Our amplifier selection guide covers the power budget calculation that determines what amplifier a given field strength target actually requires. That calculation assumes the field arriving at the EUT is genuinely uniform — and confirming that assumption, before any EUT testing begins, is itself a defined, non-trivial procedure that IEC 61000-4-3 specifies in detail. Skipping or rushing this calibration step is one of the more common sources of an invalid radiated immunity test, regardless of how correctly the amplifier itself was sized.
Why Field Uniformity Can't Be Assumed
A semi-anechoic chamber, however well designed, doesn't automatically produce a perfectly even electromagnetic field throughout its volume — reflections, chamber resonances, and the antenna's own radiation pattern all introduce variation in field strength from point to point, and that variation can be substantial enough at certain frequencies to create standing waves with significant field strength differences over a volume smaller than the EUT itself. If the field genuinely delivered to part of an EUT during testing is meaningfully weaker than the nominal calibrated level, the test understates the actual immunity requirement — the EUT could pass a test that doesn't represent genuine compliance with the field strength the standard intends.
IEC 61000-4-3 addresses this directly by requiring formal field uniformity verification before any EUT testing — not as a one-time facility certification, but as part of the documented test methodology for each specific chamber, antenna, and frequency configuration.
The 16-Point Grid
The standard defines a Uniform Field Area (UFA) — typically a 1.5m × 1.5m vertical plane, positioned 0.8m above the floor, divided into a 16-point grid (4×4) — as the reference area over which field uniformity must be demonstrated. At each of the 16 points, in sequence, a calibrated field sensor measures the actual field strength achieved for a given forward power applied to the transmit antenna, at each test frequency, for both horizontal and vertical antenna polarisation.
This calibration is performed with the EUT and its support table physically removed from the chamber — the standard explicitly doesn't account for how the table or EUT itself might affect field uniformity, only specifying that the table, if used, be built from non-conductive material so its presence during actual EUT testing doesn't meaningfully distort the calibrated field.
The 0 to +6dB Criterion
Field uniformity is judged by a defined statistical tolerance: at least 12 of the 16 grid points must show field strength within 0dB to +6dB of the target calibration value. Up to 4 of the 16 points are permitted to fall outside this range and be excluded from the calculation.
The point with the lowest recorded field strength among the 12 compliant points becomes the calibration reference for that frequency — a deliberately conservative choice.
Two Calibration Methods
IEC 61000-4-3 permits two methods, both intended to produce equivalent results:
Constant field strength calibration. At each of the 16 points, forward power is adjusted until the target field strength is achieved, and that required forward power is recorded for each point.
Constant power calibration. Forward power is established at the starting position to achieve the target field strength, then that same forward power is applied unchanged at all 16 positions, with the resulting field strength recorded at each point.
The Substitution Method — Why Calibration Data Drives the Actual Test
Once calibration establishes the forward power needed to achieve the target field strength at each frequency, the actual EUT test applies the same forward power recorded during calibration, at each corresponding frequency, with the EUT now present — this is why the method is called "substitution."
This is precisely why calibration validity matters as much as it does — an EUT test's claimed field strength is only as trustworthy as the calibration data it's built on.
When Recalibration Is Required
Recalibration is required whenever the test setup changes, and even with an unchanged setup, periodic recalibration (commonly annual) is standard laboratory practice.
A Note on Frequencies Above 1 GHz
The 16-point grid calibration applies specifically to 80 MHz to 1 GHz. Above 1 GHz, the standard uses a different windowing method with 4 calibration points rather than 16.
Practical Takeaway
Amplifier sizing answers whether a system can reach the required field strength. Field uniformity calibration answers whether the field actually delivered to an EUT during testing is the field the test report claims it is.