs, the field is uniform (±2 dB) along its length up to approximately 200 MHz. Above this frequency, the uniformity degrades due to standing wave patterns. The frequency limit where uniformity is maintained is when the wavelength is at least twice the strip line length.

Why must the DUT height be limited to one-third of the strip line height?

If the DUT exceeds this limit, it perturbs the test field, causing a stronger field than indicated by the measured forward power. This ensures field uniformity and accurate, repeatable results.

How is field strength calibrated?

Field strength is determined from directional coupler and power meter readings using calibration methods outlined in the standard. For frequencies at or below 100 kHz, an RF voltmeter can be used to measure the voltage directly between the elements.

What materials should be used for fixtures?

Non-conductive fixtures must have a very low dielectric constant (εR ≤ 1.4) to avoid disturbing the intended field distribution. Polystyrene or equivalent materials are recommended.