ashfordeOU DO-160 Radio Frequency Susceptibility (avionics/do160/radio-frequency-susceptibility)
Use when the task is RF immunity testing of airborne equipment per DO-160 section 20: computing radiated field strength from the amplifier and antenna setup, sizing the amplifier power budget with cable loss and calibration margin, converting between the dB units of the test levels, and judging conducted immunity margins against the CS114 category current limits.
Domain quick reference
- DO-160 section 20 (RTCA, EUROCAE twin ED-14G) covers radio frequency susceptibility: radiated immunity (RS101 magnetic field, RS103 radiated field) and conducted immunity (CS101 audio frequency on power leads, CS114 bulk cable injection, CS115/CS116 transients). Section 21 covers the companion emission tests (RE102 radiated, CE102 conducted).
- Far-field relation: E = sqrt(30 * P * G) / d, with E in V/m, P the radiated power in W, G the linear antenna gain, and d the distance in m. Inverting gives P = E^2 * d^2 / (30 * G), the power needed to produce a required field level.
- dB conventions: field level in dBuV/m is 20 * log10(E / 1e-6), with 120 dBuV/m equal to 1 V/m; current in dBuA is 20 * log10(I / 1e-6), with 120 dBuA equal to 1 A; power in dBm is 10 * log10(P / 1e-3), with 30 dBm equal to 1 W.
- Power flux density: the plane-wave flux S = E^2 / 377 W/m2, with 377 ohm the free-space impedance; E = sqrt(S * 377).
- Amplitude modulation: the RS103 modulated field peaks at E * (1 + m) and the average power is P * (1 + m^2 / 2), with m the modulation depth (typically 0.8).
- Calibration margin: raising the calibration level by M dB multiplies the field by 10^(M / 20) and the amplifier power by 10^(M / 10).
- CS114 categories: adjacent categories step the current limit by 10 dBuA (A through H, then J; the letter I is skipped). Published category base levels are summary reference data, verify against the current DO-160 revision before test planning.
- RS103 radiated levels span roughly 1 to 200 V/m depending on equipment category, over bands extending up to 18 GHz; the exact category table must be verified against the current revision.
- Far field: the far-field relation holds beyond the Fraunhofer distance R = 2 * D^2 / lambda, with D the antenna aperture and lambda = c / f the wavelength; at 1 GHz the wavelength is 0.2998 m.
Workflow
- Confirm the DO-160 revision, the equipment category, and the applicable immunity tests (radiated RS103, conducted CS101/CS114, or both) from the qualification matrix.
- Convert the required test level to SI: field in V/m via vm_from_dbu_vm, current in A via amp_from_dbu_a, power in W via watt_from_dbm.
- Size the radiated setup: compute the amplifier power with power_for_field_strength from the field, antenna gain, and distance, then add cable loss with amplifier_power_with_cable_loss and the calibration margin with apply_margin_db, or use required_amp_power_for_test for the full budget in one call.
- Confirm the setup is in the far field: check the test distance against far_field_boundary at the highest test frequency, using wavelength_from_frequency.
- Estimate the modulated stress: am_peak_field for the instantaneous peak and am_average_power for the average amplifier load.
- Judge conducted immunity: compare the measured injection current with the CS114 category limit via cs114_limit_dbu_a and margin_check_dbu, and record the margin and the pass-fail verdict.
Pitfalls
- Mixing dB conventions: 20 log10 for field and current, 10 log10 for power; applying the power convention to a field level is wrong by a factor of 2 in dB.
- Using average power when the AM peak matters: the modulated field peaks at E * (1 + m), above the unmodulated calibration level, and the receiver desensitization follows the peak.
- Forgetting the 377 ohm impedance when converting between field and flux density.
- Applying the margin in the wrong domain: a 6 dB field margin is 10^(6/20), about 2x the field, not 10^(6/10).
- Using the far-field relation in the near field: at distances below the Fraunhofer boundary R = 2 * D^2 / lambda the field is not plane-wave and the power estimate is invalid.
- Treating section 21 emission limits as immunity limits: emissions bound what the equipment radiates, susceptibility bounds what it withstands, and the two have different units and pass criteria.
- Confusing CS114 categories: the set steps A through H then J,
skipping I, so a category J limit is 80 dBuA above category A, not
- Trusting published category tables without revision checks: DO-160 category levels change between revisions; verify before freezing a test plan.
Behavior contract (gate 3)
The immunity math is exercised by the gate 3 contract test: scripts/test_radio_frequency_susceptibility.py against scripts/radio_frequency_susceptibility_logic.py (stdlib unittest, offline). Run: python3 scripts/test_radio_frequency_susceptibility.py
Compliance
- Standards referenced, not reproduced: DO-160 text is proprietary (RTCA); summary-only per standards-map.yaml and brief 06. The far-field relation, dB conventions, and AM power relations above are common RF engineering, and the CS114 category levels are marked as summary reference data to be verified against the current revision.
- compliance: STANDARDS-REF, gated: false.