There are these microwave sensors in car alarms that have dual zone. This means that they can detect movement both outside (a small distance) and inside of the car.

How are they implemented?

I suspect two regular microwave sensors that have different powers: one with high power for outside detection and one with low power for inside detection only.

Thank you!

Edit: link to a dual zone microwave sensor: http://www.crutchfield.com/S-D98ka8tAZol/p_220MV3/Code-Alarm-MV3.html. It's a single unit that is mounted under the plastic of the console between the driver and the passenger seats. https://www.youtube.com/watch?v=l7fZx1sU4_8 a quick description of how such a module is used in a car alarm.


2 Answers 2


It is possible that the microwave system is very similar to that used in the older microwave police speed detector (radar gun). In your example, a low-power microwave signal is transmitted by a non-directional antenna. Any near-field disturbance is reflected back into the antenna, and is detected by a direct-conversion receiver (usually just a diode). Output from the detector is low-pass filtered to nearly DC. Near-field disturbances vary that DC voltage. Variations in that voltage signal something moving.
Sensitivity is high inside the car, because mircrowaves are bouncing all around.
Sensitivity outside the car is lower, because less microwaves leak out through plastic parts and windows.
Wiki page for "radar gun" goes into a bit more detail. In this case, the frequency of the receiver's output is proportional to vehicle velocity.


You should include a link to an example of such a sensor.

People without any RF experience often think it is easy to distinguish objects based on power. Well, it's not. The RF power attenuation over short distances (centimeters to meters range) is so little that it is hard to detect. Also it is difficult to make a cheap sensor that can detect such power variations.

What is more likely is that the sensors are simply very directional meaning they emit the RF signal only in a certain direction and ignore other directions. To get the required directivity the antenna has to be shaped in a certain way. There are EM simulator programs that can help with this design. It is still a specialist field though.

  • \$\begingroup\$ Along these lines I would hazard a guess the external zone has an antenna design with larger field than the internal zone. Would they be using different frequencies as to not interfere with each other perhaps ? \$\endgroup\$
    – D-on
    Commented Aug 29, 2016 at 14:08
  • \$\begingroup\$ One could use different frequencies but that is not needed if 1) there is sufficient separation due to the antenna patterns and/or 2) the sensors do not work at a fixed frequency but scan a band and do so in an uncorrelated way (not using the same frequency at the same time and/or 3) one could use time-division, only use one sensor at a time. For example, sensor 1 is on for 100ms, then both off for 400ms then sensor 2 on for 100ms then both off for 400 ms, and repeat. \$\endgroup\$ Commented Aug 29, 2016 at 14:25

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