Radar Gun Fundamentals

After being "illuminated" by police radar a few times, I started thinking about some of the basics a bit, I am hoping for some discussion of the fundamentals.

  1. The doppler shift from a vehicle moving a few feet per second is only a few tens or hundreds of Hertz.

  1. Typical path loss would be 50dB on the forward path and 50dB on the return path or 100dB total.

So how do you keep a +100dB carrier (and its associated close-in phase noise) out of the receiver?

My guess is you simply can't do that - does that mean you need incredible dynamic range?

How do they do that magic?

Maybe the phase noise is still well correlated with the carrier...

Reply to
wavemaker
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snipped-for-privacy@pookmail.com wrote in news: snipped-for-privacy@i3g2000cwc.googlegroups.com:

25MPH translated (by my math) to 36.7 FPS,45MPH to 66 FPS,65 MPH to 94 FPS. A uP freq counter circuit could count those easily. ISTR that some radar guns have an audio output so the officer can listen to detect intentional interference(jamming).

I believe they use the Gunn diode oscillator as the detector,too.(TX/RX) They read the freq. difference off the bias voltage,I believe.

Maybe they ignore the phase noise? I dunno.

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Jim Yanik
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Reply to
Jim Yanik

formatting link

or dont drive as fast

martin

Reply to
martin griffith

i watched a video not to long ago, they were testing different sport cars to see if excessive speed could cause the radar gun to fail. I don't remember which car it was that was able to do it how ever, the speed was around 140+ MPH if memory serves? the radar gun didn't register anything going by it. so put it to the metal and go like hell. :)

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Reply to
Jamie

I once heard -- what's probably an urban legend -- that in some state the penalties for speeding were tabulated, e.g., 0-4MPH, $xx, 5-10MPH, $yy, etc., but that it stopped at something like 50MPH over so technically it you went, e.g., 120MPH you weren't breaking any laws.

:-)

Of course, in reality, even if that part could be shown in court, you'd still be ticketed for reckless driving!

Reply to
Joel Kolstad

i might have said it here before, i once got a ticket for going over

70 in Nevada, when the limit was 55. The fine was $15.

greg

Reply to
GregS

It's been done since at least 1948, with lousy klystrons and 1N21 diode mixers.

All you do is take reasonable precautions to keep the transmitting horn from leaking in to the receiving horn. If you can get 60db of isolation that's plenty. The phase noise isnt very significant as you're mixing with the same signal, so the noise mostly cancels out.

IIRC the inventor of Radar, Watson-Watt, was stopped for speeding by a radar gun once. Hoisted by your own petard, as they say.

Reply to
Ancient_Hacker

on the 10 and 24 ghz ones, the detector is in the waveguide with a iris between it and the gunn oscillator, so its a superhetrodyne. Door opener modules are simpler, and use teh same construction, so if you want a close up look at one go to

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and look at the "Gunn Modules"

BTW a door opener Gunn module, a simple FM radio for a 88 mhz FM IF and a current modulator will get you a FM set with a range of 10 or more miles on the 10.5 GHZ ham band, same for the 24 ghz band. Much farther if you install a higher power diode and use a dish or bigger horn instead of the factory horn.

Steve Roberts

Reply to
osr

...

Don't forget that you need some of the transmitter to get into the receiver to mix with the received signal and create the difference signal.

About 40 years ago I set up a simple microwave oscillator with an diode adjacent receiver and intentionally leaked some of the transmitted signal into the receiver horn by means of a small reflector. I then amplified the output of the detector with an audo amplifier - it would easily detect people walking around over a range of 40-50 feet.

kevin

Reply to
Kevin White

Was the sound higher-pitched when they were moving faster? When there were multiple targets, did you get multiple tones? _was_ it running CW? (which it kind of sounds like)

Please, tell us everything about your experiences with that - it sounds fascinating - I had no idea such a thing could be so conceptually simple.

Thanks! Rich

Reply to
Rich Grise

..

Rich,

This was when I was at Grammar school, they had a couple of pieces of microwave demonstration equipment in the Physics lab that I was playing with.

One was a reflex klystron transmitter with a horn (producing a few milliwatts of power at about 10GHz I guess) and another was a horn with a "crystal" detector (germanium diode) - no amplification or anything. A general purpose audio amplifier with speaker was connected to the receiver. A microammeter was useful for setting up.

I placed the two units adjacent and pointing in the same direction then put a small metallic object (a metal can or something I seem to remember) close to both of them and just in view of both horns so that a small amount of the transmitter signal was reflected into the receiver - this acted as the reference signal. I adjusted the position until I had a few microamps of current in the receiver.

If I put my hand in front of the horns the current would increase or decrease. Depending upon the relative phase of the reflected signal to the reference summation or cancellation would occur.

When fed into the audio amplifier I could hear a signal whose frequency was dependent on the speed of my hand movement - the frequency was pretty low and also pretty distorted so it was more a series of thumps than a tone. It would also detect people walking a few tens of feet away (the other end of the lab) with the frequency dependent on the speed of walking or running.

Modern microwave door openers are pretty much the same using Gunn diode microwave oscillators.

kevin

Reply to
Kevin White

You heard wrong. I have had a lot of speeding experience and every state I have dealt with had a law that said if you were "in excess of"

30 over (or a similar number) that was a criminal speeding violation that you had to post collateral for to get out of jail.
Reply to
gfretwell

You use a mixer to get the difference frequency.

Reply to
cbarn24050

Radar is now becoming obsolete - most cops use lasers.

Reply to
Homer J Simpson

That is not true here. The laser requires the cop's undivided attention from a speed trap. They can set the radar and forget it. The comic alarm wakes them up. They can also use the radar while they are driving. Thre laser is good for selective enforcement where they are working neighborhood streets from a hiding place. Most of our speed enforcementr is done while running other calls.

Reply to
gfretwell

On Wed, 25 Oct 2006 04:40:37 GMT, "Homer J Simpson" Gave us:

Get yer story straight. Radar is in diminished use in Law Enforcement organizations. It is still quite heavily used elsewhere.

Reply to
JoeBloe

They banned radar speed (not intersection) cameras here to get elected, so the cops use laser and stand there with it.

Reply to
Homer J Simpson

Most of the radar in SW Florida will be shot from a moving deputy cruiser while he is running other calls. They shoot you coming at them and come get you if a speeding ticket is more important than his next call.

Reply to
gfretwell

snipped-for-privacy@aol.com wrote in news: snipped-for-privacy@4ax.com:

Are you claiming that SW Florida (local PDs) does not run speed "traps" on urban streets,but only uses moving radar during ordinary patrol? That they don't monitor school zones or local city streets? That is very hard to believe.

In Central Florida,the local PD squad cars often sit in local street medians clocking oncoming traffic,they sit motorcycles up on sidewalks under shady trees in residential areas(25MPH zones),they hide cops with radar guns in cardboard boxes next to roads,or dress them as construction workers,panhandlers,etc. Moving radar is rare unless you're on a Interstate or beltway.

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Jim Yanik
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Reply to
Jim Yanik

On Wed, 25 Oct 2006 14:40:35 GMT, "Homer J Simpson" Gave us:

Far more accurate and zero error. Radar guns are subject to parallax error, let alone calibration error..

Reply to
JoeBloe

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