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Most desktop computers come with metal cases, which have a side-effect of blocking the radiofrequency emissions or electromagnetic radiation which is produced by the internal electronics.

Back when people still had analog televisions, I noticed that running a computer with no case or with an open case would cause interference with TV signals, visible as a noisy picture.

I have a Raspberry Pi sitting in a typical clear plastic case, which runs all the time. I'm wondering how much it is radiating in the RF spectrum. Is this a problem for any applications? How does it compare to a WiFi hotspot?

  • Why not try it and report back? I've never noticed interference on anything eg TV's/radios when running Pi's with no case. – CoderMike Nov 28 '18 at 20:51
  • Thanks. I don't have a TV or radio, but I may end up buying some kind of meter which could measure this. – Metamorphic Nov 28 '18 at 21:05
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"How much it is radiating in the RF spectrum?" is a question that's easy to ask, but not to answer - at least not specifically. And if you did have the specific answers, they will only have meaning if you were familiar with the testing standards, or knew the susceptibility of all the items in your environment.

Perhaps one way to answer your question is to point you to the Raspberry Pi's Product Compliance matrix. It provides compliance documents (which include emission levels) for all products in all countries for which certificates have been assigned. You may wish to peruse the documents applicable for your country.

Still another way to answer your question is through opinion and anecdotal evidence. For example, if we were having a conversation, and you had asked me this question, my reply would have been something along the lines of, "I believe RF Emissions from the RPi are quite low based on the fact that they have demonstrated compliance in the EU and USA." Anecdotally, I've yet to see or hear of an emissions issue in an RPI project that wasn't caused by "improper" wiring or interconnections.

Finally, there are some tests that can be done (relatively) inexpensively in limited portions of the spectrum. If you Google diy rf meter you'll find a number of devices that you can build or buy that will give you insights into emission levels (and therefore potential issues).

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    If you search around for "pifm", a piece of software which can generate FM signals directly from the GPIO pins, you'll find people expressing concerns that the signal strength exceeds that set for personal FM transmitters in some countries. I mention this because I would guess the various compliances are of the sort where the product is certified not to produce such and such interference under normal operation. A Pi is a bit different from a normal computer in that you could more easily get the wires to do things that would violate that premise (wittingly or not). – goldilocks Nov 29 '18 at 0:24
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    @goldilocks: Yes, good point! There are some Software-Defined Radio (SDR) projects that use PWM on one of the GPIO pins to create an rf transmitter - and a very capable one at that. WRT the OP's question though, I seem to recall reading that none of the RPi compliance tests included any hookups to the GPIO pins. In other words, connect stuff to a GPIO pin, and all bets on RE compliance are off. – Seamus Nov 29 '18 at 0:43
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    Just to be clear: Generating law defying radio signals from the pins doesn't require they be attached to anything. Just to be fair: It is really really unlikely anyone would end up doing that by mistake. – goldilocks Nov 29 '18 at 1:23
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    @goldilocks: Not sure what you mean, but... while the pin doesn't need to be connected to anything, it won't emit much RFI without an "antenna". Is that enough to "defy the law"? I dunno... I've not done the measurements, and I'll bow to your greater experience. – Seamus Dec 2 '18 at 18:42
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    I've never used it, but I have used actual fm transmitter boards, and while they may work better with an antennae, they certainly work well enough without one, and I have no doubt pifm does too. If you go and look at the discussion (but seriously, don't bother), the contention that it may break the law (a truly silly preoccupation in this case, IMO, but anyway) has nothing to do with using an antennae or not. Also: pretty sure broadcast power (which is what laws are concerned with) is a factor mostly independent of the antennae (its impedance is an influence). – goldilocks Dec 2 '18 at 19:25
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The answer is "not very much".

I purchased a "TriField" EMF Meter Model TF2 out of general curiosity.

Near the floor of my ground-floor room, the RF background reads 0.02 mW/m^2. I could detect RF radiation from a Raspberry Pi 3B+ about 3 inches from the plastic case, beyond that it was indistinguishable from background. For comparison, an Ethernet cable was only detectable at 2 inches. With the meter held against the Pi case I got a much higher reading, 10 mW/m^2.

My conclusion from measuring these things, and thinking about the physics, is that the field is decreasing much faster than the "inverse square law" we learn about in school. That law is for monopoles, where for example an Ethernet cable would have many small loops of opposite orientation which cancel out at larger distances, thus decaying faster than 1/r^2. Presumably a well-designed single-board computer would try to avoid large high-frequency current loops, even if only to save power, and so its assorted clock radiations would have similar decay properties to an Ethernet cable. This is consistent with what I found.

