> I am planning on using it as a proximity sensor similar to the
> microwave field sensors found in car alarms used in convertibles.
> Is this called a doppler effect sensor?

Yes. It senses motion and reflectivity. It won't tell you how far away
anything is.  I've used these sensors many times.

A cockroach at 6" looks the same as a man at 20'
An RF absorbing surface can move all it likes and not be detected.
It also can't discriminate between it's own motion, and the motion of
anything else. 

> It should be small 3"x3" tops, and be able to sense up to a 360 deg
> field around it.. up to maybe 8 or 9 ft.

Not actually achievable, but you can get close. 

> Any ideas? I guess using a car alarm sensor would work but they
> are kinda pricy. There must be a cheaper way to get one out there
> or a schematic I could build from scratch.

Schematics won't help much with microwaves. The PCB and enclosure ARE
part of the circuit. For some of these devices, the antenna is also
etched right into the board. Trace widths and lengths, and type of
material are critical.

The ARRL UHF and Microwave handbook will give you a good idea of what
you're up against, you'll be looking probably between 0.9 and 24 GHz.




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