Steamer, I think your question goes beyond the current state of the
art - at any price.
RADAR needs to be scanned to build up an image (linear image with a
circling antenna or 3D image with a more complicated 2D scanning
antenna, or with an electronically steerable antenna array). None of
these are labor intensive devices (unless you can machine a small
antenna with fast 2D scanning abilities). You will eventually hit the
need for lots of electronics and computing power.
As an aside, I think it was an engineer at Lockheed that bolted a
bunch of TV antennas to an outside wall here in Gaithersburg MD, about
10 years ago and processed the signals to generate radar like images
from aircraft and other items in the area. TV stations and Radio
stations put out hundreds of kilowatts from stationary antennas, and
that RF signal bouces off of remote objects and comes back to the
passive listener.
Passive RADAR issues:
-low frequencies/long wavelengths mean large antennas - Lockheed had
an outside wall decorated in antennas.
-some TV transmitters might go up to 600 MHz, so smaller antennas are
possible. Cell phones go from 875 MHz up to about 2 GHz.
-your antenna might be an array of small antennas but then you have to
electronically switch, amplify, receive and process multiple streams
of signals
-processing each signal to detect fractional differences in arrival
time which would then indicate direction of signal (in 3D) is compute
intensive. I.e. you are trading receiver simplicity -- passive
antennas -- for computational effort (this is why radar dishes swivel
- they assume they are only looking at the single transmitter source
in one direction, and distance is determined by the time it takes for
the signal to reflect back to the antenna)
-You cannot generally detect living things with RADAR. The reflected
energy is going to be small, and you will need more dynamic range
(small reflection and hence more processing power or larger antennas)
to identify these living things. (Maybe flocks of birds at airports
are one of these cases where living things are detected)
The new terahertz radars are interesting, in that the wavelengths are
so small that at least in theory you could fit an entire antenna array
in a handheld device, but that's in the far future.
http://www.space.com/businesstechnology/technology/t-ray_camera_020613.html
Cheers