Sixteen points on an invisible square: what field uniformity really means

A radiated immunity test asks a simple question: does the product keep working when it is hit by a radio signal of a given strength, for example 10 V/m? To answer it fairly, every part of the product's front face has to receive at least that strength. The antenna cannot promise that on its own. Reflections from the floor and walls, and the shape of the antenna's beam, make the field stronger in some places and weaker in others. Field uniformity calibration is how a laboratory proves, in advance, that the unevenness stays within agreed bounds.

The idea in plain words

Think of a photographer lighting a stage before a portrait. The subject will stand in one spot, and the photographer wants the light on the subject's face to be even. So before anyone steps on stage, the photographer walks a light meter across a grid of marks where the face will be and writes down each reading. If one side is much darker than the other, the lamps are moved until it is not.

A radiated immunity chamber does the same with radio waves. The "lamp" is the transmitting antenna, the "face" is the front of the product under test, and the light meter is a small field probe. The grid of marks is the uniform field area. It is not a physical object; it is a flat square of air, standing upright, exactly where the product's front face will be placed later.

Nothing is in the chamber while this is done, and that is deliberate. The calibration describes the room and the antenna, not the product. Once it passes, any product whose front face, including its cables, fits inside the square can be tested there.

The rule: twelve of sixteen, within a factor of two

Take a chamber calibrated at 18 V/m. At each frequency, the laboratory looks at the sixteen readings and asks whether at least twelve of them sit between 18 V/m and 36 V/m. The weakest of those twelve is brought to exactly 18 V/m; the strongest may be up to twice that, but no more. The standard writes the same rule as 0 dB to +6 dB, because 6 dB is simply a factor of two in field strength.

The band runs from the required level upwards, not evenly either side of it, and that choice matters. The standard accepts some over-testing, but it never accepts under-testing: every accepted point receives at least the level on the report, and none receives more than twice it.

Why 18 V/m?

The test level on a report, 10 V/m for example, is the strength of the plain, unmodulated signal. During the test that signal does not stay plain: it is amplitude-modulated by 80 % with a 1 kHz tone, which makes it rise and fall a thousand times a second. At the top of each rise the field is 1.8 times as strong as the plain signal, so a 10 V/m test briefly reaches the strength of an 18 V/m unmodulated field.

The chamber and the test equipment have to deliver those peaks cleanly, not just the average. The standard therefore asks for the field calibration to be done at no less than 1.8 times the test level. That is why a laboratory testing at 10 V/m calibrates its uniform field area at 18 V/m, and why the simulator below uses 18 V/m.

Four points of sixteen are allowed to fall outside, because real rooms have awkward spots: a corner near the floor where reflections gather, or an edge that the antenna's beam only just reaches. Which four may miss can change from one frequency to the next. If the product is small and the square is shrunk to its minimum of 0.5 m by 0.5 m, there are only four points and all four must pass.

The standard also records the field at 0.4 m above the floor, for products and cables that have to sit low. That reading is written into the calibration record for information; it does not decide whether the chamber passes.

Twice over: standing up and lying down

A radio wave has a direction of wiggle, called its polarisation. With the antenna mounted one way, the field swings side to side; turned through a quarter turn, it swings up and down. A product can be more sensitive to one than the other, so both are tested, and both have to be calibrated. The floor and walls treat the two differently, so the maps rarely look alike, and the simulator below shows only one of them. Across 80 MHz to 6 GHz in steps of 1 %, the full job comes to 435 frequencies, sixteen points and two polarisations: nearly 14,000 readings for one square.

See it change across frequency

The simulator below shows one polarisation, horizontal, with the chamber calibrated at 18 V/m. The upright view is the square as the antenna sees it: the colour runs from blue for a weak field through yellow to red for a strong one, and each of the sixteen points shows its reading in V/m. A solid line marks where the field reaches 36 V/m and dark stripes cover the area above it; a dashed line and light stripes mark where it drops below 18 V/m. A point left out carries an arrow showing which way it failed. The laid-flat view lays the same square on the floor and draws the field strength as height, so the field becomes a landscape of hills and hollows. The grey plane at 36 V/m is the limit: any point that pushes through it is left out, and so is any point that sinks below 18 V/m. Drag the slider to move through the band, drag the landscape to turn it, or click the chart underneath to jump to a frequency.

Why the picture changes with frequency

Field uniformity is not a property of any one part. It is what three things produce together: the chamber, whose size and shape decide where reflections come from; the absorbers, which decide how much of those reflections survive; and the antenna, which decides how the energy is spread across the square. Each of the three takes the lead in a different part of the band.

