Radiated Emissions Margin Calculator
Radiated Emissions Margin Calculator
How far under a radiated-emissions limit your measured or predicted electric field sits, at every frequency you care about, with the worst one called out and the extra attenuation a stated design margin would need. All nine MIL-STD-461G RE102 limit curves are built in, read from the standard’s own figures; any other limit, including the copyrighted ones, you enter as breakpoints.
Margin against an RE102 radiated-emissions limit
MIL-STD-461G RE102, Figure RE102-3, the “fixed wing internal, 25 m or more nose to tail” curve, against five field levels in dBµV/m: 38.2 at 10 MHz, 41.4 at 30 MHz, 46.1 at 100 MHz, 50.3 at 400 MHz and 54.8 at 1 GHz, with a 6 dB design margin
A limit is a straight line between breakpoints on a log axis
margin(f) = limit(f) − E(f) positive means below the line
E(dBµV/m) = Vreceiver(dBµV) + AF(dB/m) + cable loss(dB) − preamp gain(dB)
E(V/m) = 10dBµV/m / 20 × 10−6 S = E² / η0 dBµA/m = dBµV/m − 51.52
- x i
- the base-10 logarithm of breakpoint i’s frequency. Interpolating on a log axis is not a choice — it is how the limit is drawn, and interpolating linearly in frequency instead is tens of decibels wrong in the middle of a decade
- AF
- antenna factor in dB per metre, the ratio of the field at the antenna to the voltage at its connector. It is a property of that antenna at that frequency, from its calibration certificate, and it changes across the band
- η0
- the impedance of free space, 376.73 Ω. E = η0 H holds only in the far field; the power density and magnetic field printed below are only meaningful where that is true
- margin
- positive when you are below the line. This page never calls a positive margin a pass, because a limit is met by the standard’s own method on the standard’s own setup
Worked example
MIL-STD-461G RE102, Figure RE102-3, the "fixed wing internal, 25 m or more nose to tail" curve, against five field levels in dBµV/m: 38.2 at 10 MHz, 41.4 at 30 MHz, 46.1 at 100 MHz, 50.3 at 400 MHz and 54.8 at 1 GHz, with a 6 dB design margin
That curve is 44 dBµV/m flat from 2 MHz to 100 MHz, then rising to 89 dBµV/m at 18 GHz. The rising section's slope is 45 dB over 2.2553 decades, which is 19.953 dB per decade — 20 dB per decade to within 0.05
10 MHz, 30 MHz and 100 MHz are all on the flat, so the limit at each is 44 dBµV/m. The margins there are 5.8, 2.6 and -2.1 dB
400 MHz is 0.6021 decades above the 100 MHz knee, so the limit there is 56.01 dBµV/m and 50.3 is 5.71 dB below it
1 GHz is a full decade above the knee, so the limit is 44 + 19.953 = 63.95 dBµV/m and 54.8 is 9.15 dB below
The worst of the five is -2.1 dB at 100 MHz — above the limit, not below it
To reach a 6 dB design margin there the emission has to come down by 8.10 dB. At 100 MHz that is shielding and harness work, not component values
As a check on the units, 46.1 dB above a microvolt per metre is 201.8 µV/m, which in a far field is 108.1 pW/m² of power density and -5.42 dB above a microamp per metre
The nine MIL-STD-461G RE102 limit curves this page carries
| Figure and curve | Applies to | The curve, in dBµV/m |
|---|---|---|
| RE102-1, below deck | Surface ships, 10 kHz to 18 GHz | 90 at 10 kHz, −8.5 dB/decade to 56 at 100 MHz, then +20.4 dB/decade to 102 at 18 GHz |
| RE102-1, above deck and exposed below deck | Surface ships, 10 kHz to 18 GHz | 70 at 10 kHz, to 36 at 100 MHz, then to 82 at 18 GHz — the same shape 20 dB lower |
| RE102-2, internal to the pressure hull | Submarines, 10 kHz to 18 GHz | 88 at 10 kHz, −20 dB/decade to 50 at 800 kHz, flat to 100 MHz, then to 95 at 18 GHz |
