High-Intensity Radiated Fields (HIRF) testing occupies a unique position in the EMC world. Field strengths involved — up to 7200 V/m outside the aircraft for some environments — are orders of magnitude beyond standard commercial EMC testing, with pulse characteristics derived from real radar and electronic warfare transmitters. The consequences of failure are potentially a flight control system malfunctioning near a high-power radar installation.
What HIRF Is and Where It Comes From
HIRF is the electromagnetic environment experienced near high-power RF transmitters — ground and shipborne radar, navigation aids, military jamming, and communication transmitters. It includes both CW and pulsed components at peak field strengths far exceeding standard EMC immunity testing.
CS-25/FAR 25 define the HIRF environment: an external environment (up to 7200 V/m peak outside the aircraft) and an internal environment (several hundred V/m after fuselage attenuation). RTCA DO-160 Section 20 defines the equipment-level test methods, with equipment grouped by criticality.
The Two Components of the HIRF Environment
CW component represents continuous high-power transmitters. DO-160 Section 20 specifies 20–200 V/m depending on category and frequency, achievable with standard WPA-series CW amplifiers.
Pulsed component represents surveillance and tracking radars. For Category H (flight critical) equipment, DO-160 Section 20 specifies up to 1700 V/m peak inside the aircraft, 10 kHz to 18 GHz, with pulse widths around 1μs and PRF of 100–1000 Hz.
Why TWT Amplifiers Are Used for Pulsed HIRF
Producing 1000–10,000W peak power at 1–4% duty cycle demands amplifiers designed for pulsed operation. The TWWPA series covers this range across 1–18 GHz, with models delivering up to 12,000W peak power at ≥70 dB gain.
TWT amplifiers are preferred for pulse fidelity: rise time, fall time, and pulse-top droop must accurately represent the radar waveform being simulated. TWTs' broadband, non-resonant interaction between electron beam and travelling wave gives naturally good pulse fidelity.
High-Power Pulsed Systems — WWPPA for the Highest Peak Powers
The WWPPA solid-state series provides extreme peak power (up to 15,000W) across 1–18 GHz using parallel GaN transistor combining, with operational advantages like no warm-up time or tube replacement, valuable for high-utilisation facilities.
The DO-160 Section 20 Equipment-Level HIRF Test
A complete equipment-level test facility includes CW amplifiers (WPA series) for continuous-wave segments, pulsed amplifiers (TWWPA series) for pulsed segments, a switching system to route signals, isotropic field probes for calibration, and test automation software.
A Note on the Narrowband TWWPA-102010000
A distinctive entry covering only 1.2–1.4 GHz with 10,000W peak power, ≤±1.0 dB gain flatness, and VSWR ≤1.5 — tighter than the wideband series. Optimised for L-band radar simulation (Mode S interrogator, transponder, surveillance radar frequencies), providing better pulse fidelity than a wideband amplifier for test programmes requiring accurate simulation of a defined radar waveform.
Complete HIRF Test System Capabilities
Finch can supply complete HIRF test infrastructure: CW amplifiers, pulsed TWT amplifiers, high-power solid-state pulsed amplifiers, RFlight horn antennas, field calibration probes, and test automation integration support.