🚀 go-pugleaf

> Simon Travaglia, University of Waikato, Hamilton, New Zealand writes:

> I'm pretty baseline about how detectors work etc.  Can someone explain
> or mail me how the theory of it works.  (I understand the basics of 
> reflections etc, is it Dopler-ish?)

> What's to stop you syncing in with the Speed Detector and sending 
> back info to say that you are doing 500mph?

The radar uses a Gunn diode for the transmitter which drifts alot.  The
frequency is unpredictable.  Therefore you cannot measure it and jam at a
certain Doppler offset.  The most popular techniques has been the AM jammer.
You use a Gunn diode and modulate the power supply with the Doppler shift
you want the radar to read.  This does not work with moving radar, and is
very poor on stationary radar.  The radar bandwidth is usually the Audio
frequencies, so 500 MPH will be out of band and attenuated.  Probably should
stay below 100 MPH.  If I was going to build a jammer it would sweep the
whole frequency band using just a carrier, probably with a couple hundred
Watts from a TWT :-)  This would prevent any reading with a fast enough
sweep rate (can't lock-on).

The following is an article I wrote, it's unpublished except for once
being uploaded to rec.ham-radio.  It's public domain.

----------
                        Police Radar, Detectors, and Jammers

                                  Steve R. Sampson
                                     April 1988

          Introduction

               The radar systems used by  police as well as detectors  used
          by the public,  have matured and  become more sophisticated  over
          the years.    Some  radars  even have  cameras  attached  now  to
          photograph the  speeders.   The cheapest  radar detector  on  the
          market will outperform the detector of even five years ago.   All
          of them work as advertised,  that is, warn of radar emissions  on
          two or three frequency bands.   Where you spend your money is  in
          increased sensitivity  and optional  features.   The old  crystal
          detector is  fully obsolete  and superheterodyne  detectors  have
          become state of the art.

          The Radar

               The police radars  operate in  three frequency  bands.   The
          first is X-Band  with a frequency centered  on 10.525 GHz  (Giga-
          Hertz) and 13.45 GHz.   The second is  K-Band centered on  24.150
          GHz.  The third is Ka-Band  and is centered on 34.31 GHz.   These
          radars are not very sophisticated as far as frequency  generation
          and transmission go.  They generally use a Gunn Diode in a  tuned
          cavity connected to a waveguide, circulator and antenna.

          ((Figure  1 ))

          The  circulator  is  simply  a  ferrite  rod  inserted  into  the
          waveguide.   The  purpose of  the  circulator  is to  direct  the
          transmitter power out  the antenna and the  receive power to  the
          mixer.   In  older  radars a  Majic-T  performed  this  duplexing
          function.  The  circulator performs much better  (by about 6  dB)
          and allows much smaller antennas  to be used.  The mixer  circuit
          is also  quite  simple,  usually  consisting of  a  diode.    The
          reflected target  frequencys  are  mixed with  a  sample  of  the
          transmitter frequency (which comes from  some of the power  going
          around the circulator)  producing an  audio frequency  difference
          which is directly related to the velocity of the target.  Most of
          the improvements to  the police radar have  been in the  tracking
          circuitry.

               This type of radar is called Continuous Wave (CW).  Both the
          transmitter and receiver are always on so there is no need for  a
          Local Oscillator (LO).   The disadvantage  of CW  radars is  that
          they do not measure range.  For police work however, the  officer
          is only concerned  with immediate traffic  and they can  visually
          acquire the target.  With the radar tracking the fastest  target,
          the officer merely goes after  the fastest moving vehicle.   This
          procedure is not complex or error  prone as you might think.   If
          an officer can't decide  who the offender is,  they can wait  for
          easier pickings or provide a  warning to both drivers to slow  it
          down.  The whole purpose of traffic radar is to keep the  traffic
          down at a safer speed and  penalize those who refuse.  The  fines
          come in handy when it comes  time to pay for all those that  have
          killed or  injured  themselves  (gas  for  the  fire  trucks  and
          ambulances, etc).

