Detecting very low power motion sensors

[glradio]

I did some searching and couldn't find any information about commercial alarm sensors designed to be hidden in a wall.

There are probably no microwave motion sensors designed to be hidden in a wall, but there is nothing to prevent putting them there , since radio waves have the ability to pass through walls.

In fact, several microwave detectors need to be calibrated to avoid generating false alarms: if the sensor power is too high, detection may occur in the room adjacent to the one in which the sensor is placed ...

if so they may just use the power line to send signals to the controller, so no RF signalling you can detect

Microwave motion sensors always emit a radio source, then the alarm can be triggered either by cable or by radio signal to the central ...

[LaserSteve]

One of the problems with passive cavities is you may need a swept frequency source and a return loss bridge on a horn antenna to find them.  Unless carefully compensated, the resonant frequency of an open ended cavity drifts considerably in use.  near Field Body presence will have some semi-serious   effects as well.  A  SA with a low sweep rate is not a great choice here for finding these, even with a tracking generator output. 

The real way used to find them is a TSCM trick using some specialized gear and a narrow modulation or harmonic mixing technique.  A TSCM specialist would not bat an eyelash over buying a 40,000$ or 100,000$ Spec An with IF and Tracking Generator Ports.   Don't ask me in a PM, the answer is NO.

You have a web link for the widget in question?   This sounds wayyyy too much like a "Great Seal" device, unless it is some tiny self mixing PCB.  Ebay/Amazon searches would reveal many short range motion detectors and their rough frequency, which will drift because the self mixing method is somewhat independent due to direct conversion IFs. Method used for the in wall device will be highly dependent on installers  budget and the customer's whims / skillset. 

Steve

[LaserSteve]

At point blank ranges you would use broadband horn antenna sets or for hobby use, a set of Log Periodic antennas  by a fellow named  WA5VJB :

https://www.wa5vjb.com/products1.html

Waveguide beyond cutoff issues and Horn  bandwidths  are outside the scope of my post. 

This can be a very tricky problem, especially if FMCW or other modulations are used.

You may need to search up to the millimeter wave bands, and that gets tricky.

Steve

[glradio]

As stated prior, look around at 10.525GHz X-Band and if not then it may be Ku band at 24GHz. X-band can penetrate cinderblocks ...

Yeah my guess would be above the frequency the tinySA can see.  Typical frequencies for proximity radar are 10.525 GHz, 24 GHz and 77 GHz.  10 GHz is possible with a slightly higher end SDR like the SignalHound.  For the higher frequencies you are going to have to work harder or pay a lot more or both.

TinySA Ultra can detect motion sensor frequencies around 10 GHz, in these examples (see link 1 , 2 , , 4), however, the sensor is in direct contact with the spectrum analyzer.

In my case there is no direct contact, but by setting the exact frequencies at which these sensors operate I expect to see some signal ...

Another possibility would be ultrasonic, like automotive parking sensors.  If you aren't completely sure it's radar based I would look for ultrasonics.

Could ultrasonic sensors work reliably through a wall ?

Is it possible to somehow detect the emissions of ultrasonic sensors placed behind a wall ?

[glradio]

You have a web link for the widget in question ?

Unfortunately not, otherwise everything would have been simple ...

[LaserSteve]

That picture has a plastic cased SA point blank into  an unmatched waveguide.  A bare  1n914 diode would detect a signal  at that point.  In that picture the SAs front end is massively overdriven, so it sees the harmonics in the mixer. .  The SA would not see the far weaker signal of a tiny transmitter which would  be close to or below its noise floor.

Sorry, You need a better instrument or a professional.


Steve

[ejeffrey]

TinySA Ultra can detect motion sensor frequencies around 10 GHz, in these examples

I thought it only went  to 6 but I now see there is a higher frequency version. My understanding is that it has much worse sensitivity above 6 GHz so I would count on it working.

Could ultrasonic sensors work reliably through a wall ?

Is it possible to somehow detect the emissions of ultrasonic sensors placed behind a wall ?

Depends on the wall and the sensor.  Going from wall to air, reflecting, back to the wall and to the sensor is probably a tall order?  It's also possible to hide things by e.g. drilling a hole in the wall, mounting sensors in the hole, skim coating it and painting over.

A microphone with an audio signal analyzer would work as long as the microphone works up to the frequency in use.  I think the things used in parking sensors are like 40 kHz?

[glradio]

... The SA would not see the far weaker signal of a tiny transmitter which would  be close to or below its noise floor ...

That's what I think too: the power of these sensors is so low that the TinySA Ultra does not have the necessary sensitivity to see the emitted signal ...

I was wondering whether an antenna made from a short "stub" of wire (like the one shown below) of the appropriate length to receive the emission frequencies of motion sensors, would change things appreciably or whether it would be better to try a higher-end device ...

[Teledog]

Perhaps they're RCWL-0516 microwave motion sensors. Low power & can pass through drywall, etc.
They run @ ~3.181-3.2 GHz.
G'luck!