Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!thunder.mcrcim.mcgill.edu!snorkelwacker.mit.edu!usc!zaphod.mps.ohio-state.edu!ub!dsinc!netnews.upenn.edu!eniac.seas.upenn.edu!depolo
From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: POLICE hand-held RADAR units
Message-ID: <38281@netnews.upenn.edu>
Date: 25 Feb 91 23:50:35 GMT
References: <2470014@hp-vcd.HP.COM> <37842@netnews.upenn.edu> <231@alpine.gtephx.UUCP>
Sender: news@netnews.upenn.edu
Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Organization: University of Pennsylvania
Lines: 59

In article <231@alpine.gtephx.UUCP> pfluegerm@gtephx.UUCP (Mike Pflueger) writes:
>I'd like to add a comment here - and one which CAN be used as a defense
>for radar speeding tickets.  Possibly the best "technical" defense.
>
>Due to the relative received signal strengths, the capture effect, and
>radar guns' electronic bias, the stronger return will override the weaker
>one.  Received signal strength being roughly equal, the FASTER speed will
>be displayed.

I don't see the reasoning behind this.  If the two returns are the
same is strength, there is no reason why the gun would display the
faster of the two.  All of the radar guns I've played with (4 different
models) will typically alternate between two readings if there are
two targets relatively "equal" in strength.  However, as you made
mention of, some radar guns are biased to choose the faster
speed when give the choice.  This is a bad idea.  Whenever there
is a bias in the counting circuitry such as this, it leads to 
many problems with target identification.

With respect to capturing - there is no capture effect, such as in
FM communications.  In it's pure form, a radar gun is just a transmitter,
receiver, mixer, and audio-frequency counter and scaler.  The only
place capturing can occur is in the counter.  A frequency counter has
no capture-ability - it just counts.  Some radar guns use PLL's
instead of pure counters, in which case, the PLL may "capture" the 
signal, but if designed properly, the radar gun should be able
to quickly "unlock" the current signal and lock on the stronger one.

>I believe radar guns have about a 3 degree beamwidth.  At 1000'  (1/5
>mile, roughly the detection range), the radar beamwidth is 105'. 
>Consider the angles and the radar is illuminating a LONG stretch of
>the oncoming lane.

The tightest patterns are on the order of 12 degrees these days.  One of 
the most popular units, the MPH K55, had a beamwidth of over 24 degrees 
on it's initial models (Model I, aka 1979 version).  The X band K55's
they are selling now are in the 18 degree range.  You'll typically
find wider beamwidths on the X band guns due to size limitations
of the antenna horn.

>Not only is the size of the vehicle a big factor in reflected signal,
>but shape and even the construction play a part.  I can vouch for the

>There are a lot of factors involved in reflected signal strength.  Suffice
>it to say that if there are any cars near you and the cop is coming toward
>you (or shooting your lane at a shallow angle), it can be very tough to
>determine (for sure) which vehicle is being clocked.  Now, if he's shooting
>more across the lane or no one else is around, that's a different story...

If he's shooting across the lanes of traffic at more than a few degrees
off-axis, you have the cosine effect in your favor, however.  If you're
all alone, well, sorry.

								--- Jeff
--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
 depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
 University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104


Brought to you by Super Global Mega Corp .com