          Doppler Effect

               When a radar beam strikes  a stationary target, such as  the
          ground  or  buildings,   the  same  frequency   returns  as   was
          transmitted.  When a radar beam strikes a moving target  however,
          the frequency is  shifted relative to  the speed  of the  target.
          The faster a target is  moving, the greater the frequency  shift.
          This resulting frequency  shift is called a  Doppler Shift.   You
          can hear Doppler shift in many activities.  Race cars coming down
          the main stretch  for example,  have an  increasing frequency  as
          they come towards you, while  decreasing in frequency as they  go
          away.  The formula for Doppler shift in Hertz is:

          ((Formula  1 ))

          The wavelength of  the transmit frequency  in centimeters can  be
          computed using:

          ((Formula  2 ))

          For the police  radar frequencies of 10.525  GHz and 24.150  GHz,
          the wavelengths are 2.85, and 1.24 centimeters respectively.   To
          find the Doppler shift for each Mile Per Hour, simply  substitute
          one (1) for Velocity (V).   This results in  31.35 Hz per MPH  in
          X-Band and 72.06 Hz per  MPH in K-Band.  A  car moving at 55  MPH
          would cause  a frequency  shift of  1724.25  Hz at  X-Band (55  x
          31.35).   This  math is  done  electronically using  a  frequency
          counter and gate divider in the radar display.

               To accurately measure the speed of a target the radar should
          be pointed directly into the path of travel.  The reason the Race
          car frequency changes as  it goes by, is  because you are  offset
          away from the path of travel.  The angle between you and the  car
          is constantly changing.  If you get down next to the track,  then
          only one frequency will be  heard as the car approaches and  then
          recedes.  This principle is important to the police officer also.
          They must point the radar into the traffic to get the true speed.
          Any difference in angle results  in an incorrect speed  (slower).
          The  measured  speed  (Doppler  shift)  decreases  as  the  angle
          increases, until at  90 Degrees the  speed is  zero.  An  officer
          pointing the radar at the side  of your car would read zero  even
          if you were doing 200 MPH.

          ((Figure 2))

          Engineers refer  to this  resulting speed  as  Range Rate.    The
          symbol for Range Rate is R  (Dot Product, R with a dot over  it),
          and the formula to compute it is:

          ((Formula  3 ))

          What this means is that the  measured Range Rate is equal to  the
          ground speed at the speedometer; reduced by the radar angle.  For
          example a radar angle  of 90 Degrees would  result in zero  Range
          Rate (cosine  of 90  degrees is  zero).   A  radar angle  of  180
          Degrees, or pointed directly into  the oncoming traffic would  be
          minus one (-1) times the cars speed, or the true speed (cosine of
          180 Degrees is -1).  The sign information of course is  lost with
	  the Zero Hz IF (no such thing as a negative frequency).

          ((Figure  2 ))

          The Detector

               Since  the  police  radar  is  transmitting  a  signal,  any
          receiver tuned to the same frequency can detect it.  In the early
          days crystal diodes  were used with  an oscillator.   The  diodes
          were placed  in a  waveguide cavity  and  caused the  circuit  to
          oscillate when the  radar frequency was detected.   I bought  one
          and the sensitivity was poor, with range less than 1/4 of a mile.
          False alarms were  also a major  problem.   Various circuits  and
          diodes were used and sensitivity slowly increased over the years.
          It wasn't until the advent  of the superheterodyne detector  that
          realistic long  ranges  became  available.    The  comparison  in
          performance is not unlike a crystal radio to a transistor  radio.
          With the  superheterodyne circuit,  a mixing  signal is  used  to
          produce a high frequency IF (Intermediate Frequency).  This IF is
          then filtered and amplified resulting in much better  sensitivity
          and selectivity, increasing range and reducing false alarms.

          ((Figure 3))

          The circuit  complexity and  microwave  devices are  still  quite
          expensive however, and that is  the reason you pay about  $150.00
          for this type of detector.

