Radio Detector

Is it possible to create a device that would perform some arbitrary task if a radio signal being broadcasted on a certain frequency and within a certain proximity to the device is detected? If so, what would be the most reasonable way to create such a device? I'm acknowledging, of course, that the proximity measurement would be an approximation at best, but I'm wondering how accurate this approximation could get.

You're probably imagining using a threshold level for field strength. This works, but the distance is at best a guess and it's not reliable. It might be good enough for what you want though. To do this right you'll need multiple antennas. You could measure the different in field strength for a real distance estimate. More technical but perhaps easier alternatives are angle of arrival and time of flight analysis.

Any other details? Will TX amplitude be constant? Because if so, rough proximity calculation is pretty easy. Do you control the transmitter being detected? Or is it in someone else's control? Is it an off-the-shelf device, or something you built? This is certainly possible, whatever the case. If you wanted to get fancy (and way more expensive, difficult, etc) you could get really precise range and even direction using multiple phased receivers. Something like this: https://www.rtl-sdr.com/hydrasdr-preview-a-4x-coherent-rtl-sdr-for-direction-finding-passive-radar-and-more/ Buy a single sdr dongle for like fifteen bucks and start experimenting first though. Hams have a soykafload of knowledge and will probably help you, especially since what you're doing is RX only (unless you're also building the transmitter). Hams tracking down radio transmitter locations for fun is called foxhunting I think. You can search around for that to see some of the ways they make it work, often with dead simple hardware and sheer cleverness.

Also wondering what kind of "arbitrary task" you have in mind. Idgaf if it's shady or w/e, just curious. Making something activate when it detects a nearby phone? If you just need to detect any transistorized electronics in general (at very close proximity) I'd also look up what a nonlinear junction detector is. Might be relevant, maybe not. Still interesting. You can buy them online or at most "spy shops."

You can use arduino and http://www.airspayce.com/mikem/arduino/RadioHead/. For example http://www.airspayce.com/mikem/arduino/RF22/rf22_specan_8pde-example.html. This example is old and don't work with current library version, but i fix it https://pastebin.com/i80pxxSZ. >rf22.rssiRead(); Or... https://www.rtl-sdr.com/final-day-to-pre-order-a-discounted-kerberossdr/

>>2528 >>2529 Well, my idea was really to have an alarm that would activate if a nearby cop uses his/her radio, so it would essentially be a police alarm as opposed to a police scanner. Hopefully that tells you everything you need to know. I really don't know much about this sort of thing, so if you need to know anything else, just ask. I'm also not opposed to personal research, so please direct me to any resources you think could be helpful.

>>2531 Unfortunately not too many police departments use analogue radio anymore so you can't listen in as easily, but a lot use trunked communications. So at least you can see if there's activity on those bands. You can look up what police in your area use, and at what frequency range. It usually varies from town to town. Hell, you could probably just call them up and ask them. Bored old suburban men (e.g. every ham radio operator ever) follow police communications on the regs and they don't really mind. This sounds like something you could do with sdr without too much effort. You could at least watch that segment of the waterfall to see when they're transmitting, and how strong the signal is. Once you figure out what band they use where you live, buy or build an omnidirectional antenna optimized for that band. There no reason to use wideband if you know what your target is. A narrow-band setup will be far more sensitive and pick up less irrelevant noise. Of course they probably won't be transmitting constantly so you'd need to wait for them to talk before getting any info. Look up passive radar if you want to get real specific info about a radio source, like location, direction, doppler to determine speed and vector, etc. Expect passive radar to be fairly advanced compared to just monitoring signal strength through an sdr.

https://www.rtl-sdr.com/running-windows-x86-sdr-decoding-apps-on-the-raspberry-pi-3-unitrunker-winstd-c-wxtoimg-dsdplus-and-more/ https://www.rtl-sdr.com/tutorial-on-setting-up-op25-for-p25-phase-2-digital-voice-decoding/

there is a thing called a radio direction finder, kinda expensive, but ham radio operators use them to find pirate broadcasters

A triangulation device?

Wouldn't a radio spectrum analyzer do the job. You can get a usb one for relatively cheap.

>>2531 Most law enforcement is on P25 radios, a digital protocol more or less monopolized by Motorola. There's a DefCon (25?) talk about how easy it is to pick up (but not so easy to use). One of the things the speaker explained was that whether a radio was transmitting encrypted or clear, the radio always transmits its ID in the clear. If you can somehow identify the block of IDs used by the LEs you're interested in, you could possibly write a program that uses SDR to track activity. You can also ping radios and they always respond. Figuring out where they are is on you. Keep in mind that the radio protocol is also used by EMS, fire departments, and pretty much any other government agency. Also, it's probably really illegal to do anything other than RX, but then again, that probably wouldn't be the only illegal thing you'd be doing, would it?

>>2520 if you have any two points, you can compute the distance quite accurately based off of the difference in signal strength, making something precise enough might pose some problems. There are a number fo DIY cell phone and radio detector builds that can cost as little as $20-50 a unit, and multiple units can detecta location with greater accuracy. Search "Cell phone detector"

>2520 >I'm acknowledging, of course, that the proximity measurement would be an approximation at best, but I'm wondering how accurate this approximation could get. It is possible to have extremely precise distance measurement if your relay is allowed to reply to the fob. For instance, you can have your fob regularly emit a request for your relay to reply to the ping, and if the fob receives the pong within a very precise time window (nanoseconds) it can infer that it is within range and thus initiate a contact closure. Side note: you need to use better terminology and stop thinking of "a device" rather than "a system".

Related: https://montalk.net/conspiracy/55/how-to-block-microwave-mind-programming-signals

>>3875 You can also have multiple antennae and use the difference in signal strength between them to get an accurate triangulated position. The technology has been used since around WW1

https://en.wikipedia.org/wiki/The_Thing_(listening_device)

>>2868 Full schematic: https://cdn.instructables.com/FCB/2A4B/HMMFCENC/FCB2A4BHMMFCENC.LARGE.jpg

I've had an electric tester/em field screwdriver thingy for a while, and have noticed that when it lies near a cellphone, and there's an incoming call, the light would blink like it would in a presence of a live wire nearby seconds before ringtone chimes.

>>4470 Audio boxes will make sounds if the phone is near them. I think the wire acts like the antenna. On that matter you have to adjust antenna length to frequency to maximize gain.