| RE102-2, external to the pressure hull | Submarines, 10 kHz to 18 GHz | 60 at 10 kHz, to 24 at 2 MHz, flat to 100 MHz, then to 69 at 18 GHz |
| RE102-3, fixed wing external and helicopters | Aircraft and space systems — see the note above | 60 at 10 kHz, to 24 at 2 MHz, flat to 100 MHz, then to 69 at 18 GHz. Identical to the submarine external curve |
| RE102-3, fixed wing internal, under 25 m nose to tail | Aircraft and space systems — see the note above | 34 flat from 2 MHz to 100 MHz, then to 79 at 18 GHz. Nothing is drawn below 2 MHz |
| RE102-3, fixed wing internal, 25 m or more nose to tail | Aircraft and space systems — see the note above | 44 flat from 2 MHz to 100 MHz, then to 89 at 18 GHz. Nothing is drawn below 2 MHz |
| RE102-4, Navy fixed and Air Force | Ground, 2 MHz to 18 GHz | 44 flat from 2 MHz to 100 MHz, then to 89 at 18 GHz |
| RE102-4, Navy mobile and Army | Ground, 2 MHz to 18 GHz | 24 flat from 2 MHz to 100 MHz, then to 69 at 18 GHz |
Where RE102 applies, from paragraph 5.18.1
| Platform | Frequency range | Note |
|---|---|---|
| Ground | 2 MHz to 18 GHz | Figure RE102-4 |
| Surface ships | 10 kHz to 18 GHz | Figure RE102-1 |
| Submarines | 10 kHz to 18 GHz | Figure RE102-2 |
| Aircraft, Army and Navy | 10 kHz to 18 GHz | Figure RE102-3 |
| Aircraft, Air Force | 2 MHz to 18 GHz | Figure RE102-3 |
| Space | 10 kHz to 18 GHz | Figure RE102-3, whose curves are labelled in aircraft terms. Which one applies is a programme decision |
Which standard, and what may be reproduced
| Limit | Status | What this page does |
|---|---|---|
| MIL-STD-461G RE102 | US Department of Defense interface standard; a work of the US Government, distributed without charge | Reproduced. All nine limit curves are built in, read from the standard’s own figures and checked against the markers printed on them |
| RTCA DO-160 Section 21 | Copyrighted; sold by RTCA | Method and category letters named, values entered by you from your copy |
| CISPR 25, CISPR 32 | IEC documents, copyrighted | Method and class named, values entered by you from your copy |
| A programme-specific limit | Your customer’s document | The same treatment: type the breakpoints |
What moves a radiated-emissions margin, and by roughly how much
| Change | Direction | Typical size |
|---|---|---|
| A cable shield terminated with a pigtail instead of a backshell | worse | 20 to 40 dB above a few MHz; this is usually the whole problem |
| A 100 mm seam opened to 150 mm | worse | 3.5 dB, and it scales with the LENGTH |
| A display cut-out without a mesh or a conductive gasket | worse | unbounded above the aperture’s half-wave resonance |
| Both polarisations measured instead of one | worse | whatever the difference is; often 6 to 10 dB at a resonance |
| The real harness fitted instead of a short bench lead | worse | 10 to 25 dB below 200 MHz — cables radiate, boxes mostly do not |
| A clock edge slowed from 1 ns to 3 ns | better | up to 10 dB above the second corner of the spectral envelope |
| Chassis bonded with a wire instead of a face | worse | the bond is an inductor; everything referenced to it moves with it |
Margin, not a verdict
A radiated-emissions limit is a line on a chart with decibels above a microvolt per metre up the side and a logarithmic frequency axis along the bottom. Between its breakpoints it is straight, so the whole curve is described by a handful of corner points. This page carries nine of them outright — every RE102 curve in MIL-STD-461G — and takes yours for anything else.