               One problem  with the  detectors is  there are  many  X-Band
          transmitters out  there.   This frequency  is very  popular  with
          intrusion detectors and  automatic door openers  which will  also
          set off the  alarm.   For this  reason many  detectors provide  a
          'city' and 'highway' sensitivity.   What this does is reduce  the
          X-Band sensitivity  while  leaving  the  K-Band  alone.    In  my
          experience I've  found K-Band  alarms  were 99%  associated  with
          police radar, while  X-Band has many  false alarms  in the  city.
          Every grocery store has installed the new automatic sliding doors
          and the detectors can pick them out over a block away.  By  using
          the 'city' position you can knock many of these false alarms out.
          There is no  way for  the detector  to determine  whether it's  a
          police officer  or  a  door; both  emit  a  CW signal  that  look
          identical.  These sources of false alarms do not go unnoticed  by
          the police officers, and make a perfect spot to nab someone  with
          a radar detector.

          The detectors have good sensitivity from behind also.  Sooner  or
          later your detector will go  off, and like everyone you'll  start
          looking for the source.   Check that rear  view mirror every  now
          and then!

          The Jammer

               In recent years many radar  jammers have hit the streets  in
          an attempt to  create an Electronic  Counter-Measures (ECM)  war.
          These are totally  ineffective without a high  power level.   The
          most common jammer  found is called  an Amplitude Modulated  (AM)
          jammer.   It usually  consists of  a  Gunn diode  modulated by  a
          selectable frequency (Doppler shift).   The purpose of this  type
          of jammer is so a police officer would not suspect anything while
          you reduced your speed.  It's  very easy to see why this type  of
          jammer provides limited offensive capability however.  Just  like
          an AM  CB  radio, 50%  of  the  power is  in  the carrier.    The
          sidebands with the jamming information contain only a fraction of
          the transmitter  power.   That's  why  most radio  operators  use
          Single Sideband (SSB)  radios now.   On  top of  that, the  newer
          police radars lock on to  the highest speed when confronted  with
          two signals of approximately the  same amplitude.  The  effective
          jamming power will probably be  about the same amplitude as  your
          echo.  The radar ignores the jammer and locks on to the  vehicle.
          These low power (100 milliwatt) jammers only become effective  at
          very close range.   The fines for radar  jamming can run a  small
          city for the  rest of the  year however,  and basically there  is
          just no reasonable excuse for jamming tactics.

          Conclusion

               As we have  seen, the  CW police  radar is  quite simple  in
          design and  has a  significant ECM  advantage  due to  its  short
          operating range.  It performs its function easily with few errors
          or limitations.   The radar provides the  police officer with  an
          easy method of measuring speed.  The alternative is to put  marks
          on the highway, and using  a stop-watch; manually calculate  each
          vehicles speed.   This method takes  time and  gets quite  boring
          after a few minutes.   Buying yourself a  radar detector is  easy
          insurance that you won't be caught funding your state, county, or
          cities motor pool.  It's  surprising that many habitual  speeders
          do not invest in detectors.  Jammers are expensive and provide no
          advantage over  plain  detection.   The  result  is  that  jammer
          manufacturers are now trying to sell their contraptions as  radar
          calibrators for lack of  any other market.   A two dollar  tuning
          fork will perform that function just as well.
----------
          Figure 1

          A Block Diagram of a CW radar showing the basics

          Figure 2

          A picture of aiming the radar and the Range Rate (R Dot) error

          Figure 3

          A Block diagram of a superheterodyne detector

          Formula 1

                     89.36  V
               Fd =  --------
                        L

               V  = Velocity in MPH
               L  = Wavelength of Transmitter in centimeters
               Fd = Doppler Frequency in Hertz

          Formula 2

                      c
                L =  ---
                      F

               c = Speed of light (29.9792458 m/sec)
               F = Transmitter Frequency in GHz
               L = Wavelength in centimeters


          Formula 3
               .
               R = cos(theta) * V

               .
	       R     = Range Rate (Negative for closing, positive for opening)
               V     = Speedometer reading of target (true ground speed)
               theta = Radar angle off the targets heading
---------
EOF