From: "Michael A. Terrell"
Subject: Re: Radar basics?
Date: Sat, 19 Oct 2002 21:18:58 -0400
Organization: Do I have to? Well, Ok. If you insist!
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Dave VanHorn wrote:
> > > Homodyne dopplers don't know distance, and if you rotate the antenna,
> > > will tell you they are seeing motion, because effectively, you're moving
> > > world. :)
> > Hi Dave, not all radars are pulse radars. CW radar is common in the
> > military, you can get distance by adding some low frequency modulation
> > and measuring the phase shift on the returning signal.
> That's why I used the quotes. The field disturbance sensors were marketed
> years ago as "radar" alarms, but they never had any concept of direction or
> range. "RADAR" is about getting direction and range, no matter how you do
> it. FD sensors can't tell the difference between a roach at 6" or a truck
> at 20'. And that's mostly why they aren't used anymore for alarm systems.
> People always set the gain controls to "max" because obviously more is
> better, then they get mad when the system falses. PIR was a wonderful
> change, when it was introduced in the 70's finally a sensor that you really
> have to try, to screw up.
> This sensor that they were discussing at the start of this thread, is almost
> certainly an FD sensor.
> Bulldog markets them for car alarms, I think in the 2.3G range. I have a
> couple on hand.
> They used to be done down at 900 or so. for home use.
Some high end PIR detectors have a microwave detector built it to
prevent false alarms from air currents when the heat or AC kicks in. I
used some on a college campus a few years ago. The microwave sensor
kicks in for a fraction of a second to look for motion, then shuts down
if it doesn't find anything. That makes it harder for someone to locate
and disable the security system.
Michael A. Terrell