The Doppler story
 
 
  The principle behind the Doppler or Doppler Direction Finding was first discovered by Austrian physicist Christian Doppler in the nineteenth century.  He published his findings in an essay entitled “On the Colored Light of the Double Stars and Certain Other Stars of the Heavens” in 1842.
 
  The Doppler effect is readily observed in sound waves, as you have surely noticed when a train passes you with the horn sounding.  The pitch suddenly drops as the engine passes by.  The effect occurs when the distance between a source emitting a wavelength and one that is receiving it is changing, as they are approaching the wavelength appears to be shortening and as they draw apart the wavelength gets longer.  The principles behind the Doppler Effect holds true not only for sound but also for light and all forms of electromagnetic radiation including  (Radio waves).
 
  The first description of a Direction Finder goes all the way back to 1947 by H.T. Budenbom who used the Doppler modulation with the assistance of a rotary system of antenna.  Today, there are more than 300 inventions or patents recorded using Doppler's system throughout the various layers of the electromagnetic spectrum.
  Doppler direction finding for Amateur Radio began in 1978 when Terence Rogers (WA4BVY) wrote an article in QST about a practical unit using 16 leds for the 2 meters band.  Other systems with 32 and 36 leds made their appearance on the market using CMOS and TTL logic technology. W6AOP sold several kits using the CMOS version only.
  In 1981 several commercial firms began selling various products for the Amateur Radio community, like the Dick Smith Doppler Direction Finder.  A VHF Doppler system made with integrated circuits was designed by Chuck Tavaris (N4FQ).
 
  Here in Montreal Jacques (VE2EMM), user of a PIC microcontroller produced by the Microchip company, improved the existing Doppler system by adding several options that makes the Montreal Doppler 3 a very interesting project.
 
  Here are some informations on Jacques VE2EMM system :
 
  The characteristics :
1.- The MTL Doppler 3 has a 36 led display. When the center led is green the direction is
     good, and when it’s red it means that there's no signal, the last good bearing stays
     on until the next new signal.
2.- Three PICs microcontrollers are used, a 16F628A for the display, a 16F877A for the
     main circuit and a 12F675 as a frequency divider.
3.- Two audios filters are used, the max 267 is the best pass band filter and it's simple
     to use, it’s followed by the original Roanoke filter which has a very narrow total
     band-width (+/- 0.5Hz).
4.- The Doppler II phase integration and detection software is used
5.- A LM386 is used as an audio amplifier that's independent of the Doppler
6.- A simpler menu selection is used, turn a button to choose and press the switch "to
     activate"
     
7.- It can be used with 4 antennas commutated towards +V or - V or out of differential
     like 8 antennas +/-.
8.- Activating the switch sends the direction towards APRS.  The protocol is :
     CR><LF>%359/Q<CR><LF>
     Q (0<8) and is the quality of the signal taken right before the extraction of the
     phase.
 
9.- The information of the GPS passes through the Doppler and stops when the direction
     is sent to a PC.
 
  The 16 menus :
0.- During normal operation, the push-button starts only one transmission towards the PC
     and the APRS (This menu is optional, August 14 04).
 
     The first line displays : The QUALITY FACTOR (amplitude of the signal on the outlet
     side of the integrator), and the DIRECTION in degree and the number of at present
     accumulated returns.
 
     The second line posts : A bar-graph of the signal present on the output side of the
     first filter of the Doppler or the signal of the S-Meter of your radio.

1.- Normal operation, the pushbutton alternates between integration and no integration.
     The first and the second lines displays the same data as in menu #0.
2.- Reduction of the integration.
3.- Increase of the integration.
4.- Automatic calibration forwards of the car.
5.- Manual reduction of the calibration.
6.- Manual increase of the calibration.
7.- Selection of the minimum factor of quality(0<8), the direction is displayed for this
     value and more. If the quality decreases, the display freezes with the last good
     direction.
 
8.- Future Menu.
 
9.- Future Menu.
 
10.- Baud Rate selection for the TX towards the PC, 2400, 4800, 9600, 19200.
 
11.- Two choices for the bar-graph, the S-Meter or the output lead of the MAX267 filter.
 
12.- There are three calibration choices for various receivers and/or cars.
 
13.- Antennas rotation selection, CW or CCW.
14.- Two antenna choices 4 + or - and 4 differentials of 8 antennas + or -.
15.- Choice of the antenna under test (each antenna can be activated individually),