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This is a very cheap and simple bug detector that is supposed to make sound when it detect RF signals between 100Mhz-2600Mhz. I found out that it doesn't do what it supposed to do,since its sensitivity is very poor.It detects signals of some mobile phones(while some mobile phones it doesn't detect),wi-fi transmitters,garage doors remote control signals etc.at a distance of maximum 5 cm in the best case.Thats all. Regard to my description,What may be the method of that cheap bug detector:detection of magnetic fields,electric fields or maybe both?

EDIT:enter image description here This is the pcb circuit of the detector.

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    \$\begingroup\$ lol. That circuit couldn't detect a bus if it was hit by it. This is definitely utterly useless, as there's no selectivity at all but the low-pass gain-bandwidth product of the amplifier. As extensively explained in my answer, this is snakeoil bullshit. \$\endgroup\$ – Marcus Müller Dec 16 '16 at 18:30

It detects radio waves – it has an antenna. And: It's completely useless by design.

One thing to realize is that if someone clever was to construct a bug in the year 2016, it's not going to be an FM transmitter generating 100mW of output power constantly; that would both waste power and make it easy to find.

It's much more likely to be a "silent" recorder (you can record years of cell-phone-quality audio on a microSD card...) that can be triggered remotely to transmit its data only when the sniffing party considers it safe to transmit. There's a famous example of a small sculpture that contained a membrane that could vibrate with air pressure – turning it into a audio-modulated RF retroflector when illuminated with radio waves from across the street. Of course, that illumination did not happen while the friendly intelligence service was scanning the room for active transmitters.

So, having one of these is essentially snake oil. This isn't the movie, where you can rush into a room, scan it with a magical electronic device, and declare it "clean".

Another thing to consider is that even when the bug was actively transmitting, it would almost certainly use spread-spectrum technologies. Spread Spectrum is a World War II invention (aka "old news"); it's used to distribute power in a wider part of the spectrum, either to make it possible to make communication more resilient to interference, or to hide the transmission, even below the level of noise that is inevitable in every receiver at room temperature.

Yet another thing to consider is that you need much less power to transmit little information over short distance (like a bug probably does) compared to transmitting much information over large distance (like a TV tower does). In fact, if you are within 10 km of a 660 MHz TV station (secondary, or so-called "low-power") with 15 kW power, you'd get about -28 dBm ("dB Milliwatt", usual unit when dealing with radio communication devices) at where you are. If your bug chose a frequency right between two TV channels, and only needed to transmit 30 m instead of the 10 km, and the bug receiver was as sensitive as 25-year-old cellphones (-120 dBm sensitivity), the signal transmitted by the active bug would need to be 20 dB, or 100 times, weaker than the power of the TV station that you receive at the same place.
In other words, you can't "see" that bug between the TV stations without knowing that there should be nothing between the TV channels. And even if you knew, TV is just one of hundreds of things that communicate. I have absolutely no idea how you'd go and find an even mediocre clever unknown bug without military/intelligence agency-class automated signal classifiers...

No matter how you put it, if you need to be paranoid about bugs, you need to live in a copper-shielded hole in the ground, or make mechanically sure the room your sitting is free of bugs and holes through which acoustics could be extracted.

In no case whatsoever is this device capable of doing anything useful.

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  • \$\begingroup\$ In fact,i,actually,thought to use it to detect VHF/UHF signals that radiated from some transmitter and estimate their strength according to the changing of the detector sound vs distance.You says that it detects radio-waves.if we compare it to a standard EMF detector that has several measuring options:measure only magnetic fields(gauss),only electric field(v/m)and RF frequencies(w/m2).Does my detector works as the third option-detects the power of the magnetic+electric fields?even though my detector response only withing 5cm from transmitter(which means not in the far field)? \$\endgroup\$ – xchcui Dec 15 '16 at 19:32
  • \$\begingroup\$ May you refer to my last comment?since my main question is there. \$\endgroup\$ – xchcui Dec 16 '16 at 10:30
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    \$\begingroup\$ I don't understand your question. Radio waves are radio waves – you even use the word "Radio Frequency (RF)" in your third option. \$\endgroup\$ – Marcus Müller Dec 16 '16 at 10:33
  • \$\begingroup\$ RF term is just used to described EM waves between 3Khz-300Ghz.I used this term as it shown in EMF detectors(w/m2),while the option of that range frequencies are used.Since EMF detector have the option to detect only electric field(v/m)and at other option only magnetic fields(gauss),my question was if my detector responses only to the magnetic fields or only the electric fields.Since i want to use my bug detector,not to find bugs,but to estimate strength of E/M fields from appliance.So,i need to know what,exactly,this bug detector is responsed to:magnetc fields or electric fields? \$\endgroup\$ – xchcui Dec 16 '16 at 10:59
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    \$\begingroup\$ I cannot explain the fact that what you're observing is the electromagnetic wave with an antenna; that's basic wave physics. You of course are measuring the electrical field strength along the antenna, but I want to repeatedly stress that it's an effect of the wave, not of a static field. This is an antenna for propagating waves, not a near-field sensor. You're obviously lacking the physical understanding of what you're trying to make me answer. \$\endgroup\$ – Marcus Müller Dec 16 '16 at 18:29

It detects electric fields, and specifically RF (Radio Frequency) fields. You can't expect very good performance (sensitivity or selectivity) from such a cheap, over-simplified gadget. You get what you pay for. But OTOH, low sensitivity allows you to physically identify the location of the bug based on field strength.

There are different kinds of bugs. One technique we have seen used by security agencies is a wide-band receiver with a speaker. If an AUDIO BUG is detected, the sound transmitted will be picked up by the detector and amplified out the speaker. Then the sound will "feedback" through the bug's microphone.

There are much more sophisticated hand-held gadgets for detecting RF emissions. For example look at the "RF Explorer" from SeedStudio. It uses modern technology to pack much better performance into a small hand-size unit for a remarkably good price. https://www.seeedstudio.com/category/Rf-explorer-c-79.html

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  • \$\begingroup\$ Interesting feedback method. I assume that's only going to work for backpage-of-Popular-Science-esque FM Bugs. Yeah? \$\endgroup\$ – Samuel Dec 14 '16 at 17:50
  • \$\begingroup\$ @Samuel depends on how experienced you are at signal classification. I can typically spot GSM, UMTS and WiFi in a periodogramm that's wide enough, but that's a matter of experience. FSK-style modulations are typically harder to tell from PSK on a low resolution. FM transmitters are easy to spot for the "round" and non-constant spectral position; \$\endgroup\$ – Marcus Müller Dec 14 '16 at 20:01
  • \$\begingroup\$ @MarcusMüller But can you then demodulate, decode, and playback over a speaker in real time? I don't see how else you'll get feedback. \$\endgroup\$ – Samuel Dec 14 '16 at 20:03
  • \$\begingroup\$ ah, ok sorry, you were explicitly referring to the feedback-catching-method (new word creation, yay!) \$\endgroup\$ – Marcus Müller Dec 14 '16 at 20:05
  • \$\begingroup\$ well, anything that's based on knowing what the signal behaves like is based on knowledge of how the transmitter works; but it's all a bit of a wild assumption that someone who'd actively wanted to hide in the spectrum wouldn't do something extremely crazy – like doing a q-tone FSK at hopping center frequencies with both the frequencies and q and the symbol timing being a result of secret random generators that only the intended audience knows. \$\endgroup\$ – Marcus Müller Dec 14 '16 at 20:06

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