Category H
Understanding Category H
Stringent RF Limits for External Avionics
RTCA DO-160 Section 21 controls the radio frequency energy emitted by electrical and electronic equipment installed on aircraft. The goal is to prevent unintended emissions from coupling into the aircraft's communication, navigation, and surveillance receivers. Among the classifications defined in Section 21, Category H is reserved for the most critical locations. It applies to equipment installed in areas that are in direct line of sight to aircraft antennas, which typically includes the exterior of the fuselage, wingtips, tail sections, and landing gear bays.
Because there is no metallic aircraft skin to block the emissions, any RF energy radiated by Category H equipment or its cabling can couple directly into antennas. To protect the integrity of navigation aids (such as GPS, ILS, and Glideslope) and voice communication (VHF), Category H imposes the lowest allowable emission limits in the entire standard, requiring exceptional design discipline.
Design and Shielding Protocols
To pass Category H testing, designers must implement a multi-layered electromagnetic compatibility (EMC) strategy. The enclosure must act as a high-shielding Faraday cage, using conductive gaskets and finger stock to seal all mechanical seams. All aperture sizes must be minimized or covered with conductive mesh. More importantly, because cables act as efficient antennas, every signal and power line entering or leaving the enclosure must pass through an EMI filter or feedthrough capacitor. Connectors must use 360-degree shielding backshells to ensure the cable shields are grounded directly to the chassis without pig-tails, which would introduce parasitic inductance.
Key Mathematical Relations
Technical Specifications Comparison
| Frequency Range | Category H Radiated Limit | Category M Radiated Limit | Primary Protected Services |
|---|---|---|---|
| 2 - 30 MHz | 20 dBμV/m (highly strict) | 29 dBμV/m | HF Communications |
| 30 - 100 MHz | 20 - 28 dBμV/m | 29 - 37 dBμV/m | FM Broadcasting, Military Radio |
| 108 - 137 MHz | 15 dBμV/m (notch limit) | 24 dBμV/m | VHF Voice Comm, VOR Navigation |
| 329 - 335 MHz | 15 dBμV/m (notch limit) | 24 dBμV/m | ILS Glide Slope Receiver |
| 960 - 1215 MHz | 25 dBμV/m | 34 dBμV/m | DME, TACAN, Mode-S Transponder |
| 1.5 - 1.6 GHz | 20 dBμV/m (notch limit) | 29 dBμV/m | GPS L1, GLONASS, Galileo |
Frequently Asked Questions
What equipment typically requires Category H compliance?
Equipment installed on the aircraft exterior, such as weather radars, wingtip collision-avoidance lights, external cameras, pitot tube heaters, and ice detectors, must comply with Category H due to their proximity to antennas.
Why are there sharp 'notches' in the Category H limit curves?
The limit curves contain deep notches (lowered limits) in specific bands corresponding to aviation communication and navigation frequencies, ensuring that even minor harmonics from digital clocks do not desensitize critical receivers.
How does cable routing affect Category H success?
If Category H cables are routed parallel to antenna coax cables, crosstalk can transfer noise directly into the receiver. Category H requires shielded cables with full grounding at both ends and physical separation from sensitive lines.