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SAE J1113-25: Tri-Plate Line Method for Automotive Radiated Immunity Testing
SAE J1113-25, officially cancelled in July 2005, specified a radiated immunity test method using a tri-plate line (TPL) for automotive electronic components. Developed and used exclusively by Ford Motor Company, the standard covered the frequency range of 10 kHz to 1 GHz and was later replaced by other SAE/ISO standards. This article reviews the technique, equipment requirements, and engineering insights for engineers working with legacy systems or understanding prior EMC practices.
Overview of the Tri-Plate Line Method
The tri-plate line is a variation of a TEM cell without side walls. This open construction allows larger devices to be placed inside with their cables attached directly, avoiding the need for special feedthrough ports. The TPL does not suffer from side-wall reflections, enabling operation at frequencies up to 1 GHz—higher than a comparable TEM cell—provided the setup and characterization procedures are strictly followed. However, uniform electric and magnetic fields are generally limited to below 500 MHz. Because the TPL lacks side walls, it must be operated inside an RF shielded, absorber-lined chamber to prevent radiated emissions from interfering with nearby equipment and to suppress reflections that could distort the test field.
Test Setup and Equipment Requirements
Per the standard, the test setup includes a signal source, power measurement devices, a field strength meter, the TPL itself, a 50 Ω load, and the absorber-lined chamber. The DUT and its wire harness are placed on a low-dielectric support fixture centered between the septum and ground plane. The table below summarizes key equipment specifications.
| Component | Requirement |
|---|---|
| Signal Source | Residual FM <10 Hz, frequency resolution <100 Hz, harmonics and spurious <−20 dBc |
| Field Strength Meter | 3-axis isotropic probe, 50–200 V/m range, up to 1000 MHz |
| TPL 50 Ω Load | Capable of handling maximum power and frequency used |
| Non-conductive Support Fixture | Relative dielectric constant <1.4 up to 1000 MHz |
| DUT Maximum Height | ≤1/3 septum height (typically 10 cm) |
| Wire Harness Total Length | 2 m, with at least 1 m along the TPL longitudinal axis |
The wire harness exits the TPL at 90° to the axis and connects to a shielded test fixture located no closer than 100 mm from the TPL edge. The test fixture is bonded to the TPL ground plane, and any support equipment outside the chamber uses fiber-optic links or filtered wire connections to avoid coupling.
⚠️ Hazardous voltages and fields exist on and near the TPL when energized. Ensure no personnel are inside the chamber during testing.
Engineering Design Insights
🛠️ The TPL method offers practical advantages for testing physically larger components and complex cable harnesses, but achieving repeatable results requires careful attention to field uniformity and chamber layout. Field characterization is critical above 500 MHz; without proper setup, measurement uncertainty increases significantly. The absorber material must reduce reflected E-field by at least 10 dB near the TPL, and minimum distances to absorber (1 m from sides, 0.5 m from ends) must be maintained to avoid field distortion. The support fixture must be made from low-loss, low-permittivity material (εr <1.4). Bonding the TPL ground plane to the shielded enclosure using short straps or coaxial cable braids is essential to prevent ground loops and maintain field integrity.
Note: This standard was fully cancelled and replaced by other SAE/ISO standards. Current testing should follow SAE J1113-21 or ISO 11452-2.
Frequently Asked Questions
Why was SAE J1113-25 cancelled?
The standard was used exclusively by Ford Motor Company. In October 2003, Ford revised its EMC requirements and deleted the tri-plate line as an acceptable method. With no other users, the SAE committee withdrew the standard in 2005.
What are the main differences between a TPL and a TEM cell?
The TPL has no side walls, making it easier to place large devices and route cables without feedthroughs. It can also operate at higher frequencies than a TEM cell of the same size because side-wall reflections are eliminated. However, the TPL requires an absorber-lined shielded enclosure to control radiated emissions and reflections.
What are the frequency limitations of the TPL method?
Typical uniform fields are maintained only below 500 MHz. With careful characterization and strict adherence to the setup procedure, the TPL can be used up to 1 GHz, though results may show increased variability.
How should the DUT harness be arranged in the TPL?
The harness must be 2 m long, with at least 1 m running parallel to the TPL axis (the exposed section). The remainder exits at 90° to the axis and connects to the test fixture. The entire harness is supported at a constant height midway between the septum and ground plane to ensure field exposure uniformity.
