Date:	Fri, 23 Feb 90 00:13:00 EST
From:	dlb!netcom!onymouse@ames.arc.nasa.gov
Subject: Re: cop detectors

> Would it be possible to build a police radio detector that detected
> the emissions from the local oscillators of the radios?  This would be
> [Moderator tack-on: Professional rigs are normally pretty-well shielded.]

I have found it very easy to detect police hand-helds, and, sometimes, mobile
sets. Some of the recent Motorola hand-helds (the HT-220 series and on) emit
such strong signals that I have picked them up as much as 200 feet away on my
Pro-30 with it in my back pocket. The radios operate on UHF with repeaters and
I pick up the signals on the repeater output frequency. 

Of course, there are those who would say that "That's impossible!" but it
does in fact happen with my radios and I don't worry about whether it's
possible or not. It happens, it works, no problem. I do not know if it works
with all makes of radios, though. 

That is one method that may work. Others are to set your scanner to pick up
the local oscillator frequency or even the transmit frequency in case the
transmitter oscillator aways idles during receive. 

jd
onymouse@netcom.UUCP

Date:	Wed, 14 Mar 90 14:10:00 EST
From:	brian@ucsd.edu
Subject: Re: cop detectors

On the other hand, a good quality scanner will hear the local oscillator
on some of the older police radios in this town for more than a block or
two.  That's far enough away to slow down!  Typically if you know the
brand and model of radio, and the dispatch frequencies, you can easily
figure the LO freq and there you are.
	- Brian

Date:	Wed, 14 Mar 90 14:53:00 EST
From:	parnass@cbnewse.att.com
Subject: Re: cop detectors

I can tell when an Illinois State Trooper is nearby --
I hear his low power, in-car mobile repeater on 155.505 MHz,
a frequency I monitor using my scanner when I'm on the highway.

Other states and some counties and towns use similar equipment.

-- 
============================================================================
Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihuxz!parnass (708)979-5414

Date:	Wed, 14 Mar 90 15:25:00 EST
From:	Pete_Simpson@dgc.mceo.dg.com
Subject: Re: cop detectors

CEO summary:
     When I'm in my car, I always have the 2-meter ham radio tuned to 
145.27 Mhz. The state interdepartmental cop frequency is 158.97 Mhz. 
Whenever I drive through 2 towns in the area, I get an intermittent 
signal which will break my squelch. It only happens outside the PD or 
near a cruiser. They all have scanning receivers and the frequency 
difference is 13.7 Mhz, close enough to IF frequency for me. 
Apparently, there's enough local oscillator leakage out the antenna 
of some of the radios they're using to be detectable a block or so 
away. With the new wideband synthesised radios, the filtering is not 
tight enough to knock down the local oscillator signal enough 
(remember, it's only 10.7 Mhz away from the RX freq. and the band 
runs from 150 - 170 Mhz).

Date:	Wed, 14 Mar 90 16:11:00 EST
From:	geek <KRAINIER@eagle.wesleyan.edu>
Subject: Police repeater detection

    Due to a leaky memory, I do not recall all details, but about a month or
two ago I spotted an item in a "yuppie catalog" that purported to detect patrol
cars up to <insert large distance> away by picking up their repeater signals.
The device was designed to be vehicle mounted (so as to pick up police using
radar that is only turned on when you are in sight).  Of course, it relies on
the supposition that the officers left their repeaters on while in the vehicle
[they acknowledged this but claimed that most do in fact leave their repeaters
on].  The device did not broadcast what it received, it only indicated that
something was being broadcast.

    Anybody seen anything similar?  Any comments on range/feasability/other
problems?

                                                -kevin
                                                krainier@eagle.wesleyan.edu
                                                krainier@wesleyan.bitnet

Date:	Wed, 14 Mar 90 17:33:00 EST
From:	kodak!ornitz@cs.rochester.edu
Subject: Re: cop detectors

Actually, ignoring the other radios might be a trivial task if they operate on
different frequencies than the police.  The problem is that while a police
department might only operate on a small number of frequencies, they can use
any number of different brands and models of radios each having different
intermediate frequencies.  Thus, a number of local oscillator frequencies 
might have to be monitored. It might be possible in any given area for two
different models of radios operating on two different frequencies to have the
same local oscillator frequency, but it is not likely.

Unfortunately, a first intermediate frequency of 10.7 MHz is standard for a 
large number of VHF radios.  In this case the number of local oscillator
frequencies is greatly reduced.  Older models of many radios typically did
not standardize on the IFs in common use today.

A much more difficult problem is the issue of sensitivity.  By FCC rules,
local oscillator radiation must be attenuated to low levels.  These levels are
so low that detection would be nearly impossible over any useful range with
today's radios.  This was not the case with most receivers before the mid-1950s
or so (the ones with the radiating local oscillators - RF amplifier stages
were often not used ahead of the first mixer in early days).  While the
sensitivity of the detecting receivers has increased over the years, it has
been inadequate to keep up with the decreasing amplitude of radiated LO
signals.

To conclude, I believe this to be a non-problem with equipment available to
the ordinary burgler.  While a pocket scanner might receive all of the local
oscillator frequencies used by the local police, its detection range would
likely be less than a hundred feet.

 -----------------
|  ___  ________  |       Dr. Barry L. Ornitz          WA4VZQ
| |  / /        | |  UUCP:  ....rutgers!rochester!kodak!ornitz
| | / /         | |       Eastman Kodak Company
| |< < K O D A K| |       Eastman Chemical Company Research Laboratories
| | \ \         | |       P. O. Box 1972, Building 167B
| |__\ \________| |       Kingsport, TN  37662       615/229-4904
|                 |  INTERNET:   ornitz@kodak.kodak.com
 -----------------

Date:	Wed, 4 Apr 90 13:29:00 EDT
From:	netcom!onymouse@claris.com
Subject: Re: cop detectors

Of course, if a department doesn't want a deal on a quantity purchase of 
radios, the department may well buy several different makes/ models. Most
departments do buy one make and model, though, and do so in quantity. This
 way, they can get a discount and only need pay for one service contract.
It simpifies the problem of figuring out which freqs to check out.

>            ...  While a pocket scanner might receive all of the local
> oscillator frequencies used by the local police, its detection range would
> likely be less than a hundred feet.

OK, I confess: I am not picking up the LO from the hand-helds. The signal
is always on the repeater frequency. I don't know where it's coming from
inside the radio but I can detect it as much as 1/4 away with a good antenna.
(I'm about to see if I can pick up weaker sigs with an LNA after tha antenna.)
How did Motorola get away with producing radios like this, I don't know but
I don't really mind.

jd
onymouse@netcom.UUCP