Where the built-in numbers came from. Not from memory, and not from a summary. Each of the four figure pages was rendered at 300 dpi and the drawn curves traced column by column. The level axis was calibrated on the drawn gridlines rather than on the tick labels, and the labels were then checked against those gridlines as a separate anchor; the frequency axis was fitted to the logarithmic minor ticks, fifty-five to sixty-four of them against two parameters. Seventy readings are asserted against the constructions the page implements. This matters because a limit line that is a few decibels wrong is worse than no limit line at all — you would have no reason to doubt it.
Figure RE102-3 and spacecraft. The figure is captioned “RE102 limit for aircraft and space system applications” and carries three curves, every one of them labelled in aircraft terms: fixed wing external, fixed wing internal under 25 metres nose to tail, fixed wing internal 25 metres or over. Paragraph 5.18.1 makes RE102 applicable to space from 10 kHz to 18 GHz and says nothing about which curve. There is no rule to be read off the page, so the answer is that the programme’s EMC control plan or the contract says which one, and this page will not guess. If you are designing satellite hardware and you do not yet know which curve you are working to, that question comes before every number here.
Both polarisations, above 30 MHz. Paragraph 5.18.2 requires the limit to be met for both horizontally and vertically polarised fields, and the test procedure requires the antenna to be oriented for both. The number you enter above 30 MHz should therefore be the WORSE of the two, at the worst antenna position. A vertical-only sweep on a bench is a useful diagnostic and is not the measurement.
From what the receiver reads to what the limit is written in. An EMI receiver reads dBµV at the end of a cable. The limit is in dBµV/m. The difference is the antenna factor, plus everything the signal lost on the way and minus everything a preamplifier added. That conversion is on this page, but only as a mode, and with a warning attached: antenna factor is a function of frequency and of the individual antenna, so one number is honest inside one antenna’s band and nowhere else.
What a positive margin is, and what it is not. It is the distance between a number you measured and a number on a figure. It is not a compliance statement. RE102 is met by measuring the equipment under test the way the standard says — its ground plane, its bonding, its 2 metres of exposed leads, its antenna at 1 metre from the test setup boundary at 120 cm above the floor, its receiver bandwidths and dwell times from Table II, both polarisations above 30 MHz, at every antenna position the setup’s size requires — and a bench sweep with a near-field probe is not that. What a margin is good for is engineering: knowing how much to find, and where.
Where the emission actually comes from. RE102 covers the enclosure and all interconnecting cables, and on most hardware the cables win by a wide margin. A box with good seams and a harness whose shields are pigtailed to the backshell will fail on the harness. That is why the cable radiated emissions page and the transfer impedance page exist, and why the aperture page matters more than the wall thickness one. For the conducted half of the same problem, the conducted emissions margin page is this page’s sibling and works the same way.
Frequently asked questions
Where do the RE102 curves on this page come from?
From MIL-STD-461G’s own Figures RE102-1 through RE102-4, read by rendering each page at 300 dpi, calibrating the level axis on the drawn gridlines and the frequency axis on the logarithmic minor ticks, and tracing the drawn curves. Seventy readings are checked against the implemented breakpoints and agree to better than 0.46 dB. MIL-STD-461G is a work of the US Government, distributed without charge, so its curves may lawfully be reproduced.
Which RE102 curve does a satellite use?
The standard does not say. Paragraph 5.18.1f makes RE102 applicable to space from 10 kHz to 18 GHz, and paragraph 5.18.2 points at Figure RE102-3, which is captioned for aircraft and space systems — but all three of its curves are labelled in aircraft terms and none of them mentions spacecraft. Your programme’s EMC control plan or your contract specifies which one applies, and this page will not choose for you. Guessing here is the most expensive mistake available on this subject.