By contrast, when measuring the Pi's WiFi antenna (while doing a ping -f -I wlan0), it was difficult to detect any decay at all. At first it seemed that the values were increasing with distance; then I decided to use a wooden yardstick, rather than a metal measuring tape, which turned out to be changing the readings. Eventually, twisting and turning the meter and looking for areas of local maximum (the wavelength of 2.4GHz is 5 inches), I came up with some tentative plausible-looking numbers.

RF radiation from WiFi:
1ft     1.1 mW/m^2
2ft     0.6-0.7
3ft     0.5
4ft     0.3

I imagine these could be somewhat far off, since the numbers jump around a lot, but the gist is that the WiFi signal decays much more slowly than the small "unintentional" radiation from the Pi's unshielded processor.

I hope the meter I got is good, it is supposed to measure from 20MHz to 6GHz. For some backgorund, one of the first things I noticed when the meter first arrived was that cell towers produce much more radiation than cell phones. Standing outside or by a window I get a 5mW/m^2 signal from the cell tower a few blocks away; it increases to 15 or 20 near the tower itself. But I'm lucky to pick up 0.1 mW/m^2 a foot away from my phone when it's making a call. I read somewhere that this is due to the fact that towers have better reception with their larger antennas; phones can whisper to towers, while towers have to shout to phones. I can't find the link at the moment. Most of the information I find online states that the opposite should be true, for example the "American Cancer Society" states, without any references "The amount of exposure from living near a cell phone tower is typically many times lower than the exposure from using a cell phone."

It could be that this meter is peculiar; it is definitely consumer-grade. However, it is instantly responsive. The fact that the signal increases when I stand near a window, or near a WiFi router, is unmistakable.

The values I measured are much smaller than the 1.5 W/kg levels used in the 2016 National Toxicology Program study on rats, perhaps 1000 times smaller - not sure how to convert units - although according to another meter I borrowed, with a higher maximum reading, you could approach those levels sitting a couple feet from a WiFi router.

I'm not sure why, two decades ago when my housemate owned a TV, I noticed TV signal interference when operating a PC with no case. In America, VHF is 54-216 MHz, UHF is 470-890 MHz. The CPU of my computer, an iMac, would have run at 200-700 MHz. My current laptop produces 1 mW/m^2 at several inches, which is quite a bit more than the Pi. It seems possible that the differences have a lot to do with the RAM which must run at high frequencies and which is located quite close to the the CPU on a Pi, not so close with the iMac or with my current laptop.

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I am also a radio amateur and have noticed broadband interference from a Pi3B. In my case it appears in my Flex 6600 waterfall as closely spaced horizontal lines at the top and bottom of the 40M band. Other bands may also be affected, but the noise is less pronounced. If I disconnect the HDMI cable thing are marginally better, but if I put 3 turns of the power cord through a type 31 ferrite, things are markedly better and acceptable to my use.

It is unclear which clock in the Rpi3B is causing this effect. It does make me wonder how they measured compliance with FCC Part 15. Nonetheless, the problem seems solvable.

One other point, the interference seems to be worse when the RPi3B is in an aluminium case. Perhaps a good ground on the case might be helpful.

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I am a radio ham and I have been unable to use LF/MF/HF (Long to Short Wave) due to an incredible amount of noise. Over a few months, I have tried different antennas which make no difference at all.
Today I finally found the problem, My Raspberry Pi 3B.

The noise was being 'transmitted' through the HDMI cable, disconnecting it dropped the noise to almost zero. There is also an amount of noise traveling down the power supply cable into the mains and strangely moving the Pi and its power supply to a different 'ring-main' in my house, this has reduced the noise even further.

I don't know if a metal case around the Pi would make a difference if the noise is radiated via the HDMI cable (monitor on or off makes no difference) and even just touching the HDMI port with a piece of wire caused a massive increase in noise. Maybe it is the video chip that is causing it, but it is so severe that it prevented me from listening to any AM (LW/MW) radio at all (BBC Radio 4 on 198kHz is completely swamped by noise) and not practical to listen to anything else. So, in short, the Raspberry Pi 3B is extremely noisy and wonder if it is actually certified!

  • Wow that's a nice first post. I had to look up: LF is 30 to 300 kHz, MF is 0.3 to 3 MHz, HF is 3 to 30 MHz. So my meter which only goes down to 20 MHz indeed misses these bands. Did you try putting a choke around the cables? I did some investigations of my own near AM 520 kHz (the minimum on my radio) and found Pi 3B and 4 to cause interference, but never from more than a few feet away. Sometimes a buzzing, sometimes just white noise or silence. Worst offender in the house was a switching power supply to an audio amp, also an LCD display - not the Pi's. Would be interested to hear more. – Metamorphic Jan 14 at 18:12

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