• At the low end, the chamber takes the lead. At 80 MHz a wavelength is almost 4 m, longer than the square is wide. Energy that bounces off the floor and walls arrives at the square alongside the direct wave, and the two add up in some places and cancel in others. The absorber on the floor between the antenna and the square is there to tame exactly this, and it works less well the lower the frequency.
• In the middle, the absorbers change jobs. Most chambers line their walls with ferrite tiles, which do the heavy lifting at low frequency, and foam cones, which take over higher up. Around the hand-over, the walls reflect a little more, and the map gets busier.
• At the top, the antenna takes over. A horn antenna concentrates its energy more tightly as the frequency rises, like a torch focused to a spot. The centre of the square stays bright while the corners fade. This is why the edge and corner points are usually the ones left out above a few gigahertz, and why the standard also offers, above 1 GHz, a method that checks the area in separate 0.5 m windows instead of one large square.

For a small number of frequencies below 1 GHz, the standard lets the band stretch from twice the required level to about three times it (from +6 dB to at most +10 dB), still never below the required level, for no more than 3 % of the frequencies, provided the wider figure is stated in the test report. At 18 V/m, that is up to about 57 V/m instead of 36 V/m. In a dispute, the factor-of-two rule takes precedence.

Uniformity also decides how much amplifier power you need. The test level is set at the weakest of the accepted points, so the drive is raised until that point reaches the required level. Where the field is even, the weakest point sits close to the rest and the amplifier has an easy job. Where the field is lumpy, or the corners fade, the drive has to rise until the weakest point is lifted to the level, and everything else in the square rises with it. The same test in a less even room asks more of the amplifier. That is why there is no single amplifier power that suits every scenario: it depends on the chamber, the absorbers and the antenna together, as well as on the test level, and it is sized for the combination, not read from a table.

A good solution is never one outstanding part. It is the know-how of how the four interact: the chamber, the absorbers, the antenna and the amplifier. Change one, and the other three have to be looked at again.

One example of a matched set

There is no single right combination. The chamber size, the antenna, the amplifiers and their power all follow from what you test and at what level: a laboratory testing consumer products at 3 V/m needs a very different set from one testing vehicle electronics or medical equipment at much higher levels. As one example, this is a set we would put forward for a product laboratory testing to IEC 61000-4-3 up to 6 GHz at moderate levels. Whatever the combination, the principle is the same: the chamber, the absorbers, the antenna and the amplifiers are chosen together, the software that runs the test is set up for that combination, and the 16-point calibration is done with all of them in place.

The chamber and absorbersFrankonia SAC-3 Plus L 3 m semi-anechoic chamber with a dome roof and a 2.0 m quiet zone, designed as one with its absorber lining and specified for IEC 61000-4-3 at 3 m.

The antennaAR ATR26M6G-1 26 MHz to 6 GHz in one antenna, so nothing is swapped at 1 GHz and the set-up recorded in the calibration stays the same across the band. Polarisation changes without removing it from the positioner.

Test softwareNexio BAT-EMC Runs the 16-point field uniformity verification and the radiated immunity test to IEC 61000-4-3 from one interface, with drivers for Teseq and AR equipment and for field probes, and produces the report automatically.

Signal and controlTeseq NSG 6000A Signal generator from 9 kHz to 6 GHz, switch network for up to five amplifiers and EUT monitoring in one unit.

Amplifier, 80 MHz to 1 GHzTeseq CBA 80M1G E series Class A, one band from 80 MHz to 1 GHz, four models from 125 W to 1000 W.

Amplifier, 0.8 GHz to 6 GHzTeseq CBA 6G D series Class A, 30 W to 400 W, with a built-in calibrated directional coupler for forward power.

ICX Lab is the authorised distributor of AMETEK CTS products, of which Teseq and AR are brands, of Frankonia chambers and of Nexio BAT-EMC software. We size each set to the application and test level, then supply and install it as one, and verify it with the 16-point method. Quotations on request.

What the calibration gives you besides a pass

The sixteen readings are not only a verdict. At every frequency the calibration also records how hard the amplifier had to drive the antenna to produce the field. During the test, the laboratory uses that record to set the same field without a probe in the way. This is why the calibration only holds while the room stays exactly as it was: the same antenna in the same position, the same cables, the same absorber on the floor. Move any of them and the record no longer describes the room.

The standard expects the full calibration to be repeated every year, and whenever something in the room changes: absorber replaced, antenna moved, amplifier or cables exchanged. Before each batch of testing, the laboratory checks that the calibration still holds.

What to check in your own records