Why is there no limit line below 2 MHz for internal equipment?
Because Figure RE102-3 does not draw one. The figure has a single sloping section below 2 MHz — the 60 to 24 dBµV/m line, which joins the lowest curve’s flat continuously — and the 34 and 44 dBµV/m flats simply begin at 2 MHz. Army and Navy aircraft and space are applicable from 10 kHz, so there is a genuine gap in the figure. Below 2 MHz this page holds those two curves flat at their first breakpoint, which is an assumption, and it says so rather than presenting it as the standard.
How do I turn a receiver reading into dBµV/m?
Add the antenna factor in dB per metre, add every decibel the signal lost between the antenna connector and the receiver, and subtract any preamplifier gain. This page will do it, but only within one antenna’s band: antenna factor changes with frequency, by 10 dB or more across a biconical’s range, so a single number is not honest across a band change. Use the calibration certificate’s value at the frequency you are looking at, not a typical curve.
Do I enter the horizontal or the vertical reading?
Above 30 MHz, the worse of the two, at the worst antenna position. Paragraph 5.18.2 requires the limit to be met for both polarisations and the test procedure requires both to be measured, so the level that decides the margin is the higher one. Below 30 MHz the rod antenna measures the vertical component and the question does not arise.
Is a positive margin a pass?
No. It is the distance between a level you entered and a line on a figure. A limit is met by a measurement made to the standard’s own method — its setup, its ground plane, its bonding, its antenna positions, its bandwidths and dwell times, its operating modes — and it is a test house that says so, not a calculator. The word this page uses is margin, and it means margin.
Can I use this for DO-160 Section 21 or CISPR 25?
Yes, with the last option in the limit selector: type the breakpoints from your own copy. The arithmetic is identical, because every radiated-emissions limit is drawn as straight segments on a log-frequency axis. What is not identical is the measurement distance, the antennas, the detectors and the chamber, so a margin against one standard says nothing about another.
Why is the sloping section 15.6 dB per decade and not 20?
Because that is what the figure draws. The lowest curve on Figure RE102-3 runs from 60 dBµV/m at 10 kHz to 24 dBµV/m at 2 MHz, which is 36 dB over 2.301 decades — 15.65 dB per decade. The traced ink agrees with that construction to better than 0.16 dB across the whole section. The rising sections above 100 MHz are close to 20 dB per decade but are also fixed by their printed endpoints: 19.95 dB per decade on Figures RE102-3 and RE102-4, and 20.40 on Figure RE102-1.
Related calculators
References
- MIL-STD-461G, Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, 11 December 2015, superseding MIL-STD-461F. A US Department of Defense interface standard and a work of the US Government, distributed without charge. The limit curves on this page are read from Figures RE102-1, RE102-2, RE102-3 and RE102-4, with the applicability from paragraph 5.18.1, the polarisation requirement and the pointer to the figures from paragraph 5.18.2, and the antennas, the 1 metre antenna distance and the 120 cm antenna height from paragraph 5.18.3. Figures RE102-5 to RE102-8 are test setups and carry no limits.
- RTCA DO-160G, Environmental Conditions and Test Procedures for Airborne Equipment, Section 21 Emission of Radio Frequency Energy. Copyrighted and sold by RTCA; its category letters and limit curves are not reproduced here. Cited for the method and the category structure only.
- CISPR 25, Vehicles, boats and internal combustion engines — Radio disturbance characteristics — Limits and methods of measurement for the protection of on-board receivers. An IEC document, copyrighted; its five severity classes are named here, its limit values are not reproduced.
- Ott HW. Electromagnetic Compatibility Engineering. Wiley, 2009. Chapter 12 Digital Circuit Radiation for common-mode cable radiation and Chapter 18 Precompliance EMC Measurements for what a bench sweep can and cannot tell you. Chapter and section titles checked against the author’s own published contents listing.