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I have a couple of MacBooks. Using a Trifield EMF meter with radio/microwave capability, if I hold the meter near the laptop (where I understand the wifi antenna is located), I receive blips ranging from 4-10 milligauss. In Radio/Microwave mode it does blips from 0.1 to about 1.5 mW/cm^2).

The router itself, near its antenna, does movement between about 0.5 to 2.5 (0.5mW/cm^2 to 2.5/cm^2). No wireless devices are connected, but the wireless antenna is turned "on" (cannot be turned off).

I work in a high security field and am wondering if the wifi is actually transmitting even though it is set to "off" in the operating system. E.g., due to a root kit or other virus.

Can other components in a MacBook cause quick fluctuations of this much? I would expect it to remain mostly constant if Bluetooth/wifi were not transmitting.

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  • $\begingroup$ How is this got anything to do with ham radio? $\endgroup$ – K7PEH Jan 16 '16 at 17:16
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    $\begingroup$ Well, interference emitted by digital devices is on-topic, as is RF in general. However, it seems like the answer to this question will be based entirely on understanding the behavior of specific WiFi hardware and operating systems, so it does seem like a poor fit. @K7PEH Since you believe it is off-topic, you could vote to close it — you have the ability. $\endgroup$ – Kevin Reid AG6YO Jan 16 '16 at 18:16
  • $\begingroup$ Kevin, Well, I have never voted to close anything on SE, never bothered to look up how to do that. As far as on or off topic, there are a lot of off-topic posts and I don't usually make a comment about being off-topic. This question might be best for something covering Mac computers or Mac software. I think RF in general (for a topic of a question) is fitting if it has something to do with amateur radio, but not in general. For example, solving Maxwell for retarded potentials can be said to deal with RF but more suited to Physics.SE rather than this forum. $\endgroup$ – K7PEH Jan 16 '16 at 19:42
  • $\begingroup$ I figured you guys would know more about the RF and hardware side of what a wifi signal looks like than the guys at the Apple Exchange. They deal mostly in software issues. $\endgroup$ – rcd Jan 17 '16 at 5:13
  • $\begingroup$ This would be more appropriate for the Electronic Engineering stackexchange. $\endgroup$ – Rob Jan 27 '16 at 14:10
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We would expect no emissions from a disabled WiFi or BT adapter, but as you suggest, all bets are off if your system is compromised.

Your broadband detector is only a crude way to check for emissions, because you have no idea of the frequency involved. The computer has a lot of digital circuitry and clocks that emit RF energy at low levels -- not just WiFi and BT. Some devices also have NFC (near-field communications) that you could pick up. You need a spectrum analyzer or other more accurate receiver to truly assess what's going on.

You could also check the output as the computer boots up -- before it would activate the WiFi or BT.

Very high security installations require "TEMPEST" qualified computers, that are specially shielded and RF-suppressed. With standard computers, it's possible to pick up a screen image or other info using a (fairly nearby) sensitive receiver.

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I find it very likely that you are picking up the computer itself and not the expected radiators you are listing. If instead of a Spectrum Analyzer which shows a field strength across a range of frequencies, you use a Frequency counter, that could tell what you are seeing with the other meter which is only showing a general field strength. My guess is you are seeing the clock, the frequency will be the core bus frequency. I suggest the clock because its the strongest regular signal emitted.

However, any time you run electricity through unshielded circuit traces your going to get some level of RFI. You could even be picking stuff up off the power adapter since it is also a switching supply, and in a plastic box it isn't really shielded either. The old PC's with 50mhz clocks used to harmonize 'nicely' with bottom end of 6 meteres

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