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Is it possible to build a system of antennas to find the rough direction of a radio frequency emitter such as a WiFi router, a Bluetouth tag, or, a 5G cell tower?

I cannot use distant antennas to use triangulation. If several antennas are needed, they should be close (within 50cm) to each other.

From wikipedia, it seems that such a system is possible: https://en.wikipedia.org/wiki/Direction_finding

It also seems possible using Bluetouth 5.1 angle of arrival, but I cannot find a vendor.

JRE
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oli
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    A warm welcome to the site. Your question asks 'is it possible' but then cites evidence that it is. So your question appears to actually be 'can you teach me about how to build one'. The site is a Q&A site, not a discussion forum and not a free design house or online personal tutorial. People will help you take the next step if your question shows you've already done as much as you possibly could - which yours doesn't, I'm afraid. Please edit your question and greatly improve it. The better the quality of your question, the better the quality of the answers it will attract. Again, welcome. – TonyM Jan 28 '22 at 16:24

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Is it possible to build a system of antennas to find the rough direction of a radio frequency emitter such as a Wifi router, a Bluetouth Tag, or, a 5G cell tower?

By the fact that direction finding works: yes.

But note that Wifi, bluetooth, 5G exist in very strongly non-line-of-sight scenarios means that the directions you'll find will have little to nothing to do with the place where the emitter stands.

I cannot use distant antennas to use triangulation. If several antennas are needed, they should be close (within 50cm) from each other.

You do not get to choose the distance freely. There's practical lower limits, below which you get into ambiguities. Luckily, 50cm is larger than half the wavelength of all these technologies, so a (pretty much optimal) \$\lambda/2\$ spaced array is feasible.

But again, if you're in a scenario where the propagation is not only a single direct line (like between a satellite and a satellite dish array), then you will get multiple directions, and/or directions that don't necessarily point in direction of the emitter, but just in direction of one path.

It also seems possible using Bluetouth 5.1 angle of arrival,

It is exactly what AoA does.

Wifi is especially tricky, because it's very common that access points and mobile stations these days use MIMO techniques to dedicatedly combine physically independent paths in a way that allows the other end to form multiple, independent data streams (to get more data in the same bandwidth across, in the end). But that means that you can get a coherent wavefront at the receiving end that "looks" as if it came from one direction, where there's neither the actual transmitter nor a single propagation path.

In the end, what I'm saying is, yes, it's possible to build a measurement system for incident wavefronts' angles, but these angles will not mean what you'd assume they mean.

Marcus Müller
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  • Hello Marcus. Thank you for your answer. If this is possible to measure the incident wave front's angle, would you have any hint about how to build it or buy a device that does it? – oli Jan 29 '22 at 06:25
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    yes, you need to construct a receiver with multiple coherent receive channels, connected to at least three antennas that are not on one single line. From there it's classical angle of arrival estimation – big topic, hundreds of tutorials. Too broad for for a single question, and certainly too broad for a comment in StackExchange. – Marcus Müller Jan 29 '22 at 18:42